i

ABSTRACT

Dipturus batis, the largest of the European Rajiforms, was once common around the coasts

of Britain (Brander, 1981). In recent years due to fishing pressure, its numbers have

receded dramatically from the Irish Sea (Brander, 1981) and the North Sea (Walker and

Hislop, 1998). Little is known about the life history characteristics of this species,

therefore, to better understand its biological characteristics a study was done, mainly using

the data collected by the Glasgow Museum Tagging study, but also with data collected by

the author. The data was split into three groups, A, B and C after it became clear that bias

due to fishing effort and local changes in bathymetry influenced the results as a whole to

greatly to be analysed. Results showed a significant association between the weight and

depth for female fish in all areas (A, B and C), with larger fish being caught in shallower

depths, this correlation was strongest in area A (Spearmans Rank Correlation 2-tailed test,

p=<0.01). The same correlation was found for all males in the study area as well as

(p=<0.05) and there was a strong association in area A (p=<0.01). Using age at maturity as

11 years old (Brander, 1981, Ellis and Walker, 2005 and Little 1995), weight at maturity

was estimated to be between 33-42kg for males and 69-96kg for females, these estimates

are substantially higher than those quoted in the literature and suggest, especially for

males, that either D. batis matures earlier, or is heavier than previously thought at

maturation. It was found that the economic worth of D. batis in area C is higher than its

commercial value and that charter boat trips annually bring £292,500 to the local economy.

Keywords: D. batis, Rajiformes, seasonal migration, maturation, depth, weight, economic

worth.

ii

ACKNOWLEDGEMENTS

My thanks go to:

Dr Alastair Lyndon from Heriot Watt University who supervised this project, his continual

enthusiasm, assistance and guidance throughout this study was invaluable and greatly

appreciated.

Richard Sutcliffe from Glasgow Museums for initially allowing me the use of the Glasgow

Museum Tagging Study data, which he himself has collected since its initiation, but also

for the continued help and assistance he gave during the project.

SNH, SSMEI Sound of Mull Project and Argyll and Bute Council Local Biodiversity

Action Plan for funding the project.

Dr Jane Dodd for accepting me to undertake the research and for her continued enthusiasm,

help and support throughout this study, it made the collection of data in Oban run much

more smoothly. Thanks also to all staff at the Oban SNH office who’s patience and support

was much appreciated.

Dr Sarah Cunningham for all of her help and support from beginning to end, especially her

guidance with GIS software

Ronnie Campbell for all the boat time he provided for research trips. His continued help

and support throughout the write up of this report was greatly appreciated.

I would also like to thank the following people for their contribution to this report: Davy

Holt, Professor Jon Side, Adrian Lauder, SSACN, all the anglers who have contributed to

the Glasgow museum tagging project throughout its course

iii

CONTENTS

Table of Contents Page number

ABSTARCT i

ACKNOWLEDGEMENTS ii

TABLE OF CONTENTS iii

TABLE OF FIGURES AND TABLES v

ABREVIAITONS vii

CHAPTER .1 INTRODUCTION 1 1.1 Background on D. batis 1

1.2 Distribution 1

1.3 Feeding 3

1.4 Migratory Behaviour 4

1.4.1 Seasonal depth migrations 4

1.4.2 Population movements and aggregations 5

1.5 Life History 6

1.6 Pressures 8

1.7 Potential for Re-populating 11

CHAPTER 2. MATERIALS AND METHODS 12 2.1 The Study Area 12

2.2 Data Collection 13

2.2.1 Collection of biological data 13

2.2.2 Extra Field Work undertaken towards this project 16

2.2.3 Measurements from landed fish 18

2.2.4 Collection of economic data 19

2.3 Data Analysis 20

2.3.1 Population segregation 20

2.3.2 Life history 21

2.3.3 Movement 22

2.3.4 Population estimates 23

2.3.5 Economic importance of D. batis 24

2.3.6 Value of skate compared to commercial market 25

2.3.7 Statistics 26

CHAPTER 3. RESULTS 27 3.1 Behavioural Ecology of D. batis 27

iv

3.1.1General Observation of the population throughout the study area 27

3.1.2 Seasonal observations of the population throughout the study area 33

3.1.3 Areas A, B and C 37

3.1.4 Annual migration 48

3.1.5 Population estimates 52

3.1.6 Fishing mortality 52

3.2 Economic importance of D. batis 53

CHAPTER 4 DISCUSSION 58 4.1 Maturation 58

4.2.1 Population segregation 61

4.2.2 Areas A, B and C 63

4.3 Migratory Behaviour 64

4.4 Population Estimates 66

4.5 Economic Importance 68

4.6 Life History 69

CHAPTER 5. CONCLUSIONS 70 Conclusions from this study 70

Further areas for Study 71

REFERENCES 73

APPENDICES 78 Appendix 1 Individual Growth rates 78

Appendix 2 Growth rate Scatter plots 79

Appendix 3 Spearmans Rank Results 81

Appendix 4 Distance travelled by each fish 82

Appendix 5 total number of fish caught each year 83

Appendix 6 Tables used for population estimates 84

Appendix 7 Logsheets used for data collection 88

Appendix 8 Questionnaires 89

Appendix 9 Weight Charts 91

v

Table of figures

Figure 1.1 Map of D. batis Distribution 2

Figure 1.2 Relative numbers of eggs laid per species on an annual basis 7

Figure 1.3 Increase of smaller species in commercial landings 10

Figure 2.1 Map of the Study Area 12

Figure 2.2 Photograph of the Jumbo Rototags 13

Figure 2.3 Diagram of restrictive nature of Rototag 13

Figure 2.4 Diagram of D. batis showing tagging site for Rototag 13

Figure 2.5 Floy Dart Tag 14

Figure 2.6 Needle Applicator for Floy Dart tag 14

Figure 2.7 Diagram of D. batis showing tagging site for Dart Tag 15

Figure 2.8 Fishing gear used for catching D. batis 16

Figure 2.9 Diagram of D. batis showing Gaffing sites 16

Figure 2.10 Diagram of D. batis showing measurements taken 17

Figure 3.1 Map showing the distribution of both sexes throughout the study area 28

Figure 3.2 Histogram showing the number of Males caught in each weight class 29

Figure 3.3 Histogram showing the number of Females caught in each weight class 29

Figure 3.4 Scatter Plot showing weight related depth distribution for males 31

Figure 3.5 Scatter Plot showing weight related depth distribution for females 31

Figure 3.6 Sex ratio over time 32

Figure 3.7 Map showing the seasonal distribution of males 33

Figure 3.8 Map showing the seasonal distribution of females 34

Figure 3.9 Median depth of males and females 35

Figure 3.10 Growth rate of male D. batis 36

Figure 3.11 Growth rate of female D. batis 36

Figure 3.12 Sex ratio in areas A, B and C 38

Figure 3.13 Histogram showing numbers of males and females caught in area A 39

Figure 3.14 Histogram showing numbers of males and females caught in area B 39

Figure 3.15 Histogram showing numbers of males and females caught in area C 40

Figures 3.16 Histograms showing numbers of males in each weight class for all areas 41

Figures 3.16 Histograms showing numbers of females in each weight class for all areas 41

Figure 3.18 Map showing depths fish were caught at 42

Figure 3.19 Median seasonal depth values for males and females in Area A 43

Figure 3.20 Median seasonal depth values for males and females in Area B 44

Figure 3.21 Median seasonal depth values for males and females in Area C 44

Figure 3.22 Seasonal weight related depth distribution for females in area A 46

Figure 3.23 Seasonal weight related depth distribution for males in area A 46

Figure 3.24 Seasonal weight related depth distribution for females in area B 47

Figure 3.25 Seasonal weight related depth distribution for males in area B 47

vi

Figure 3.26 Map of longest migration routes 48

Figure 3.27 Map showing localised travel of males 49

Figure 3.28 Map showing localised travel of males 49

Figure 3.29 Map showing localised travel of females 50

Figure 3.30 Map showing localised travel of females 51

Figure 3.31 Mean distance travelled by males and females 51

Figure 3.32 Population estimates 52

Figure 3.33 Fisheries mortality for D. batis 53

Figure 3.34 Percentage of participants on D. batis angling trips compared to Highlands

and Islands 56

Figure 3.35 Percentage contribution of charter trips from Oban to Highlands and

Islands watersports Industry 56

Figure 3.36 Amount spent on the top four water sports in the Highlands and Islands 57

Figure 3.37 Comparative value of fish to t he sport and commercial fishing industries 57

Figure 4.1 Sigmoid growth curve 60

Figure 4.2 Diagram of hypothetical movement 65

Table 3.1 Estimated amount of money brought to the local economy by anglers 54

Table 3.2 Estimated value of an individual fish 54

Table 3.3 Estimated value of fish to area C 55

vii

ABBREVIATIONS USED THROUGHOUT THIS REPORT:

CEFAS: Centre for Environment, Fisheries and Aquaculture Science

FRS: Fisheries Research Service

ICES: International Council for the Exploration of the Sea

IUCN: International Union for the Conservation of Nature

MarLIN: Marine Life Information Network: The website of the Marine Biological

Association of the UK and Ireland

SAH: Scottish Angling Homepage

SNH: Scottish natural Heritage

SSMEI: Scottish Sustainable Marine Environment Initiative

SSACN: Scottish Sea Anglers Conservation Network

1

1. INTRODUCTION

1.1 Background on D. batis

The Common skate Dipturus batis (formerly Raja batis or Raia batis Linnaeus 1758)

was once common around the coast of the British Isles (Brander, 1981 and Fowler &

Cavanagh, 2005) but due to increased fishing efforts, both commercial and sport

angling, their numbers have rapidly decreased (Brander, 1981). D. batis is a favoured

sport fish and has been pursued by anglers for many years, this has led to its depletion

around the coast of Orkney, where a productive sport fishery has reduced its numbers

(Anon, 2000). The number of D. batis caught in commercial trawls began to decline in

the 1920s and, after a brief recovery period during World War II, they had all but

disappeared from North Sea trawls by the early 1980’s (Walker & Hislop, 1998). One of

the main reasons that these industries have impacted D. batis so much is partly due to

our lack of knowledge of the species. The species is currently on the IUCN red list as an

endangered species throughout its range (IUCN, 2007) and the UK Biodiversity Action

Plan (Anon, 1999) although neither of these offers it any protection in terms prevention

of landing and killing.

The aim of this project was to gain more insight into the life history

characteristics and behaviour for the species as well as assessing the population around

the Isle of Mull, this was accomplished by using the data set provided by Richard

Sutcliffe at the Museum of Glasgow. This report also looks at the economic value of D.

batis to the local economy to the Oban area.

1.2 Distribution

D. batis is the largest of the European Rajiform (IUCN, 2007) with females reaching a

length of 285cm (approx 120kg, weight estimated from weight chart in appendix 9) and

males, 205cm (approx 75kg, weight estimated from weight chart in appendix 9) (Ellis

and Walker, 2005) (length is measured from snout tip to tail tip and can be seen in

Figure 2.10), although there is evidence from commercial catches suggesting that they

may grow up to 180kg in weight. In common with many other species of

elasmobranchs, D. batis is slow growing, late to mature and has relatively low fecundity

(Brander, 1981). They can be found in the northeast region of the Atlantic, ranging from

the Mediterranean near Madeira and northern Morocco up to the Barents Sea and across

2

the Atlantic to Iceland (MarLIN, 2008) (Figure 1.1). They are found at varying depths

throughout their range but are most commonly found in water around 200m deep on the

edge of continental shelves (IUCN, 2007), although in the Barents Sea in the north of

their range, they have been found in water deeper than 300m, and were most prevalent

at depths of 550-800m (Dolgov et al 2005b). A CEFAS trawl study around the British

coast classed D. batis as a deep-water species, distributed along the outer continental

shelf edge of the Celtic and northern North Sea (Ellis et al, 2005). However, this does

not represent the distribution of the species fully as it is often taken by shore anglers,

suggesting that the preferential depth for D. batis cannot be easily assigned, it has also

been hypothesised that depth distribution may be linked to the life history of the species.

They are tolerant to temperature variation as shown by Dolgov et al (2005a) in the

Barents Sea, where it the species occupied a wide temperature range even within a

small geographic region.

Figure 1.1: Map of the known distribution of D. batis throughout Europe. Range marked in orange fill. Adapted from ZeeInZicht.

3

1.3 Feeding

It is recognised that D. batis has a varied predatory diet ranging from benthic feeding on

crustacea and molluscs (Little 1995) to demersal species such as cod (Dolgov, 2005c).

In a study on the stomach contents of different species of skate in the Barents Sea by

Dolgov (2005c) it was shown that fish contributed approximately 70% of the diet for D.

batis with young cod, haddock, halibut and long rough dab being prevalent. Fisheries

waste was also a major constituent of their diet, contributing 25% by mass, Dolgov

(2005c) noted that this could not be seen as an indicator of poor abundance of prey

species, but was more indicative of the large amount of available fisheries waste in the

area due to high fishing intensity. There are some noted morphological differences in

tooth shape between the sexes, these have been noted and substantiated by numerous

anglers. Males appear to have more cuspidate teeth associated more with pisciverous

fish, while females have more molariform teeth, more in common with bottom feeding

fish, these differences in tooth shape suggest different feeding strategies, which might in

turn suggest different habitat selection due to different prey items. As mentioned, D.

batis is known for having a varied diet, but the possibility that this might vary with sex

has never been studied and the sex of the fish studied in the Barents Sea was either not

looked at or not disclosed. If it is the case that there is a difference in the feeding

regimes between the sexes, then we may expect to see a difference in behaviour,

especially migratory. It is possible that males, relying on migratory fish species as prey,

follow the migration routes of fish such as cod while females, having a more varied diet,

are able to stay within a smaller hydrographical region, opportunistically feeding on the

benthos and in the demersal water column. However, it is known that males do feed on

the bottom as anglers use benthic targeting equipment, and dead bait for skate fishing

and regularly catch males. Dolgov’s study looked at 39 specimens of D.batis over 6

years, not a large number, which is perhaps a reflection of decreasing population levels.

There was no breakdown of results by size for D. batis in his study so no change in diet

for size/age is noted; other species however which were caught in larger numbers did

have results broken down into stomach contents by length. For the Thronback Ray

(Raja Clavata) it can be seen that as the fish grows, its diet changes, smaller fish having

a more benthic based diet, 70% consisting of largely polychaetes and benthic

crustaceans these diet components not being present in larger fish, where the diet

consists mainly of fish (Dolgov et al, 2005c). If we assume a similar behavioural change

in D. batis we might expect a similar change in diet, although we may anticipate fish to

4

appear in stomach contents at an earlier age due to the larger size of D. batis allowing it

to handle larger prey items at earlier. If this is true, then we would expect only larger

fish to migrate to follow fish stocks, while smaller immature fish may remain in the

same place, feeding on benthic invertebrates.

1.4 Migratory Behaviour

1.4.1 Seasonal depth migrations

The annual movements of D. batis are unknown and previous tagging programmes have

been aimed at trying to understand any patterns in the movement of adult fish. However,

since it is mainly anglers undertaking the tagging, it tends to be the larger, adult fish that

are targeted. The initial thinking was that, due to their shape, they remain in the same

place throughout the year, although Stephen (1929) proposed the idea that skate of all

sizes and sex undertake annual migration to deeper water throughout the winter,

returning to the same shallower waters in the spring time, other species of Rajiform

have been shown to make such annual migrations. One of the best examples of this is

shown research carried out by Hunter et al (2005 a & b) focusing on the migratory

behaviour of R. clavata. Electronic data tags measuring depth and temperature every 10

minutes were used, which revealed that this species made an annual depth migration

(Hunter et al, 2005a). The animals moved from deeper water (20-35m) during the

winter months (November –February) into shallower water (<20m) in spring and early

summer months (March – June). This supported previous studies (Walker et al 1997)

and the timing was in direct relationship to the R, clavata reproduction cycle (Holden,

1975). If we put forward the hypothesis that D. batis performs similar migrations to R.

Clavata, then we would expect to see an annual movement of the species from deeper

water (600-200m) during winter months to shallower water (200-50m) in summer

months in direct relationship to its breeding cycle, which is thought to be between

March and June (MarLIN, 2008). Hunter et al (2005b) also focused on the movements

of a single mature female R. clavata. Using the information gathered by an electronic

data tag and a tidal location method (Hunter et al, 2005b), a migration route for this

individual could be plotted over 423 days. A clear pattern was found as the female

moved to the shallower water within the Thames Estuary in spring through to late

summer time, presumably to spawn, then to the deeper waters off the Dutch coast

during autumn and winter months. It repeated this migration 1.5 times, being released in

London and recaptured off the Dutch coast. Although this was only one animal and

5

therefore not representative of the whole species, it does highlight that annual migration

can occur within the Rajiformes and that some individuals are likely to remain in a

regional population. The migration work of Hunter et al, (2005a&b) on R. clavata

could explain the noted recapture of D. batis specimens within the same area (Little,

1995), Although it is known that some of the population remain in shallower coastal

waters during winter months as some specimens are caught (Little 1997&1998) and

there is anecdotal evidence from anglers that they catch D. batis during the winter

months, although few assumptions about the decline in caught and recaptured fish

during the winter months can be made as this may be down to decreased fishing effort

alone (Little, 1995).

1.4.2 Population movements and aggregations

It is unknown if D. batis populations are closed or recruit from other regional

populations; it could be that individuals of both sexes move between populations. If this

inter-population recruitment does take place it could explain why some fish were caught

further away from their original catch site, they may have been en-route to another

breeding area; there have been no genetic studies to prove or disprove this though.

Trawl surveys undertaken in the North Sea have shown that skates and rays can have

reasonably discrete species distributions, with each species occupying a slightly

different environment, often depth related (Walker and Hislop, 1998) which suggests

that species congregate, either all year round situation or only during spawning. The

Glasgow tagging study has previously shown some possible congregation areas for D.

batis on the west of Mull (Anon, 2000) where animals seem to gather and then disperse

in northerly and southerly movements.

As well as species segregation between Rajiformes, it is also thought that there

is a degree of segregation within the same species based on sex, size and maturity (Ebert

et al, 2008). It is unclear as to what extent this segregation takes place and whether

segregated groups all move within the same population area, or whether each group

occupy different habitats, based mainly on feeding and predator avoidance. In support

of different groups occupying different areas, anecdotal evidence from anglers report

that in the Crinan basin, off the west coast of Scotland, there seems to be a population of

larger juvenile males and few if any females are caught. However, whilst the anglers

claim these male fish to be larger specimens, the angling gear used would very rarely

catch smaller individuals so a full picture of the resident weight range is unclear. Some

6

trawl surveys have caught mostly immature fish (Walker and Hislop, 1998) suggesting

possible congregation of this age group in this region, although the data are not clear

enough to determine if this is a single sex congregation or not. It was also noted in a

Barents Sea study that some long line catches could compromise 100% of D. batis and

other times, no D. batis were caught (Dolgov, 2005b), further suggesting that the

species moves in groups, there was no breakdown of these catches with regard to sex or

size mentioned.

D. batis has also been shown to travel individually; a trawl survey along the

edge of the continental shelf off Norway caught all its specimens of D. batis singly

(Dolgov et al, 2005a). All of these differing reports suggest that the distribution and

segregation of D. batis is complicated and little is fully understood. However, it is likely

that in shallower coastal waters, mature populations come together for spawning as

suggested by Little (1995 and 1997). Using the data collected through the Glasgow

tagging programme, his study produced an interesting pattern regarding the movements

of males into and out of the area. The proportion of males caught increased in the

spring, suggesting an increase in numbers and declined again in the early summer, then

increased again in September/October (Little, 1997). There is evidence from other

Rajiform species that as their population levels decrease, populations have aggregated in

certain key areas, with fewer pressures allowing for increased survival, these are

becoming known as essential fish habitats (Payne et al (2008). If D. batis follows this

survival behaviour we would expect to see a population increase within an area, as well

as an increase in the diversity of sex, size and maturity as groups come together.

Currently there is not enough information to map out segregation within species

accurately.

1.5 Life History

Growth rates for D. batis have been shown to be 1-9kg per year for males and 4-13kg

per year for females (Little, 1998) and they are generally thought to live to 50yrs of age

(IUCN). Both sexes reach maturity at approximately 11 years old, with males being

125cm long (Fowler & Cavanagh, 2005) and females 180cm (Walker & Hislop, 1998).

The number of eggs laid per female is dependent on her size but is estimated to

be approximately 40 eggs (Brander 1981) which get deposited on the sea bed once

every two years (Little 1995), which is low compared to other European species of

Rajiform. In a study by Koop (2005) on five Raja species (Raja brachyura, R. clavata,

7

R. microocellata, R. montaggui and R. undulate) it was shown that the average

fecundity of females was 117 on an annual basis.

However, the skate studied were all smaller species than D. batis (R. brachyura is the

largest of the group at 125cm long, less than half the size of D. batis) and lay smaller

eggs. If the fecundity of D. batis is compared to a Rajiform of similar size such as

Dipturus trachyderma (Roughskin Skate), from the southeastern Pacific Ocean, which

grows up to 258cm long (D. batis 285cm) there is similarity in their fecundity with D.

trachyderma laying between 28 – 68 eggs, depending on the size of the female. This is

similar to the estimate per female for D. batis, supporting it as a reasonable estimate.

The exact breeding time for D. batis is unknown and is hard to estimate because

Rajiformes are able to mate many weeks in advance of the females laying their eggs.

The females are able to store the sperm and then lay eggs throughout a specific season

Figure 1.2 (Koop, 2005). In the case of D. batis is in the spring and summer months

(Ellis and Walker, 2005 and MarLIN, 2008). If we assume that sex segregation occurs,

then breeding can only occur when the sexes mingle, according to previous analysis of

the Glasgow data, this would appear to be in spring time when the proportion of males

increases, this assumes that the sexes do not come together in another area (Little,

1995). The eggs are 14.4-24.5cm long (Ellis and Walker, 2005) and the length of time

before the eggs hatch varies from 2-5 months dependent on temperature (Little, 1995).

Figure 1.2: Relative numbers of eggs laid per month of five species of rays in the ‘Ray Reef’ of Dolphinarium Harderwijk, 1993-2001. Raja brachyura, R. clavata, R. microocellata, R. montagui and R. undulate. Adapted from Koop (2005)

8

These life history characteristics suggest that instead of a ‘bloom’ of hatchling skate,

there is a more constant recruitment throughout the spring and summer. A more in depth

study of the spawning cycle of D. batis could confirm fecundity, mating, laying and

hatching season as the timing of the reproduction cycle of different species of Rajiform

can vary considerably. This is highlighted in a study by Koop (2005) in which the

reproduction of different species of captive Rajiform where looked at. All species

displayed a similar reproductive cycle, although the timing and duration of this cycle

varied considerably with some species laying large numbers during April to June season

while other species lay lower numbers but more consistently between November and

July Figure 1.2.

The young of D. batis hatch from the eggs at approximately 22cm long with a

13-15cm wingspan (Brander 1981). They have fully developed spinal columns and

sturdy spines (Walker and Hisplop, 1998) the movement of these hatchling fish is

unknown as they are too small to be targeted by sport anglers and capture by

commercial fishing is unrecorded. If D. batis does segregate by sex and size, then we

would expect to see a mass movement of young out of the area. It is thought that

breeding, laying and hatching occurs within the Firth of Lorne and Sound of Mull due to

the presence of mature males and females during the spring and summer months (Little

1998) and anecdotal evidence from commercial trawl skippers confirming D. batis egg

cases are brought up in trawl nets.

1.6 Pressures

Elasmobranchs are well known for their vulnerability to commercial fisheries, and of

these, skates and rays are arguably the most sensitive (Dulvy et al, 2000). For the same

level of mortality among Rajiform species, it is expected that the longer the maturation

period (Jennings et al, 1998) and the larger the body size (Dulvy and Reynolds, 2001)

the greater the population decline. This places D. batis as one of the most sensitive

species of Rajiform as it is a strong K-strategist (Stevens et al, 2000), has the longest

maturation period for any species of Rajifrom and is the largest species in Europe. Even

the hatchlings of D. batis are vulnerable to commercial fishing due to their size and

morphology (Brander 1981). It is also true that species in higher trophic levels are more

susceptible to decline (Jennings et al, 1999) and the size of D. batis puts it at a higher

level than other Rajifrom species due to the size of prey it can consume, including

smaller species of Rajifrom (Dolgov et al, 2005c). Current estimates suggest that the

9

population of D. batis could be declining by as much as 35% a year (IUCN, 2007). This

large depletion of the population probably leading to significant changes in distribution

(Oddone and Vooren, 2005) as is with the case of R. clavata which appears to be

aggregating in ‘Essential Fish Habitats’ (Payne et al 2008) these are areas were

pressures are fewer and the population is more likely to survive. The history of D. batis

rebuilding former populations is not promising; after the initial depletion of skate

around Orkney the population has shown little sign of restabilising itself after 30 years

(Anon, 2000). This may be down to populations of D. batis being isolated so there are

very few if any resident mature fish to enable repopulation. Elasmobranch species with

the most potential to rebuild populations are short lived, early maturing inshore coastal

species (Stevens et al, 2000) such as smaller species of Rajifrom, which after

exploitation could rapidly (compared to D. batis) rebuild population levels.

One of the main problems is that the UK commercial fishing fleet do not log

Raja catches as individual species, instead grouping all catches of Raja species under

‘Skate and Ray’ (Dulvy et al 2000). Not only does this make it impossible to set catch

quotas for specific species (Agnew et al, 2000) but it also makes determining the impact

a commercial fishery has on a particular species using landing data very inaccurate. D.

batis, however, is vulnerable to commercial fishing (Stevens et al, 2000), and is often

taken as by-catch from long line, benthic and demersal trawl fisheries (Agnew et al,

2000). All skates and rays have some commercial value and commercial vessels will

retain them especially as the exclusion of one species from a fishery is not an easy task.

There is currently only a Total Allowable Catch limit on skate and rays set in the North

Sea but this has historically always been above the total landings for this group (Payne

et al, 2008), rendering it a very ineffective management tool.

It has been shown that in the Irish sea, while larger species of skate such as D.

batis have reduced in number due to commercial pressure (Gallagher et al 2004),

smaller species such as R. montagui (spotted ray) and L. naevus (cuckoo ray) have

increased in number. This may be due to the reasons mentioned previously, with the

smaller species being able to rebuild their population more rapidly than larger species.

This change in community structure has also been noted in the North Sea (Gallagher et

al 2004). The proportion of species being landed is constantly changing and whilst the

landings of skate and rays as a whole may not be substantially declining, the proportion

of D. batis may be declining, but this cannot be detected due to mixed species landing

data (Figure 1.3).

10

Figure 1.3: Hypothetical situation: Increase in proportion of smaller species being landed masking the decline of larger species being landed

Another reason for the sudden increase in proportion of smaller skate could be due to a

change in their life history. It has been shown that long term exploitation of

elasmobranches can lead them to change their life history (Tanaka et al, 1990, Walker,

1999), most noticeably, their growth rates (Stevens et al, 2000) and age of maturity

(Tanaka et al, 1990) which means that these individuals will have a greater chance of

surviving to breeding age (Licandeo et al, 2007). As these are selective attributes which

are partly inheritable, it is expected that the population will evolve over time (Stevens et

al 2000). Walker (1999) showed that this was true of certain Raja species (although not

as yet noted for D. batis), although even if larger species do change life history patterns,

the evolutionary nature of these changes on the population will take longer to occur in

more K-selective species such as D. batis putting them at a further competitive

disadvantage.

As mentioned previously, D. batis was once common around the coasts of the

UK. However, even at the beginning of the 1900s it was clear that the population levels

were rapidly declining. In a study by Rogers and Ellis (2000) survey trawl data from the

Irish and North Seas (in which specimens were identified by species) from 1901-1907

and 1989-1997 was compared. Even in the 1901-1907 D. batis was absent from the

North Sea and only had low numbers in the Irish Sea (maximum 1 per hour trawl time

(Roger and Ellis, 2000)) showing the reaction of the population to early commercial

11

pressure. In the trawls conducted between 1989 and 1997, D. batis was absent from all

trawls. This has led to D. batis being declared extinct from the Irish Sea (Brander, 1981)

and numbers disappearing completely from North Sea trawls (Walker and Hislop, 1998

and Philippart, 1998). Trawl data for the Celtic Sea up until 2003 showed that D. batis

was still prevalent in the area (Walker & Hislop, 1998). However, the trawl survey

around the coast of Britain showed that D. batis was found in the northern North Sea

and Celtic sea only, in water 84-271m deep (Ellis et al, 2005), a lot shallower then the

fish in the Barents sea. It is important to note that during these trawls, there was very

low occurrence of immature fish.

1.7 Potential for Re-populating

For a species with such a late maturation age and a low fecundity such as D. batis,

survival to maturation is the most important factor (Walker and Hislop, 1998). The

response to exploitation showed by some species of Rajiformes of increased fecundity

would do little to increase the population size of D. batis as it is so susceptible to

immature animals being caught. Far more important for species survival is the number

of individual females reaching maturity. However, in a species with such a slow

maturation period the initial survival of adults is paramount for population growth. This

was shown to be the case for D. laevis (Barndoor skate), another large Rajifrom

populating the Western Atlantic (Frisk et al, 2002). It has been documented that the

species is robust and can survive the trauma of commercial capture well (IUCN) so if

commercial vessels can be encouraged to return D. batis, there is a good chance the fish

will survive, dramatically reducing fisheries mortality for this species.

12

2. MATERIALS AND METHODS

2.1 The Study Area

The data used in this thesis were collected around the Isle of Mull, which lies between

6º23’18” - 5º38’45” and 56º39’25” - 56º15’52” and is located km off the West coast of

Scotland (Figure 2.1)

Figure 2.1: Study Area around the Isle of Mull, western Scotland, with ICES rectangle numbers marked for the three used for Fisheries mortality analysis. Also marked a the three Skate angling areas, A, B and C used to split the Glasgow data into geographic areas.

This area was chosen due to the availability of historical angling records dating back to

1975 collected by various anglers in the area and compiled by Richard Sutcliffe,

(Sutcliffe, 1994) at Glasgow Museums who kindly made these data available through

SNH and they comprise the majority of those used for further analysis presented here.

There were two phases of skate tagging in the area (1974-1988 and 1988-present day),

Oban Isle of Mull A

B

C

42E3 42E4

41E4

57

56.5

56

-7 -6.5 -6

Passage of Tiree Loch Aline

Sound of Mull

Firth of Lorn

13

originally initiated (in 1974) by Dr Deitrich Burkel as a result of a dramatic decline in

population numbers around the Shetland and Orkney Isles (Little, unknown date).

2.2 Data Collection

2.2.1 Collection of biological data

Figure 2.2: Photograph of the Jumbo Rototags used in phase 1 of the Glasgow Museum Study. Picture courtesy of Richard Sutcliffe.

Figure 2.3: Diagram representing the Jumbo Rototags initially used during phase 1 of the tagging study. Source: http://www.ketchum.ca/imgs/livestock/p7/versa.gif. viewed on 12/08/2008.

Figure 2.4: Showing the tagging location on the skate for the Jumbo Rototags.

Tag number

Project ID

14

In phase 1 (1974 – 1988) Dalton Jumbo Rototags (Little, unknown) were used (Figure

2.2) (normally used for ear tagging livestock). However these proved to be ineffective

because, firstly, the tags offered a large surface area which allowed fouling organisms to

attach, rendering it unreadable, while secondly, the constricting nature of the tag

restricted growth (Figure 2.3) in the area of the fish around the tag, causing sores. The

build up of fouling matter on the tag, which included hard shelled organisms such as

barnacles, along with the rotational movement of the tag also caused damages to the

skin of the animals. (Little, 1995). The Jumbo Rototag was placed on the trailing edge

of the wing of the animal on either side as shown in Figure 2.4.

Figure 2.5: Showing the design of the current Floy Dart Tag with the barbed anchor at the left end of the tag. Picture courtesy of Richard Sutcliffe

Figure 2.6: Showing the Needle Applicator and the insertion of the tag into the needle ready for application. Picture courtesy of Richard Sutcliffe

15

Figure 2.7: Showing the tagging areas on the fish for the new Flow Dart Tag. The body core surrounding the internal organs is outlined in Grey

Due to the unsuitably of the Rototags, they were replaced by Floy FT-1 Dart tags

(Figure 2.5)(Little, unknown) in 1988. These tags have a single anchoring point, do not

cause injury and offer very little surface area for fouling. A needle applicator (Figure

2.6) is used to insert the anchoring point beneath the skin but above the muscle. The

changeover of tags also represented the end of phase 1 and the beginning of phase 2. At

the time of writing, Floy dart tags are still in use, although they have undergone a design

change by the manufacturer, shortening the tags and changing the colour from orange to

yellow. Floy Dart Tags could not be secured in the area previously used, as the tissue

here was thin and the area subject to a lot of movement during swimming, which had

the potential to dislodge the tags. Consequently they were secured in the more muscular

areas towards the centre of the body (Figure 2.7), but avoiding areas above the internal

organs. These areas provide a more secure anchor point for the tags and also are subject

to less movement, reducing the risk of the tags being lost. (pers. Comm. Davy Holt).

When Fish were initially caught and if they were not already tagged a tag was anchored

to the fish, if a Rototag was present, this was replaced with a dart tag. Log Sheets

provided by Glasgow museum were filled in for each new tag used. The details

recorded were: tag Number, date caught/tagged, time, length (inches), longitude and

latitude, location in words, sex, wingspan (inches), weight (pounds), depth the fish was

caught at (depth of location opposed to length of line out), tidal state (open to taggers

interpretation), bait used, tagger/captor and comments (such as condition of fish, any

unusual features or circumstances). If the fish was already tagged, then the same details

were taken but instead of a new tag being fitted, the number form the existing tag was

16

recorded and the tag left in the fish. There are some circumstances where the tag may be

replaced, if the existing tag is a Jumbo Rototag then this is replaced with a Floy Dart

Tag or if the existing Dart tag is loose and likely to fall out then this should try to be re-

anchored or if the anchoring point is damaged then the tag is replaced.

2.2.2 Extra Field work undertaken towards this project

Figure 2.8: Fishing gear used to catch D. batis and prevent deep hooking and cause minimal damage in the situation where the main line snaps, the weight and majority of line being detached from the hook. Picture Courtesy of Ronnie Campbell.

Figure 2.9: Showing the gaffing sights on D. batis

Swivel to main line

Line between split link and main line. 40cm

Swivel with split link15mm plastic pipe

approximately 30cm. Snap swivel tied on with cable tie for attachment of lead weight (1.5-2kg)

Line between split link and main line. 240cm

Size 10 hook

17

Figure 2.10: Showing the length and Wingspan measurements on D. batis

For the Field work undertaken in this project the Floy Dart Tags (Figure 2.6), provided

by Glasgow Museums, were used along with a needle application tool (Figure 2.7). Log

sheets were created (see appendix 7) which recorded name of skipper, date, weather,

start time of fishing, end time of fishing, time fish caught, longitude and latitude (in

WGS1984) existing tag number, new tag number, sex, depth (of location), weight

(pounds or kg), length (inches or cm), width (inches or cm), tidal state, bait and

comments. Also included were two images representing the dorsal and ventral side of

the fish to for the purpose of marking any areas of damage, this damage could be further

described in the comments box. The addition of start and end time is to allow for future

analysis of fishing time or effort as this is one thing that is currently missing from the

Glasgow data set. These logsheets were given to all skippers in the area running D. batis

targeted charters. Any completed log sheets were collected at the end of the sampling

period. Sampling took place form the angling boat “Laura Dawn” skippered by Ronnie

Campbell operating from the Oban Times Pier in Oban between the dates 06/06/2008 –

26/06/2008 (not fishing every day). Fishing trips started at 9am and lasted 8 hours until

5pm, the amount of time spent fishing for D. batis varied each day due to a number of

external influences. Firstly, the weather was the deciding factor as to the location for

fishing as some, although offering better fishing, were very exposed and were un-

fishable in rough weather so the travel distance between Oban times Pier and the fishing

location varied considerably which affected time spent fishing. Also, the first part of the

18

trip was spent fishing for bait fish, usually mackerel. The length of time spent fishing

for mackerel depended on the number of fish caught, needed a minimum of 20 (personal

observation) to use as bait for D. batis and also if the anglers bought any bait on board

with them such as frozen squid, salmon then not as many mackerel were needed. The

fishing gear used can be seen in Figure 2.8, this was attached to a nylon main line (50-

60lb approx 500 metres long) run from good quality reel with harness links attached to a

50lb class rods, there were commonly 6 rods over the side of the boat, but this was

condition dependent, in rougher weather were the boat was subject to greater

movement, fewer rods were put out to prevent entanglement with each other and the

anchor rope. Bait was routinely checked as other benthic and demersal organisms in the

area removed the bait from the hook. The bait was changed upon every inspection to try

and maximise the scent trail along the sea floor. When a Specimen of D. baits was

caught, the fish was brought to the surface and landed on the boat using gaff hooks, one

for smaller fish, two for larger specimens, through the muscular tissue on the wings of

the fish (Figure 2.9) avoiding the main body cavity. This procedure has been shown to

cause no lasting damage to the fish and is the currently accepted method of landing the

fish (Sharktrust, 2004) although some boat anglers chose not to land the fish at all,

preferring to take measurements over the side of the boat while the fish is still in the

water.

2.2.3. Measurements from landed fish

Once the fish was landed, the length of the fish from the snout tip to the tail tip and the

wingspan, the longest possible width remaining perpendicular to the length axis (Figure

2.10) were measured. The weight was often estimated using weight tables (appendix 9)

as the fish are often too big to be accurately weighted at sea and the vast majority of

anglers do not wish to kill the fish to take it back to shore for an official weight to be

taken. Weights, wingspans and lengths were collected in imperial units as the equipment

the skippers use is in these units as in the angling community, fish are still described in

pounds and the weight charts use length and wingspan units of inches to estimate weight

(in pounds) As information on the relationship between length, wingspan and weight

has gradually been compiled over the years weight tables have been created for each sex

to allow anglers to accurately estimate the weight of their fish from the length and

wingspan measurements, these tables can be viewed in appendix 9. Sex is determined

by the presence of claspers in male fish (Hamlett, 1999) although this can be harder to

19

spot in smaller specimens where immature males have very underdeveloped claspers.

The current practice for anglers is to try and limit the time the fish stays out the water to

three minutes maximum (pers. comm. Ronnie Campbell, Davy Holt and Adrian Lauder)

Readings of position and depth should be taken before the fish has been landed, during

the time the angler is playing the fish on the line, this is often in excess of 20 minutes

due to the length of line out, but these readings can be taken afterwards, before the boat

moves and should ideally be taken directly after the fish has been released. During the

course of the field work which compromised of 12 days at sea, a total of six fish were

caught and tagged, three aboard the Laura Dawn.

2.2.4 Collection of Economic data

As part of the project, it was important to find out how much the population of D. batis

was worth to the local economy as not only do 5 charter boats from the area run regular

trips to fish for the species, but the anglers paying the charter also contribute to the local

economy. To try and find out the worth of the species locally a socio-economic survey

was undertaken. Questionnaires were handed out to the charter boat skippers with

questions aimed at finding out how much they relied on the population of D. batis for

their income. The information they gave also gave insight into how many charter trips

they ran per year and which months they ran fishing trips for D. batis.

The skippers were also given a questionnaire for the anglers to fill in (examples

of both questionnaires can be found in appendix 8). These questionnaires were aimed at

trying to find out if anglers were staying in the area, and if so, which type of

accommodation they used and the group size of the party (including non-anglers). It was

also designed to establish the expenditure of the group in the area, in order to evaluate

out how much each charter boat contributed to the local economy. Aboard the Laura

Dawn, these questionnaires were often filled out by the author directly, while the

skippers from other boats gave the questionnaire out at the beginning of the trip and

collected them at the end. A modified version of the anglers questionnaire, aimed to

target all anglers of D. batis including shore based anglers and anglers with private

boats was given to the SSACN to pass on to their members and also to Davy Holt to

distribute on the skate tagging forum, directed through the SAH. All filled in

questionnaires were returned via email or fax to the author

20

2.3 Data analysis

Before any analysis of the data took place, the Glasgow data fully updated and filtered

for any errors, erroneous and missing data. They were then converted to metric units

(metres, centimetres and kilograms) using the following conversion factors:

1lb = 0.453 592 37 kilogram and 1 inch = 2.54 cm

The majority of the data collected has been measured in imperial units, in all instances,

the data has been converted to metric units (metres, centimetres and kilograms). All

entries for longitude and latitude were converted to decimal degrees, the method for this

varying depending upon the original form of entries. The majority of data were entered

as degrees and decimal minutes DD° MM.MM” although some entries were in degrees,

minutes and seconds DD°MM”SS’. To convert these into decimal degrees the following

formulae were used:

Decimal Degrees = Degrees + minutes/60 + seconds/3600 or

Decimal Degrees = Degrees +MM.MM”/60

The calculations for these were carried out in Microsoft© Excel© 2003 spreadsheet.

2.3.1 Population Segregation

In order to determine if there was any segregation within the population of D. batis in

the Mull area, the data were split into two groups by sex and weight class which were

then both compared against depth in Microsoft© Excel© 2003 and geographic location

(determined by longitude and latitude) in ESRI ARCview. Initially, a Spearmans-rank

correlation was used in the software SPSS to see if there was any realtionship between

weight of fish and capture depth the fish was caught for each sex. Further splitting of

the data into seasonal groups, winter, spring, summer and autumn, allowed for annual

patterns and movements to be highlighted. Each season covered three months with

winter grouping catches in November, December and January, Spring grouping

February, March and April, Summer grouping May, June and July and Autumn

grouping August, September and October. The months for each season were chosen

selected based on three variables, namely months spent fishing by different anglers and

the breeding cycle of other species of Rajifrom taken from the work carried out by

Koop (2005) and are also linked to the breeding cycle of R. clavata as shown in the

21

studies by Hunter et al 2005 a&b (the data were analysed again in Microsft Excel and

ARCview).

The data collected by the Glasgow Museum project were mostly collected by

anglers and as such cannot be considered random sampling, since each angler who

contributed data probably have preferred fishing areas decided by their own personal

fishing history in the area and limitations on where they moore and operate their boat

from. Also, different anglers fish during different times of the year with charter skippers

mainly fishing during summer and Autumn, whereas the pleasure anglers fish all year

round. In order to distinguish any movement patterns displayed by the skate from

patterns created due to angling effort, the data were split into three geographic locations

based on angling activity. These areas were the Sound of Mull (around the mouth of

Loch Aline), referred to as Area A, the Passage of Tiree, referred to as Area B and the

Firth of Lorne below the Junction of the Sound of Mull and Loch Linnhe, referred to as

Area C. These areas are shown on Figure 2.1. Although these areas do not contain all

fish caught, they do represent the majority of catches of D. batis in the study area and

contain the majority of the records with geographic location given. The variables sex

and weight class were then compared to depth within these geographic constraints.

2.3.2 Life History

It has been shown that other Species of Rajifrom have changed their life history

characteristics in response to mortality pressure (Tanaka et al, 1990 and Walker et al,

1999). We wanted to know if this might be the case for D. batis in the study area. As the

Glasgow Museum data has been collected over the last thirty years it was possible to

compare historical information to more recent data. The first thing looked at was the

proportion of males and females and how this altered over time. As anglers target both

sexes equally when they are fishing, it was assumed that fishing effort for both sexes

was equal as no evidence on geographic preference by either sex could be found and the

sexes showed no preference to bait, so proportions of each sex should be representative

of population. Recapture data within the same year was discounted as this might count

the same individual more than once and could lead to false values. Sex ratios for each

year were looked at within each area, as fishing within each area covered different time

periods.

Another variable which was looked at was growth rate of each sex of the

species. Individuals captured more than once with weight data for each catch incidence

22

were included in this analysis. An average value of kilograms change per day was

calculated by dividing weight change by days between captures. This was compared to

growth rates from a smoothed set of data (method outlined below) as most species of

elasmobranchs have different growth rates throughout the year (Kusher et al, 1992). To

smooth the data out and discount large changes caused by annual growth patterns,

changes caused by feeding regimes and the carrying of gametes by females, the total

weight change per individual between initial capture and final recapture was divided by

total day’s freedom to gain an average growth rate in kg/day. In both cases, the figure

for the growth rate was paired with the initial starting weight. The equation of this line

was marked using Excel and used in calculations to create Figures for an estimated

growth curve. Assuming a start weight of 0.5kg the line equation from the growth rate –

weight scatter plot was used to work out the growth rate in kg/day for this weight of

fish. 0.5 was used as an initial start weight by adapting the weight charts in appendix 9

as this figure was added onto the original starting weight (0.5) to give the weight after

one day. This process was repeated for 5475 days (15 years, assuming 365 days per

year) to create growth data for each sex over a period exceeding the cited maturation

period of 11 years (Brander, 1981). This process was repeated four times, twice for each

sex using the smoothed and unsmoothed results.

2.3.3 Movement

The second objective of this project was to try and ascertain if individuals of

either sex undertook any migrations as displayed by individual R. clavata in the

southern North Sea (Hunter et al, 2005b). The recapture data were used to test this. Any

fish that had been caught more than twice i.e. had two recaptures after its initial tagging

catch were spatially analysed using ESRI© ARCview, individuals again being analysed

by sex categories. The distance between each capture location for the same individual

was marked in km and for each fish a total distance between capture sites was

calculated, this was used to see if there were any annual migration patterns for either sex

and to see if there was a difference between the sexes for total distance between capture

points. The literature on maturity size for both sexes of D. batis is unclear and

unconfirmed, so there is no definite weight split to distinguish mature from immature

fish, to overcome this, the data was split into weight classes of 10kg intervals - 0-10kg,

11-20kg, 21-30kg, 31-40kg, 41-50kg, 51-60kg, 61-70kg, 71-80kg, 81-90kg, 91-100kg

and 101-110kg and compared to each other.

23

2.3.4 Population Estimates

As there is no unit of fishing time or effort the only model which could be used

to estimate the population was the Schnabel model (McCallum , 2000). This is similar

to the Peterson method but deals with ongoing sampling as opposed to just two discrete

sampling episodes. The present data were split up into sampling periods so that

recaptures and total catches could be compared over a given time. Three sampling

periods were selected as the basis of population estimates. For all sampling periods, the

same assumptions were applied, these being that the population of D. batis was closed,

the mortality rate was zero, the tagged animals were not more vulnerable to recapture,

death and had no competitive disadvantage compared to untagged fish, there was

complete mixing between tagged and untagged individuals and finally, that none of the

tags were lost. All of the above assumptions will lead to overestimates of the population

but will provide a starting point for further analyses and evaluation of restricting

parameters such as mortality.

Daily sampling period

The first population estimate used all the recapture data except for recaptures occurring

on the same day, but does include fish caught in consecutive days. This is perhaps too

short a time to allow mixing between the tagged and untagged individuals and is likely

to provide the lowest population estimate.

Annual sampling period

For the second population estimate, each year was treated as a discrete sampling

episode, discounting recaptures of fish within the same year, as this discounted many of

the recaptures, as many fish were caught within the same year, this was expected to give

the highest population estimate out of the three methods applied.

30 Days sampling period

The third method is aimed at giving an intermediate estimate by including many of the

recaptures but trying to leave enough time for mixing of the tagged fish with the rest of

the population. As we know so little about the movements and behaviour of D. batis it is

hard to give an exact time period to allow the fish to mix within the population, a time

period of thirty days was used to allow for mixing of the population. This time length

was chosen as most recaptures within a month occur at the same point or within the

immediate vicinity of the original catch point, while recaptures after one month show

greater dispersal which implies that they have mixed with the rest of the population.

24

Fishing Mortality

In order to gain some level of understanding into the mortality from commercial fishing

in the area data provided by FRS in Aberdeen were used as it shows the total skate and

ray catch for the ICES squares 42E3, 42E4 and 41E4 which cover the study area (Figure

2.1). The original population estimates from the above sections are for the whole area so

to work out fishing mortality for this population estimate, the ICES data was added

together for each year to get a total skate and ray landings for the whole study area.

Previous studies on Rajifrom species have run survey trawls in the Irish, Celtic, Barents

and North Sea (Dolgov et al, 2005b and Ellis and Walker, 2005) and have noted skate

and ray landings by species. These studies gave catch levels for D. batis in the form of

either percentage of total catch or average number of individuals caught per hour

fishing. With the information from these studies, a range of hypothetical catch levels as

percentages could be applied to the FRS data to see what the landings of D. batis from

commercial fishing maybe, in kg of gutted D. batis. To convert this number into

individuals proved difficult as the commercial fishing is indiscriminate regarding sex or

size as all life stages of D. batis are vulnerable to being caught (Brander, 1981). The

Glasgow Museum Study was unsuitable to provide an average weight for D. batis as the

data is collected by anglers who target larger specimens. An average weight for D. batis

was gained by calculating the mean value of 50% maximum male weight and 50%

maximum female of fish caught in the area, this gave a mean weight of 88.495kg. The

kg catch was divided by this number to get a hypothetical value for individuals caught

by commercial fleets. This figure was felt to be too vague to apply to the population

estimates so has been left on its own as estimated mortality.

2.3.5 Economic Importance of D. batis

Although the study area focuses on the Isle of Mull and surrounding area, the

only number of viable returns for the economic survey all came from Oban, so the

results are focusing on the economic importance of D. batis to Oban. With the data

collected from the economic survey, the following results were worked out using

Microsoft Excel formula with Minitab used for all statistical analysis. Firstly an average

based on mean values, as the data was parametric as shown in Minitab using the

Anderson Darling test (P value = <0.05), was gained for Money spent in area and an

average based on the median value, as the data was shown to be non-parametric (P

value = >0.05), for money spent on fishing tackle in the area were added together to get

25

a total average spend figure. Then an average, based on mean values (P value <0.05) for

group size was calculated and the total spend figure was divided by average group size

to get average spend per person excluding charter boat costs.

The combined total for chartered days was estimated to be three hundred

between the four charter boats operating from Oban, this was estimated from the returns

of the skipper questionnaires (appendix 8). This multiplied by the average spend per

person per day gave a total contribution of the customers partaking in fishing charters

targeting D. batis to the local economy per year. This was then compared to an

economic report on tourism in the Highlands and Islands (George Street), within this

report was a section on watersports with total money spent in the Highlands and Islands.

The figures gained from the study could then be used to assess how much money skate

fishing contributes this as a whole. The figure for average spent per person on charter

was compared to other watersports, such as fishing, scuba diving and sailing, to see how

much money people spend on each activity.

To assess the value of the Charter boat industry in Oban an average cost of

charter hire per day was estimated from the returns of the skipper questionnaires and

personal observation. This value was divided by the average group size to calculate the

average spend on charter hire per person per day. This figure could then be used to

calculate the total amount spent on chartered trips per year assuming 300 days at sea

total.

2.3.6 Value of Skate compared to commercial market

The SSMEI for Mull project was interested in the average value of individual D. batis in

area. A population estimate had been worked out earlier in the thesis with varying levels

of mortality. The Highest population estimate, i.e. without any mortality added, would

give the lowest value per individual so this was used as a benchmark to estimate an

individual worth for D. batis. The total value of the charter boat industry was divided by

the population estimate to give a value per individual figure. In order to compare this to

current commercial values (which are given in £ per kg), the average weights as used in

the fishing mortality analysis were used to work out the average worth per kg of D.

batis to the Charter boat industry in Oban. This value was then compared to values

taken from the internet.

26

2.3.7 Statistics

The Anderson-Darling test for normal distribution and Levenes test for equal variances

were used in Minitab to determine whether further testing should be parametric or non-

parametric. In all instances in this study, data proved to be non-parametric. Spearmans

Rank tests in SPSS were used to compare the association between variables. Variables

used were depth and weight of fish (appendix 3).

As all the variables looked at were non-parametric, median values have been

used for averages opposed to mean values as they more accurately represent the average

values of the variables.

27

3. RESULTS

3.1 Behavioural Ecology of D. batis

3.1.1 General Observations of the Population throughout the Study Area

The observed distribution of D. batis within the study area (Figure 3.1) shows that

concentrations of specimens were concentrated in three main areas, the Firth of Passage

of Tiree, the Sound of Mull and the northern end of the Passage of Tiree. These areas

are ringed in black in Figure 3.1. The sexes were distributed equally throughout this

range with neither showing preference for a particular area. Male specimens appeared to

be more closely grouped in the Passage of Tiree and were absent from the west coast of

the Isle of Mull while females were more widely distributed, including a scattering

along the west coast of the island.

There is a noticeable peak in the number of males being caught between 40.01

and 60kg with the largest number of male fish caught belonged to the 40.01-50kg

weight class as shown in Figure 3.2. There were very few fish caught in the 60.01-70kg

class. The numbers of females caught from each weight class shows a bimodal

distribution (Figure 3.3) with numbers of individuals peaking in the 10.01 – 20 kg and

70.01 – 80 kg weight classes. Very few fish heavier than 90.01 kg were caught.

28

Figure 3.1: Map showing the distribution of both sexes of the D. batis population around the Isle of Mull. The main map shows all captures of D. batis and the lower, smaller maps show captures of males and females.

Area A: Based on fishing effort

Area B: Based on fishing effort

Area C: Based on fishing effort

29

Figure 3.2: Histogram showing the number of male specimens caught in the study area for each weight class between 1975 and 2008. Each weight class has a 10kg interval and the maximum weight for adult male specimens of D. batis in the area was 74kg.

Figure 3.3: Histogram showing the number of female specimens caught in the study area for each weight class between 1975-2008. Each weight class has a 10kg interval and the maximum weight for adult female specimens of D. batis in the area was 102.06kg

30

Figure 3.4 shows the weight related depth distribution of male fish. There was a

significant association between the weight of the fish and the depth it was caught (result

after performing Spearmans Rank correlation 2-tailed p = <0.001). There are two

distinct groupings at different depths, group I, between 25 and 75m and group II,

between 75 and 145m (Figure 3.4). The association between weight and depth can be

seen when looking at the weights of fish caught in each group, group I only had fish

greater than 10kg caught in it while group II had fish between 2-70kg, encompassing

nearly the full range of weights showing that while large fish (greater than 10kg) are

caught at in all depths of water, smaller animals (less than 10kg) are only caught the

deeper water range encompassed by group II. In group I, the depth distribution of fish

between 40 and 60kg is greater (between 25-75m) than fish outside this weight range,

which are all caught at a similar depth (50-60m), there is no similar pattern in group II

with all weights of fish being distributed throughout the depth range (75-145m).

The weight related depth distribution for female fish (Figure 3.5) showed no

association between these two variables (result after performing Spearmans Rank

correlation 2-tailed p = >0.001). There are once again two distinct s depth groups, again

labelled group I, 25-75m and group II, between75-145m. In group I only female fish

weighing between 7kg – 83kg were caught, while in group B, fish between 1.36-

102.6kg were caught, showing larger weight range in group II. It can also be seen that in

group I that there is an even distribution of weights, with no weight being substantially

more represented than another, while in group II there are two groups based on weight,

one between 1.36-30kg and the other between 60-90kg, these two groups are ringed in

blue on Figure 3.5.

31

Figure 3.4: Scatter plot showing the depth distribution of male fish in relation to weight. Depth groups are circled in black

Figure 3.5: Scatter plot showing the depth distribution of female in relation to weight. Black circles are showing depth groups and blue circles are highlighting the bimodal distribution of weight in depth group II.

-250.00

-200.00

-150.00

-100.00

-50.00

0.00

0.00 20.00 40.00 60.00 80.00

Dep

th (m

)

Weight (kgs)

I

II

-250.00

-200.00

-150.00

-100.00

-50.00

0.00

0.00 20.00 40.00 60.00 80.00 100.00 120.00

Dep

th (m

)

Weight (kgs)

I

II

32

The proportion of males within the study area appears to have declined throughout the

tagging programme as shown in Figure 3.6, with the large drop between 1988 and 1990

where the proportion fell from 70% down to just over 20%. Before this large decline,

the sex ratio was even, with fluctuations, around 50:50, sometimes with a higher

proportion of male fish, sometimes, more females. After 1990, the proportion of males

did not rise above 45% and is currently at its lowest recorded value of below 20%.

Figure 3.6: Showing the change in sex ratio over time. Dashed line showing the trend for the proportion of males

33

3.1.2 Seasonal observations of the population throughout the study area

When the geographic location of males was plotted with seasonal variation (Figure 3.7),

a pattern could be seen. The numbers of males caught in summer and autumn were

significantly higher than the number of males caught in the spring and winter. It can

also be seen that there is an absence of males from the Passage of Tiree during the

winter and spring seasons, while a large number of males were caught in this area

during summer and autumn.

Figure 3.7: Map showing seasonal distribution of males throughout the study area with Seasons defined as winter: November-January, spring: February-April, summer: May-July and autumn August -October.

When the geographic location of females was plotted with seasonal variation (Figure

3.8), it could be seen that the numbers of females caught in summer and autumn was

significantly higher than the number of females caught in the spring and winter. Another

34

pattern that could be seen, although not as distinctly as with males, is the dramatic drop

in numbers of females caught in the Passage of Tiree during the winter and spring

seasons while a large number of females were caught in this area during summer and

autumn.

Figure 3.8: Map showing seasonal distribution of females throughout the study with Seasons defined as winter: November-January, spring: February-April, summer: May-July and autumn August -October.

Figure 3.9 shows that both sexes displayed a change in median depth throughout the

seasons, with both sexes being caught at a similar, deeper median depth in the winter

and spring and a shallower median depth during the summer. The median depth for

males was 118.5m deep in the winter and spring and 92m deep in the summer. Females

showed a similar pattern with average winter and spring depths being around 122m

Winter

Autumn

35

during the winter and spring months and 102m in the summer months. During winter,

spring and summer, the average depth for females was lower than for the males with the

largest difference in the summer season. The median depths for both sexes were the

same for autumn and lie approximately halfway between the median spring and median

summer depths.

-140.00

-120.00

-100.00

-80.00

-60.00

-40.00

-20.00

0.00Winter Spring Summer Autumn

Season

Dep

th (m

)

Females

Males

Figure 3.9: Showing the median depth of male and female fish throughout the study.

Winter Spring Summer Autumn

36

Figure 3.10: Line graph showing the estimated Growth Curves for Males using Smoothed and Unsmoothed data for the first 15 years of the animal’s life.

Figure 3.11: Line graph showing the estimated Growth Curves for Females using Smoothed and Unsmoothed data for the first 15 years of the animal’s life

Comparing the two estimated growth curves for males (Figure 3.10), the unsmoothed

data curve shows a faster initial growth rate, which gives weight differences of up to

12.5kg between the ages of 4 and 9 between the two curves (smoothed and

37

unsmoothed). This shows signs of levelling after 40kg to give a difference between the

two curves of less than 5kgs. The lessening of the difference between the curves is

caused as the smoothed data curve continues to rise relatively steadily, displaying more

linear progression, showing that after the age of six, males experience a higher growth

rate. The smoothed curve shows only a very small decline in growth rate between the

ages of 7 and 15 years.

Looking at the two estimated growth curves for the females, smoothed and

unsmoothed, there is a large difference between them which continues to get larger as

the fish get older instead of converging like the male growth curves in Figure 3.10. The

initial growth rates for females are very rapid, although this is not clear in Figure 3.11

due to the scale of the axis, the smoothed curve, showing the upper estimate, shows

females reaching a maximum of 30kgs towards the end of their first year, while the

estimated weight predicted by the lower, smoothed curve shows female weight to be

approximately 18kgs after the end of 2 years. Assuming a maturation age of eleven

years old (Brander, 1981) the maturity weight for males is 33-42kg and for females,

maturity weigh would be 69-96kg.

3.1.3 Areas A, B and C

As Figure 3.1 shows, there are three distinct clusters of records; this next section looks

at these areas individually in an attempt to remove local variability from the analysis.

Figure 2.1 in materials and methods shows the geographic split based on angling

activity which are similar to the three groups distinguished in Figure 3.1, it is these three

areas, A, B and C that are used in the following figures.

When the sex ratio in each geographic area was compared, it was found

that there was a higher proportion of females in Area A, 70% than in Areas B or C

which both had similar proportions 55 and 52% respectively, this is clearly shown in

Figure 3.12. These proportions were taken from total numbers of males and females

caught in each area as a cumulative total from all years.

38

Figure 3.12: Column Graph showing the sex ratio in Areas A, B and C

The distribution of males and females in each weight class varies throughout the three

study areas. Figure 3.13 for area A shows a large increase in the number of females

caught in the two weight classes between 60-80kgs, with up to 50 fish per weight class

caught. The weight classes show little variation between the numbers caught, ranging

between 20 (0-20kgs) and 12 (40.04-50kg). Area A had the largest female fish caught in

the study area, weighing 102.6kg. The distribution of males in area A shows a steady

increase in numbers of fish caught up to the 50.01 – 60kg weight class, with no

substantial increase in numbers between each weight class. There were no larger males

than 60kg caught in this area.

In area B (Figure 3.14) there is no substantial peak of numbers of females in any

weight class such as apparent in Area A, with the largest number of females caught

being 40 in the 20.01-30kg class. Males, however, show a large increase in numbers, up

to 70 fish, in the 40.01-50kg and 50.01-60kg weight classes, this a substantial increase

of almost 50 fish from the weight class below (30.01-40kg). Numbers of males caught

in other weight classes are relatively low outside the two peak weight classes, with

numbers of fish caught not rising above 22.

Area C (Figure 3.15) shows large peaks in the number of fish caught for both

sexes. With females, the largest numbers of fish, are caught in the 10.01-20kg weight

class (26), this declines in subsequent weight class to a low of 2 fish caught in the

39

40.01-50kg and 50.01-60kg weight classes. The number of fish then rise again to a high

of 12 individuals caught in the 70.01-80kg class. Males have a substantial rise in

numbers in the 40.01-50kg weight class with 29 individuals being caught. This quickly

drops over the next two weight classes.

0

10

20

30

40

50

60

0-10 10.01-20

20.01-30

30.01-40

40.01-50

50.01-60

60.01-70

70.01-80

80.01-90

90.01-100

10.01-110

Weight class (kgs)

Num

ber

Caug

ht

MaleFemale

Figure 3.13: Histogram showing numbers of males and females caught from each weight class in area A within 10 kg weight classes

0

10

20

30

40

50

60

70

80

0-10 10.01-20

20.01-30

30.01-40

40.01-50

50.01-60

60.01-70

70.01-80

80.01-90

90.01-100

10.01-110

Weight Class (kgs)

Num

ber

Caug

ht

MaleFemale

Figure 3.14: Histogram showing numbers of males and females caught from each weight class in area B within 10kg weight classes

40

0

5

10

15

20

25

30

35

0-10 10.01-20

20.01-30

30.01-40

40.01-50

50.01-60

60.01-70

70.01-80

80.01-90

90.01-100

10.01-110

Weight Class (kgs)

Num

ber C

augh

t

MaleFemale

Figure 3.15: Histogram showing numbers of males and females caught from each weight class in Area C within 10kg weight classes

Figure 3.16 shows the number of males caught per weight class from each area

compared against each other. It can be seen that most fish were caught in area B. In all

three areas there is a similar pattern of gradually increasing numbers of male fish caught

in each weight class until the 40.01-50kg class, which contains the highest number of

fish caught for all three areas. This is most noticeable in area B where the numbers

rapidly increase. The numbers of males in this weight class in areas A and C do rise, but

not as sharply as area B. In all three areas, the peak of the numbers is within either the

40.01-50kg class (areas B and C) or the 50.01-60kg class (area A).

There is no clear pattern in weight class proportion displayed by females (Figure

3.17), all three areas do have some level of bimodal distribution but the highest

representation varies between weight classes. In area A, the largest numbers of females

were caught in the 70.01-80kg weight class, in area B 20.01-30kg and in area C 10.01-

20.01kg.

41

0

10

20

30

40

50

60

70

80

0-10 10.01-20 20.01-30 30.01-40 40.01-50 50.01-60 60.01-70

Weight Class (kgs)

Num

ber C

augh

t

Area AArea BArea C

Figure 3.16: Histogram showing numbers of males per weight class in Areas A, B and C

0

10

20

30

40

50

60

0-10 10.01-20

20.01-30

30.01-40

40.01-50

50.01-60

60.01-70

70.01-80

80.01-90

90.01-100

10.01-110

Weight Class (kgs)

Num

ber

Caug

ht

Area AArea BArea C

Figure 3.17: Histogram showing numbers of females per weight class in Areas A, B and C

42

The map in Figure 3.18 shows that most fish caught at less than 75m deep were caught

in area B near in the Passage of Tiree apart from one individual caught from the shore in

area A. Fish caught at more than 145m deep were only caught in Area C in the Firth of

Passage of Tiree. Fish caught between 75m-145m deep were caught in areas A and C.

Figure 3.18: Map showing distribution of depths fish were caught at throughout Areas A, B and C. Figure 3.19 shows the median depth for fish in area A, which display similar seasonal

depth distribution to the fish caught throughout the whole study area, with both males

and females moving to shallower water in the summer. There is a difference of 11m

between the summer median depth for males and the summer median depth for females

in area A. The median depths for autumn are very similar for males and females, around

107m, being deeper than the median summer depth for males, approximately 114m, and

shallower than the median summer depth for females, approximately 103m. Winter and

spring depths are similar for each sex and only vary by a maximum of 2.5 metres, with

females being caught at a median depth of 116.5m and males, 119m. Both sexes were

caught at the deepest median depth in the spring.

43

-125.00

-120.00

-115.00

-110.00

-105.00

-100.00

-95.00

-90.00Winter Spring Summer Autumn

Dep

th (m

)

MaleFemale

In area B, shown in Figure 3.20, there were no fish caught in the winter and only

1 female caught in spring. The median depth value for the female caught in the spring is

a lot shallower than the median depth for females in the summer. The median catch

depth for both males and females is the same in the summer, approximately 56.5m deep

and in both cases, this rises to a shallower median depth in the autumn, females being

caught at a median depth of approximately 54.75m and males 53.5m. This is the

opposite pattern that we see in area A.

In area C, the sexes display very different seasonal variation in the depth they

were caught in, Figure 3.21. In areas A and B, although the actual median value

differed, the seasonal pattern of depth caught in was the same for each sex between the

seasons. Females show progressively shallow median depths throughout the year, with

their shallowest median depth (111.25m) in autumn and the deepest in the winter

(115.8), while males were caught at their shallowest median depth in the spring

(112.8m), and their deepest during the autumn (116.28m). This is the exact opposite of

the seasonal depths in Figure 3.19.

Figure 3.19: Column graph showing the median depth values for males and females in area A

44

-58.00

-56.00

-54.00

-52.00

-50.00

-48.00

-46.00

-44.00Winter Spring Summer Autumn

Dep

th (m

)

MaleFemale

Figure 3.20: Column graph showing the median depth values for males and females in area B

-117.00

-116.00

-115.00

-114.00

-113.00

-112.00

-111.00

-110.00

-109.00

-108.00Winter Spring Summer Autumn

Dep

th (m

)

MaleFemale

Figure 3.21: Column graph showing the median depth values for males and females in area C

Figure 3.22 shows a scatter plot of depth against weight with seasonal data for females

in area A, the association between weight and depth was shown to be significant using

Spearmans Rank, 2-tail correlation test, p=<0.01. The general trend shows larger fish,

50kg+, being caught in shallower water. This is especially noticeable during the

summer, where larger animals are found in shallower water, this can be seen in Figure

45

3.22 ringed in black. The fish caught in the shallowest water is 70kg in weight, heavier

fish were found in deeper water again. In winter, there are three distinct groups, the first

being 7 animals up to 25kg in weight caught in approximately 120m of water, the

second group, two individuals between 20 and 40kg, being caught in 76m of water and

the third group, 5 individuals, between being caught in 115m of water. These three

groups are ringed in blue on Figure 3.22. Most females are caught at the same depth

(between 130-140m deep) in the spring with a few outlying results, mainly between 50-

80kg, in shallower water. Most females caught in the autumn were above 60 kg in

weight and were caught at shallower depths than most fish caught in the spring and

summer time. There was one small specimen caught in the autumn under 10kg at a

depth of 87m, shallower than any of the larger females fish. Larger males, (35kg+) in

area A, in common with females, were found in shallower water up until 55kg, after this

weight, they are caught in deeper water as within the circle region on Figure 3.23, once

again, the association between weight of male fish and the depth was found to be

significant (Spearmans Rank 2-tail correlation test, p=<0.01). This is most apparent in

the summer, where fish between 35-60kg were caught at shallower depths, no fish

lighter than 35kg were caught in less than 100m of water. Two groups are seen in the

spring, with a fish between 5 and 45kg all being caught at a depth of 120m while larger

fish 50-55kg are caught between 90-120m (ringed in blue on Figure 3.23).

Figures 3.24 and 3.25 show the seasonal weight related depth distribution for

males (Figure 3.24) and females (Figure 3.25) in area B. There were no winter results

for either sex and only 1 spring result for females. Looking at the female data for

summer and autumn, we can see that smaller individuals, less than 40kg, are caught

between 50-60m, heavier females, 40kg+, have a greater depth range between 28-72m.

In the summer, these larger animals, 40kg+, were more prevalent at shallower depths,

50-30m deep. Only one individual weighing less than 40kg was caught at a depth of less

than 50m. The autumn female data appears to follow a linear progression (the ranges of

this are marked with black lines on Figure 3.24) with larger fish being found at

progressively shallower depths. The association between weight of female fish in area B

and the depth they were caught at was found to be significant to a 0.05 level (Spearmans

rank correlation test, p=<0.05). This is not true for males, although it appears that some

larger fish, up to 50kg, are found at shallower depths, heavier fish are found at

increasingly deeper depths down to 60m. The depth range for summer increases with

weight, with males above 40kg being caught between 30 and 75m, below 40kg, the fish

46

are concentrated between 50-60m. There was a significant association between weight

and depth for females in area C also (p=<0.05) but the scatter plot for this area did not

show this and so scatter plots fort area C have not been included.

Figure 3.22: Scatter plot showing seasonal weight related depth distribution for females in area A

Figure 3.23: Scatter plot showing seasonal weight related depth distribution for males in area A

47

-80.00

-70.00

-60.00

-50.00

-40.00

-30.00

-20.00

-10.00

0.00

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00

Weight (kg)

Dept

h (m

)

Spring Summer Autumn

Figure 3.24: Scatter plot showing seasonal weight related depth distribution for females in area B

-80.00

-70.00

-60.00

-50.00

-40.00

-30.00

-20.00

-10.00

0.00

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00

Weight (kg)

Dep

th (m

)

Summer Autumn

Figure 3.25: Scatter plot showing seasonal weight related depth distribution for males in area B

48

3.1.4 Annual Migration

Figure 3.26 shows the longest confirmed distance travelled by D. batis between catch

sites. There have been longer distances recorded, but these are by commercial boats and

their accuracy is dubious (Little, 1995) and so are not been presented here. Fish 1 and 3

where originally tagged in the Sound of Mull and then travelled south to the Firth of

Clyde. Fish 2 was originally tagged in Ballycastle Bay in Rathlin Sound then move

north to the Firth of Passage of Tiree where it was subsequently captured the following

year. These distances are all in excess of 200km. The median distance travelled by male

fish was 6613m and 746m for females (taken from total distance travelled by all fish

recaptured more than once). It can be seen from these median values that males, on

average, travelled greater distances than females.

Figure 3.26: Maps showing the longest recorded ‘anger to angler’ distances travelled by male and female fish. Fish 1 and 3 were caught in the Sound of Mull and then the Firth of Clyde, fish 2 was caught in Ballycastle bay, Ireland, then in the Firth of Lorn.

The two movement patterns shown in Figure 3.27 show that the fish were being

caught within the same area on a routine basis. The pattern between catch sites is

an approximate figure of eight shape, with fish moving away from the initial catch

site, and then moving back towards it after subsequent catches. The maximum

distance between recapture sites was 5km

Fish 1 Fish 2 Fish 3

49

Figure 3.27: map showing catch sites of two male D. batis in the Sound of Mull and the Firth of Lorne. The lines and arrows shown on the map do not show the movement path of the fish, the fish is likely to have moved beyond the pathway of the line. The lines show direction of movement between the points and help to provide a time series between catch numbers of each fish.

Figure 3.28: Map showing the catch sites of two male D. batis in the Sound of Mull. One specimen is marked with blue circles and blue lines, the other with blue squares and black lines. Again, the lines are not representing the exact movement of the fish, but represent the direction travelled and time series of recaptures.

50

Female fish show a similar pattern as male fish with individuals seeming to move away

from their initial capture site, then move back towards it after the first re-capture. The

fish in Figure 3.29 was shown moving south in the Firth of Passage of Tiree from its

original capture site, before moving north into the Sound of Mull, between the two

geographic areas C and A. This was a longer distance (20km) between re-capture sites

than displayed by males in Figures 3.27 and 3.28. The two females in Figure 3.30 show

similar catch site locations to the male fish caught within the same geographic location.

The fish move away from their initial site of tagging, only to move back towards it after

the first recapture. The fish marked by red circles shows a repeated movement pattern

along the Sound showing an almost circular movement.

Using median averages, the distance between catch sites was greater for males

than it was for females as shown Figure 3.30. On average, the distance between catch

sites or males was 6613m, while for females, this was 736m, this is a difference of

5877m (5.877km).

Figure 3.29: Map showing the catch sites of a female D. batis Between the Passage of Tiree and the Sound of Mull. The lines are not representing the exact movement of the fish, but represent the direction travelled and help represent the time progression of recaptures.

51

0

1000

2000

3000

4000

5000

6000

7000

Sex

Ave

rage

dis

tnce

bet

wee

n ca

tch

site

s (m

)

Male Female

Figure 3.30: Map showing the movements of two female D. batis within the Sound of Mull.

One is marked by red triangles, the other red squares. The lines are not representing the exact

movement of the fish, but represent the direction travelled and help represent the time

progression of recaptures. Figure 3.31: Median distance travelled overall between catch sites for male and females

52

3.1.5 Population Estimates

Figure 3.32 shows population estimates for the area using different assumed sampling

periods. All the estimates show an increasing population with highest and lowest

estimates for 2008 of 3318 and 1961 respectively; a difference of 1356. The middle

population estimate shown is 2341. All lines show a rapid increase in the population

after 1990 with the population continuing to rise until 2008. The annual sampling

estimate shows an initial population decrease after 1979, which is not apparent in the

other estimates. The annual sampling estimate also shows a slight decline in numbers in

2004, which is the same time period as a year of ‘no growth’ for the estimate gained

from the 30 day sampling period. All estimates show a decrease in population in 1985

with numbers dropping by a maximum 200 individuals.

Figure 3.32: Line graph showing population estimates for the whole study area. Each line shows a population estimate assuming different study periods. The red line is an estimate assuming an annual sampling period. The green line assumes a 30 day sampling period which allows for mixing. The blue line is an estimate assuming a 1 day sampling period, using all records except those caught on the same day. All estimates are with the assumption of no mortality.

3.1.6 Fishing Mortality

The general trend is for the number of landed D. batis to decrease (Figure 3.33). This is

true of all assumed proportions of total landings. The highest number of D. batis landed

Population Estimates

0

500

1000

1500

2000

2500

3000

3500

1978

1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

2000

2002

2004

2006

2008

Year

Num

ber o

f Fis

h

All Recaptures

30 Day mixingAnnual

53

was in 1984 when 198 specimens were landed. This dropped by 62 the following year to

136. After 1984 the levels of D. batis landed did not rise above 85. There was another

dip (down to 49) in numbers being landed in 1994; this, however, recovered in 1996 to

73 fish landed. The lowest estimate shows 0.166 fish landed per year in 2007 while the

upper estimate is 16.65. This is substantially different from the lower estimate of 1.56

and 156 in 1980, almost by a factor of 10.

FIsheries Mortality for D. batis with different % of Total Catch Scenarios

-40

10

60

110

160

210

1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

2000

2002

2004

2006

Year

Num

ber o

f Ind

ivid

ual f

ish

0.10%0.50%1%5%10%

Figure 3.33: Line graph showing the estimated landings of D. batis by the commercial

fishing industry in the ICES rectangles 42E3, 42E4 and 41E4 assuming different

proportions of total landings. Assuming an average weight of 88.495kg for D. batis

3.2 Economic Importance of D. batis

From the results of the economic questionnaires it can be seen that charter trips for D.

batis contribute £292,500 to the local economy every year (based on results for 2008).

Of this, £135,000 goes to the charter boat industry and £157,500 goes to the other

businesses in Oban. Each member of an angling party spends £141.53 per day in Oban,

£65.2 of this goes towards charter hire, the rest is spent within Oban town on food and

drink, fuel, accommodation, other activities and goods.

54

Table 3.1: Showing the estimated amount of money that the D. batis angling fishery brought to Oban n 2008. The table shows the breakdown of this figure and how it was achieved by showing average spent per person, number of people a year the fishery brings to Oban and how much of this money goes to the local economy.

Amount in £'s

Total average money spent per trip per group excluding charter 1195.83 Total average spent per group per day excluding charter 525.00 Average spent per person per day excluding charter 76.21 Average spent per person on charter 65.32 Total spent in area per person per day 141.53 Estimated number of people per year 2066 Total spent per day fishing per charter boat 975.00 Total spent annually in area (300 days charter fishing) 292500.00 Contribution to local economy 157500.00 Charter boat industry Gross income 135000.00

Using the median number from the population estimates of 2342 from the entire study

area (this number excludes mortality and therefore is an overestimate; any value placed

on the worth of an individual fish is likely to be an underestimate) we can see that a

single specimen is worth £124.89 in total with a breakdown of worth to local economy

and the charter boat industry shown in Table 3.2

An estimate of the worth of fish in Area C is also useful, as this is the area most

frequented by the charter boats based in Oban (personal correspondence, Ronnie

Campbell) and so the population of D. batis in this area is the target of most charter boat

anglers. The population estimate for area C is 169 and, as the money spent in this area

and on the charter boat industry is all from Oban, we can assume that at least 80% of

this money is spent on targeting fish in Area C. The values for ‘total spent in the area’,

‘contribution to local economy’ and ‘charter boat income’ are used for the calculations

of fish value in Area C, as seen in Table 3.3.

Table 3.2: Showing the value of each fish throughout the study area, broken down into value as a whole, value to local economy and value to charter boats.

Value of

individual fish Value to local economy £67.25 Value to Charter boat industry £57.64 Total Value of fish £124.89

55

Table 3.3: Showing value for fish in area C, the area heavily targeted by Oban based charter boats assuming an estimated worth of 80% of total money spent.

When the number of people who come to Oban to fish for D. batis (2066) is compared

to the number of people who participate in all water sports in the Highlands and Islands

(165, 000( George Street Research)), it can be seen that 1.25% of these participate in D.

batis charter trips from Oban.

Of the total revenue spent in the Highlands and Islands by water sports

participants’ 1.56% of that is spent in Oban by charter boat customers. On average,

charter boat customers spend £26.03 per person per day more than do other water sport

participants. The most money is spent on water activities. The money spent on each

activity is shown in Figure 3.36 where D. batis angling trips are the third most

expensive activity after Scuba diving and canoeing/kayaking.

0% of money spent in: Amount £'s Minimum Worth of individual fish

Total Spend on Area C 234000 £1384 Contribution to local economy of Area C 126000 £745 Contribution to charter boat of Area C 108000 £639

56

99%

Oban angling trips for D.batis

Total number of peopleparticipating in watersports in the Highlandsand Islands

Figure 3.34: Percentage of watersports participants who fish for D. batis from Oban. Actual figure is 1.25% of total watersprt participants from the highlands and Islands.

Figure 3.35: The percentage of total revenue to area brought by anglers fishing for D. batis. Actual figures are 1.53%

57

Figure 3.36. Amount spent on each water based activity. Adapted from George Street Research

If the value of D. batis from area C and the entire Isle of Mull population are compared,

it can be seen that fish in area C are much more valuable to the local economy than fish

in the Isle of Mull population (including area C) (Figure 3.37). If this is then compared

to the current market value for skate (currently £2.40 per kg) which includes all species

because skate is sold as skate and is not species specific, it can be seen that although the

total value of the population is less per kg than the current market value, the value of

fish in area C (£13.84 per kg) is comparably much larger than the market value.

Figure 3.37: Comparative value of D. batis. Comparing current market price for skate (Grimsby

Fish market, 2008) (mixed box of flesh, so all species included) price per kg of Isle of mull stocks,

using lowest population estimate and price per kg for fish in Area C base on population estimate for

area C and assumed 80% spend of total revenue in area C.

0

2

4

6

8

10

12

14

16

Value of f ish in £ per kg

Pri

ce p

er k

g

Fish MarketIsle of MullFrith of Lorn

58

4. DISCUSSION 4.1 Maturation

Broadly speaking, a population of any species can be broken down into four main

groups, mature males, mature females, immature males and immature females, so it is

useful to first get growth parameters for these groups for D. batis so the population can

be studied with reference to them as we may reasonably expect D. batis to segregate its

population based on sex, size and maturity as it has been shown that other Rajifrom

species do so (Ebert et al, 2008). The current literature gives the age of maturation in D.

batis as 11years (Brander, 1981) with males being 125cm long (Fowler and Cavanagh,

2005) and females between 140cm (Walker and Heesen, 1996) and 180cm long (Walker

and Hislop, 1998). Using the weight charts in appendix 9, we can estimate the weight

range that these lengths give. For males, the estimated weight would be 12.7-16.33kg

and females, the lowest estimate would be between 17.23-21.77kg and the upper

estimate between 41.28-51.71kg. Using the estimated growth curves in Figures 3.10 and

3.11 which were taken from recapture data for animals caught more than twice, it can be

seen that the estimated weight range for 11year old fish in both sexes is substantially

higher than those estimated from lengths quoted in the literature for D. batis. The

growth curves given in Figures 3.10 and 3.11 are liable to be overestimates as they take

into account weight change caused by food in the gut in both sexes and the spawning

cycle in females. If fish are initially caught with an empty stomach and subsequently

caught just after feeding, there will be a large weight gain recorded which will

subsequently give a large weight gain from which the growth curves are estimated from.

This effect will be compensated to some extent by fish that were initially caught directly

after feeding with a full stomach then subsequently caught with an empty stomach,

causing the growth rate to be low if not negative, A similar situation occurs with

spawning females as female D. batis produce up to 40 eggs a year (Brander, 1981),

measuring up to 24.5cm long, assuming an average brood size of 20 eggs, and a weight

range of 10-20 g per egg, this could result in an apparent weight change (likely

negative) of 200-400g per fish (likely negative). The other variable which could affect

the estimated growth rate is the use of the weight charts to estimate weight. The

common practice is for anglers is to use the weight charts (personal correspondence

Davy Holt, Ronnie Campbell and Adrian Lauder) as opposed to weighing the fish with

scales at sea (which is in any case difficult on a moving boat). Apart from the 5% error

59

that the weight charts give, there is also error produced by inaccurate measuring of the

fish, as in rough sea conditions, or when measuring the fish in the water, the accuracy of

the measurements may be lessened, even an error of 2.54cm (1 inch) either way can

lead to a difference in weight estimate of up to 11kg (estimated from female weight

charts, appendix 9). This error can also be introduced by intermediate measurements

(i.e. 30.5 inches), the number will be rounded up to the nearest whole number (personal

observation), giving overestimates in weight although the anglers try to keep the

measurements as accurate as possible, allowance for errors should be taken into

account. These errors will overestimate the weight of the fish, in turn overestimating

growth rate.

These combined factors lead to an overestimate of weight gain per day, the

smoothed growth curves were produced to reduce this overestimate, they do however

reduce the growth rate of smaller fish as the growth rate is taken as an average over time

and any early periods of rapid growth are reduced by subsequent slower growth. The

growth curves for males (Figure 3.10) show these differences very clearly, with faster,

logarithmic growth being displayed by the unsmoothed data and lower, more linear

progression displayed by the smoothed. For the females however there is no

logarithmic progression for either estimated growth curves and the smoothed data curve

shows more rapid growth than the unsmoothed curve. The reasons that the opposite of

what we would expect occurred could be due to annual growth patterns displayed by

females, if we look at the growth lines for each fish shown in appendix 1, we can see

that females experience no growth between November and February, these periods of

no growth would show up on the unsmoothed data giving zero growth rate values which

would lower the average value causing the unsmoothed growth curve to be lower than

the smoothed growth curve as the smoothed growth rates would be averaged against

less zero values as the weight change is from a longer period and less likely to be zero

or negative.

Neither of the growth curves show the traditional sigmoid shape (Figure 4.1) but

this is to be expected as D. batis is strongly K selective (Stevens et al 2000) investing

resources into producing well developed hatchlings.

60

Figure 4.1: Sigmoid Growth curve.

This will remove the lower section of a typical sigmoid growth curve (ringed in black in

Figure 4.1) as this occurs within the egg case. This is shown to be the case with other

species of Rajifrom such as D. trachyderma (Licandeo et al, 2006) and Bathyraja

parmifera (Alaska skate) (Matta and Gunderson, 2007) and the curve that was produced

for the growth of these two species is similar in shape to the growth curves in Figure

3.10 (smoothed) and 3.11 (both) for D. batis suggesting that they are reasonable

estimates. As the estimated weight at maturity taken from the growth curves is between

33-42kg for males and 69-96kg for females we can assume, due to the overestimating

nature of the growth curves, that the lower value is closer to the true weight. This is

further supported by the scatter plots of weight against growth rate in appendix 2 where

after these estimated weights of maturity, animals started experiencing weight loss,

33kg for males and 64 kg for females, this suggests onset of maturity, as more energy

gets put into gamete production and weight loss is not uncommon in mature animals,

especially during breeding seasons (Ref?). These estimates are significantly different to

the current literature which, compared to this study, appear to be underestimates,

especially in the case of males suggesting that maturation for D .batis occurs at an

earlier age, or that D. batis are heavier upon maturity than previously thought.

Time

Wei

ght

61

4.2.1 Population Segregation

The number of animals caught in each weight class throughout the whole area shows

that there is a peak in the numbers of males (Figure 3.2) in the weight classes 40.01-50

and 50.01-60kg. When compared to the estimated weight at maturity it can be seen that

these two weight classes encompass putative mature fish, so it appears that a large

proportion of all males caught in the study area are probably mature. The numbers of

females in each weight class (Figure 3.3) shows bimodal distribution with a large

number of immature females being caught that were between 10.01-30kg and 60.01-90

kg. The largest number of females are in the 60.01-70 and the 70.01-80kg classes which

are mature fish when using the growth curves and allowing for the over estimate. The

high proportions of mature males and females in the region suggest behaviour in

common with other Rajifrom species whereby mature individuals move to shallower

coastal waters during the breeding season (Walker et al, 1997 and Hunter et al 2005b).

Using the growth curves it is possible to estimate the age of the fish in the 10.01-20kg

class to be between 6 months and 2 years old which suggest they are a generation below

the mature adults. If D. batis does move to shallower water to breed and lay eggs, then

the eggs will hatch in the shallower water and it is possible that they remain in this

depth and use it as a nursery until they are strong enough to move into deeper, offshore

waters to mature as we know that D. batis does occur in deep water up to 800m (Dolgov

et al 2005b) but there was no classification of the weight of the animals found at this

depth so it isn’t possible to estimate how old they are and this can’t be used to support

an offshore migration of smaller animals. A possible reason that there is no bimodal

distribution of males could be explained by 2 year old male fish being a maximum of

10kg (using the lower estimate from the growth curves, Figure 3.10) and are too small

to be caught by anglers in the area who target larger fish. The change in diet shown by

R. clavata shows smaller animals consuming largely benthic polychates and amphipos,

this diet would allow smaller fish to retreat to deeper waters and continue feeding while

also avoiding predation. As the fish grow and become less vulnerable to predation, they

also change their diet to more active prey, such as migratory fish, and they will move

into shallower water where these fish are present. (Dolgov, 2005c). It is known that D.

batis preys on Spurdog (Squalus acanthias) in the area (pers. comm. with local

fisherman), this is too large a prey item for smaller D. batis to take and the population

62

of S. acanthias and the Lesser Spotted Dogfish (Scyliorhinus canicula), of which there

are high numbers in the area, would compete with smaller skate to which they have

similar diets, both feeding opportunistically on macrobenthic fauna including hermit

crabs, cockles and whelks (Dolgov, 2005 and Lyle, 1983). These feeding patterns

support the theory that young D. batis move into deeper water to feed and avoid

competition and predation, and then move into shallower water in accordance with a

change of diet and fill a predatory niche with little competition from other species. This

is only a hypothesis and the results from this project cannot support this fully, but it is

an area that would benefit from further research with some targeting of fish smaller than

10kg in weight as well as sampling of D. batis in deeper, offshore water.

An initial look at the geographic distribution of both sexes of D. batis

throughout the study area shows three main areas of clustering at the Passage of Tiree,

the mouth of Loch Aline in the Sound of Mull and the Firth of LornLorn (Figure 3.1)

and it would be easy to assume, when linked to the large proportion of mature fish that

these are possible breeding sites for D. batis. However, with more investigation into

fishing effort it is revealed that the clustering pattern is purely down to fishing effort

throughout the area, with anglers focusing their fishing in these three regions. What is

clear from the geographic distribution throughout the study region is that both male and

female fish are prevalent throughout the area.

Fishing effort is also responsible for the grouping at depths, group I and II, seen

in Figures 3.4 and 3.5. By looking at the depth fish were caught at in each area, A, B

and C it can be seen that all catches in less than 75m were in area B, less than 145m in

area C and equal distribution of fish being caught between 75-145m between areas A

and C (Figure 3.18). Each angler tends to fish within the same area i.e. anglers fishing

area A will rarely fish in area B, they also fish at different times of the year which

appears to be the cause of the variance in seasonal geographic distribution. The main

angler who fished area B only fished the months of March to October with peak fishing

times around June/July (personal correspondence, Brian Swinbanks) while the main

angler fishing area A is often out all year (personal correspondence, Davy Holt) and

anglers fishing area C are generally out all year but with effort focused between March

and October (personal correspondence Adrian Lauder and Ronnie Campbell). This

explains the absence of fish in winter from area B and the large numbers in summer and

autumn; this also offers an explanation for the observed shallower median depth in these

seasons (Figure 3.9).

63

The obvious biasing of the data by the methods used to collect it makes it

unsuitable to draw any conclusions on population segregation regarding depth and

geographic distribution as a whole. However, if we remove the variable of local

bathymetry and fishing effort by splitting the data into areas where the angling effort

has been focused so all fish caught are subject to the same effort and the same localised

bathymetry, where depth is more likely to be by choice rather than by location, then

actual depth changes displayed by the fish can be looked at. This was done for the fish

in nominal areas A, B and C shown in Figure 2.1 with each area being analysed

separately.

4.2.2 Areas A, B and C

The median depth for each sex between the areas show that both sexes in area A are in

deeper water during the winter and spring (November – April) and in shallow water

during the summer and autumn (June – October), in area B there were no records of

depth for winter and only one record for spring making it unrepresentative, the fish were

at their deepest during summer, although no depth data were available for winter and

spring so no comparison against these seasons can be made. In area C males and

females appear to follow different movements with females being at their deepest in the

winter, progressing to their shallowest depth in the autumn, males however are found in

deep water during the winter, their shallowest depth in the spring, and progressively

deeper water over the summer reaching their deepest depth in the autumn. These

patterns on their own might suggest that area A could be a breeding and/or hatchery

area as the fish are moving to shallower water later in the year in accordance with their

thought breeding season (Fowler and Cavanagh, 2005). Area C, with the rise of animals

to shallower depth in the spring might also be a breeding area from which the males

then descend to deeper water again, while the females remain to lay eggs. In an attempt

to support these hypotheses, the depth distribution was plotted against weight with each

season on the same area plot, the scatter plots for area C showed no patterns of depth

related to weight although, in general, the depth of fish appeared to be slightly shallower

in summer seasons, this was supported by a significant association between the weight

of female fish and the depth they were caught in (Spearmans rank, p=<0.05), there was

no association for the male fish in the area, more reslts from area C need ot be recorde

before any firm conclusions can be drawn. In area A there was a distinct cluster of

females larger than 60kg (borderline mature fish) found in shallower depths during the

64

summer, the records for autumn only included one fish smaller than 60 kg which

happened to be caught in shallow water, assuming this result to be unrepresentative then

a similar pattern during the autumn of larger fish at shallower depths can be seen. For

males, this pattern is even clearer, with no fish less than thirty five kilograms being

caught in less than 100m of water. During all four seasons, but most noticeably in

spring and summer, the larger male fish (mature animals of 35kg and above) were found

at shallower depths, this is similar behaviour displayed by other Rajifrom species

(Walker et al, 1997, hunter et al 2005a) during breeding seasons, supporting the

hypothesis that area A could be a potential breeding or hatchery ground with mainly the

larger, mature fish being found in the shallower depth, to strengthen this hypothesis,

there was a significant association between weight of fish (both male and female) and

the depth they were caught in (p=<0.01). Area B showed similar patterns to area A for

both sexes, although only females showed a significant association (p=<0.05), with

mature fish rising to shallower water during the summer and autumn, this was again

more distinct with males with no male fish less than 35kg being found in water

shallower than 50m. Although area B is locally shallower than area A, it might be

supposed that it is not the actual depth of water that is important, but the movement

from deeper to shallower water that matters, in order for mature adults to congregate

away from immature individuals or groups. This is assuming that both areas A and b are

used for breeding purposes and cannot be assumed to be true. There were no records for

winter and only one for spring so no comparisons against these seasons can be drawn

These are hypothesis based on the known behaviour of other species of Rajifrom

and although are only suggested by the data in this report are worth consideration and

further research. There is not enough information to draw any conclusions for area C.

4.3 Migratory Behaviour

Other species of Rajifrom do undertake annual migrations moving into offshore waters

during winter and summer months and moving back into coastal waters during the

summer months this movement appears to be linked to the breeding cycle, with the

population moving to coastal waters during the breeding season (Walker et al 1997 and

Hunter et al 2005b), it would be reasonable to assume from this that the fish breed and

lay eggs in shallower, coastal waters (Hunter et al, 2005a). The records of migratory

behaviour for D. batis are few, all using the Glasgow museum data, the conclusions

drawn from these papers were that individual D. batis are associated with a regional

65

population with a few individuals performing long migrations (Sutcliffe, 1994 and

Little, 1995). The results gained from this study generally support these hypotheses as

there are several examples of fish being caught repeatedly within a small geographic

area, displaying recapture patterns that suggest confinement to a small area,. Caution

must be used; however, when looking at the recapture sites as, although they show were

the fish has been caught and were the fish has moved to, they do not show movement in

between the catch sites, we may only be looking at a small section of a larger pattern.

Take the example in Figure 4.2, if the fish normally performs the migration shown by

the solid black line but we only sample it within the boxed section, we might assume

that the fish only moves within that area and the assumed route is shown by the dotted

line. So although it cannot be said from the recapture data that the fish remains within

one area throughout its life, it can be said that it returns to the same area on a regular

basis.

Figure 4.2: Diagram to of hypothetical migration route with small sampling area in the centre of the route. Example of misleading recaptures.

The long migrations shown in Figure 3.26 prove that not all fish remain within a small

geographic area. It cannot be assumed that the fish have moved from one population to

another as we might only be sampling two points on a circular route, this is shown in

Figure 4.2 where the two red circles represent sampling points, if we only catch a fish

once at point X and once at point Z afterwards, it would be easy to assume that the fish

had moved to point Z when in actual case the points are on a continual route. Of the fish

that have been confirmed to make long journeys two, a male and a female, are recorded

as having moved from the Isle of Mull to the Firth of Clyde in a southern movement, the

other is a female moving from Ballycastle Bay (Ireland) to the Isle of Mull in a

northerly movement. It is possible that these fish are performing similar routes between

X

Z

66

the Isle of Mull and the Firth of Clyde but due the timing of catching, different

conclusions for each fish could be drawn. The frequency and number of these

migrations is unknown, partly due to the fact that fishing effort for D. batis is focused

around Mull so the low record of long movements may be due to the lack of targeting

outside of the study area rather than due to low numbers of fish moving. The other

records of long migrations are from commercial vessels and there is a certain amount

doubt about the location of the catch sites (Sutcliffe, 1994). A tagging programme in

other areas, such as the Firth of Clyde where some of the Isle of Mull fish have moved

to might increase our understanding of the movements of D. batis along the west coast

of Scotland and would give insight into the regularity of these movements and whether

or not they are one way migrations leading to recruitment into other populations or

circular movements such as displayed by R. clavata in the Southern North Sea (Hunter

et al 2005b).

4.4 Population Estimates

Figure 3.32 suggest that the population of D. batis in the area is increasing. After talking

to local fishermen, this does not seem unreasonable as some would agree that the

population has been increasing over the last decade. Yet it would be unwise to assume

from these figures that the population of D. batis in the area is increasing as there are

other factors which can affect the population estimates, one of these is tagging effort. In

the beginning of the project there were few anglers participating in the tagging

programme, the common practice was to return the fish to sure for confirmation of

weight landbased. This became less common and from the early 1990’s and more

anglers began to participate in tagging in 1993, a concentrated tagging effort began in

the Sound of Mull and in 1994, the Firth of LornLorn in; these years corresponds

directly to the beginning of a rapid increase in population levels shown on Figure3.32, it

can therefore be assumed that tagging effort is a major variable in the population

estimates and it cannot be assumed that the increase in population size shown in Figure

3.32 is due solely to an increase in population. If the graph for population estimate

shown in Figure 3.32 is compared to the graph in appendix 5 showing the number of

fish caught each year discounting recaptures within the same year, it can be seen that a

decrease in numbers caught directly relate to years with a decrease or zero increase in

population, this further demonstrates that population estimates are heavily related to

tagging effort. Little (1995) produce a population estimate of 60 -90 from the Glasgow

67

data, this was at the beginning of the increased tagging effort and so was likely to be an

underestimate due to the lack of data being returned. We now know that the population

is at least 110 fish as this was the maximum number of fish caught in one year and this

would assume that 100% of the D. batis in the area had been landed. We can, however,

use the population estimate in 2008 as a start point for future estimates and might prove

a useful baseline to compare to any future research, but it must be emphasised that this

is an overestimate and should not be used for any management plans, nor should it be

assumed that there is a resilient population of D. batis in the area.

If there is an increase in the population, this might not be solely attributable to

recruitment through breeding, since there have been examples of Rajifroms

congregating together in what is referred to as essential fish habitat (EFH) (Payne et al,

2005), it could be that the waters around the Isle of Mull are an EFH for D. batis. This

would certainly explain why, in most other areas around the UK, D. batis is reported as

being virtually extinct (Brander, 1981, Roger and Ellis, 2000). Although at initial glance

this could partially explain the large increase in population, the bias put on the data by

fishing effort does not permit this conclusion to be drawn directly, although it cannot be

discounted and needs further research with data collection more suitable for population

estimates.

On a cautionary note, all estimates are overestimates, as the Schnabel model

used does not add in any level of mortality. It also assumes that the population around

the Isle of Mull is closed, we know that this is not the case as there are examples of

individual animals both leaving and incoming to the area with up to 200km between

catch sites (Figure 3.26). This shows that there is some degree of mixing outside the

study area in this project although to an unknown degree. Another assumption for the

model is that fish caught and tagged recover 100% and are at no competitive

disadvantage to uncaught fish. This may not always be the case. One of the problems

reported by some skate anglers is ‘deep hooking’ This is where the hook is taken down

into the stomach and can either tear the inside of the stomach, resulting in death of the

fish, or can remain impaled in the lining, causing feeding problems and weight loss. The

main reason for deep hooking is the use of inappropriate fishing gear sometimes used by

private boat angler or shore anglers, especially those not directly targeting D. batis.

Deep hooking is sometimes a problem that cannot be avoided, due to the nature of skate

landing on the bait and consuming it without performing a ‘run’ alerting the angler to

‘strike’ the fish, which would tend to set the hook in the mouth. Taking this into

68

account, it is fair to assume that some of the fish returned may be at a disadvantage to

other fish and also may have a higher mortality rate(Committee on the Recreational

Fisheries et al). There is a fishing code of conduct for D. batis which should be

followed to minimise stress and damage to the animal, increasing its chances of survival

after capture (available from Richard Sutcliffe at Glasgow Museum Resource Centre)

The estimated fisheries mortalities all show the same pattern, a declining number

of Rajifromes being landed. Due to the nature of Rajifrom landings all being grouped

under ‘skates and rays’ the estimate for the proportion of this catch being D. batis works

around assumed proportions of total catch and assumed average weight for D. batis

which itself assumes that equal numbers from each weight class were caught. This

makes the estimates liable to be inaccurate and not reliably representative of the actual

D. batis take. It is likely that landings of D. batis have decreased, but to what extent is

unknown without species specific landing data. Assuming a level of 10% of total catch,

which is higher than any of the percentages in research trawls performed in other seas

(Dolgov et a, 2005, Wlaker and Heesen, 1996 and Ellis and Walker, 2005), the pattern

for most elasmobranch species in the southern North Sea is of declines in landings

(Philippart, 1998), which is also true for the skate and ray landings from around Mull.

Fishing effort in the area has not decreased substantially, and in fact the most days spent

out of harbour were between 1994 and 1999. Reported landings of skate and rays do

increase during this period, but not above levels pre-1986, suggesting that the numbers

of skate and rays in the area are decreasing, most likely in response to fishing pressure,

to which they are particularly vulnerable (Dulvy et al, 2000). Since D. batis is one of

the most sensitive species to fishing pressure (Stevens et al, 2000) we can assume that

there has been a proportionally greater drop in the numbers of D. batis being taken,

compared to other Rajifroms.

4.5 Economic Importance

The returns from the economic survey were not sufficient to conduct in-depth analysis

into the economic value of D. batis to the area. Nevertheless, there were enough to

make some general overviews as to how much money anglers brought to the area. For a

relatively small industry, the charter trips bring a lot of people to the area, most of them

travelling especially to the area for D. batis angling trips. Although the proportion of all

water sport participants is only 1.25%, it must be remembered that skate angling is a

specialist sport, with many anglers coming to Oban especially for it, staying for several

69

days and fishing for most of them. Proportionally, the amount of money spent by the

anglers chartering boats is higher than the average spend by other water sport

participants and contributes 1.53% of total income to the Highlands and Islands. It is

hard to work out what percentage of money coming into Oban from water sports skate

angling represents as there are no figures by town or even council area as to how much

money is contributed by the local economy. The figures for contribution from skate

anglers are estimates and would need more results to substantiate them; however, it is

clear that the money brought into Oban by skate anglers is relatively large per capita.

It appears from the results that fish in area C are far more valuable to the local

economy than they are to the commercial industry, and it must also be remembered that

skate caught by anglers and returned can be caught again, which doubles the value of

that fish as two charter boat trips have enjoyed the benefit of catching that animal,

which will in turn encourage anglers to return to the area to fish for D. batis again.

Although the average estimated value of each fish throughout the whole study area is

lower than the current commercial price per kg, it must be remembered that this value

was obtained from a probable overestimated population level, which in turn,

underestimates the value of the fish, so it is likely that the true value of D. batis is

higher than that quoted in this here (£1.14 per kg)

4.6 Life History

Although the Glasgow museum provides an excellent historical record of D. batis in the

area, there were not enough records for every year with all the necessary data recorded

to accurately assess any change in the life history of the population in response to

pressure. Other species of elasmobranchs have been shown to do this (Tanaka et al,

1990), changing growth rates and age of maturity (Tanaka et al, 1990 and Stevens et al,

2000). Although it is possible that D. batis could change its age at maturity by

increasing its growth rate and lessening its age of maturity, there is no evidence that this

is the case. The results from this study strongly suggest that the age of maturity for D.

batis around the Isle of Mull is 11 years old. This is the same age for maturity given 20

years ago (Brander, 1981) and cannot be seen to have altered. Due to the long life, late

maturity and strong K selectivity of D. batis any changes to life history characteristics

would be expected to occur over several generations so we would not expect any

changes to happen quickly.

70

5. CONCLUSIONS

5.1 Conclusions from this study

In conclusion, this investigation was able to show that immature and mature individuals

of D. batis are present throughout the northern Passage of Tiree, the Sound of Mull and

the Firth of Lorn. There was no observed geographic preference of either sex throughout

the study area, although seasonal preference for one area was not attainable from the

results due to the seasonal change in fishing effort in different areas.

There were clear trends showing annual movement of mature individuals of D. batis, of

both sex, from deep water to shallow water in relation to the cited breeding cycle (Ellis

and Walker, 2005) in certain areas which strongly suggested the preference of D. batis

to breed and lay eggs in shallower water during the spring and summer in areas A and

B. There was no preference to an actual depth displayed by either mature or immature

individuals of either sex, this appeared to be related to local bathymetry. There is a

strong suggestion that larger animals are found in shallower water, this association was

shown to be significant for females in all areas, with the strongest association in area A

(Spearmans Rank 2-tailed correlation test, p=<0.01) and males in area A and the study

area as a whole (p=<0.01). Careful management of areas A and B might aid the

rebuilding of the population around the Isle of Mull, this in turn, due to proven

movements of D. batis, might help the population growth along the west coast of

Scotland.

It is also clear that the Isle of Mull population is not closed, this can be seen by the long

migrations of both sexes shown in figure 3.26. It is unknown to what extent these

migrations occur and whether or not they are one way or cyclic.

This study also calls into question previous estimates for length and associated weight at

maturity for D. batis. It is true that either D. batis matures at an earlier age than

previously thought, or, that, the previous estimates for length at maturity from other

papers are under estimates.

No clear conclusions can be drawn from the population estimates as the results are

dependent onto many assumptions and are heavily influenced by the number of tag

71

returns. What is clear is that the tagging data needs more information, such as a record

of time spent fishing for D. batis to enable a more in depth study into population levels

is needed to determine the current status of the Isle of Mull population. As with

population estimates, as we have no official record for the landings of D. batis it is

unclear what impact the commercial industry has on the Isle of Mull population as this

study assumes landing levels. Landing data recording individuals of D. batis is needed

to look into the fisheries mortality.

Charter trips for D. batis bring a substantial amount of money to Oban, but it is unclear

what proportion of total income this contributes to Oban itself, although to the

Highlands and Islands as a whole, it is a relatively large amount for such a small

industry. What is clear is that individual fish in the area are worth more to the angling

industry than they are to the commercial market and, due to their ability to survive the

trauma of netting, are worth returning to the sea upon capture.

5.2. Further Areas for Study

The main conclusion drawn from this study is the need for further research into D. batis,

not only in the waters around the Isle of Mull, but around the whole UK. If this species

is to be preserved effectively, more knowledge on its behaviour is desperately needed to

ensure its future, this study has opened the doors to many areas of potential research and

I would suggest the following areas as targets for further study:

• Genetic study. Looking into the mixing between regional populations of D.

baits. This would indicate to what level populations mix and whether or not

regional populations are closed or not. This is essential as closed populations are

more vulnerable to fisheries mortality and are less likely to survive in exploited

areas.

• Study into the annual migration of both sexes of D. batis. Using electronic data

tags or/and sonar tags to track the movement of individual skate more closely to

see how common long migrations are and if they are cyclic by nature or one

way. Also will show any annual migrations.

72

• Study into the weight classes found in various areas of sea including offshore.

This will further our understanding into the movements of young D. batis and

will substantiate if they do indeed move into deeper water offshore to mature.

• Sampling for D. batis throughout all waters around Mull. This will lead to better

understanding of any geographic congregation areas for D. batis and will help in

the allocation of breeding and hatchery areas for the species, allowing more

effective management.

• An in depth study into the breeding cycle of D. batis to confirm the months of

breeding and egg laying for the species, again, to allow more effective

management.

• A more in depth study into the economic worth of D. batis to the Oban area,

looking for contribution to local economy.

• Collection of egg case data via trawl surveys and commercial trawlers. This will

help us to understand the egg laying nature of D. batis and if females do

congregate in a common hatchery, where these areas are.

73

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Anonymous (2000) Tagging of common skate (Dipturus batis) off the west coast of

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Brander, K. (1981) Disappearance of common skate Raja batis from Irish Sea. Nature.

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Dolgov, A.V., Drevetnyak, K.V. and Gusev, E.V (2005a) The Status of Skate Stocks in

the Barents Sea. Journal of Northwest Atlantic Fisheries Science. 35. 249-260 Dolgov, A.V., Grekov, A.A., Shestopal, I.P and Sokolov, K.M. (2005b) By-catch of

Skates in Trawl and Long-Line Fisheries in the Barents Sea. Journal of Northwest Atlantic Fisheries Science. 35. 357-366

Dolgov, A.V., (2005c) Feeding and Food Consumption by the Barents Sea Skates.

Journal of Northwest Atlantic Fisheries Science. 35 495-503 Dulvy, N.K and Reynolds, J.D. (2002) Predicting Extinction Vulnerability in Skates.

Conservation Biology. 16 no.2. 440-450 Ebert, D. A and Compagno, L.J.V. (2007) Biodiversity and systematics of skates

(Chondrichtyes: Rajiformes: Rajoidei). Environmental Biology of Fishes. 80. 111-124

Ebert, D.A and Sulikowski, J.A. (2007) Preface: Biology of skates. Environmental

Biology of Fishes. 80. 107-110 Ebert, D.A, Compagno, L.J.V and Cowley, P.D. (2008) Aspects of the reproductive

biology of skates (Chondrichtyes: Rajiforms: Rojoidei) from southern Africa. ICES journal of Marine Science. 65. 81-102

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Ellis, J.R., Cruz-Martinez, A., Rackham, B.D., Rogers, S.I. (2004) The Distribution of Chondrichthyan Fishes around the British Isles and implications for Conservation. Journal of Northwest Atlantic Fisheries Science. 35. 195-213

Ellis, J.R., Dulvy, N.K., Jennings, S., Parker-Humphreys, M. and Rogers, S.I. (2005)

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common or blue skate Diptururs batis (Linnaeus, 1758). Sharks, Rays and Chimeras. IUCN 333-335

Gallagher, M.J. Nolan, C.P and Jeal, F. (2004) Age, Growth and maturity of the

Commercial Ray Species from the Irish Sea. Journal of Northwest Atlantic Fisheries Science. 35. 47-66

George Street Research and Jones Economics (2003/2004). Economic Impact and

development opportunities for outdoor and Environment related recreation in the Highlands and Islands: Research Report: Watersports. Provided by Sona Magill at the Scottish Association of Marine Science.

Grimsby Fish Market. Price List. Available from:

http://www.grimsbyfishmarket.co.uk/frames/Prices.html. Viewed on 15/08/2008 Hamlett, W.C (1999) Sharks, Skates and Rays: The Biology of Elasmobranch Fishes.

The Johns Hopkin University Press. Baltimore. 1 Hedges, Andrew. 2003 Convert Latitude/Longitude to Decimal. Direct link:

http://andrew.hedges.name/experiments/convert_lat_long/ viewed on 26/06/2008 Holden, M.J. (1975) The Fecundity of Raja clavata in British waters. Journal de

Conseil. 36. 110-118 Hunter, E., Buckley, A.A., Stewart, C. and Metcalfe, J.D. (2005a) Migratory behaviour

of the thornback ray, Raja clavata, in the southern North Sea. Journal of the Marine Biological Association of the United Kingdom. 85. 1095-1105

Hunter, E., Buckley, A.A., Stewart, C. and Metcalfe, J.D. (2005b) Repeated seasonal

migration by a thornback ray in the Southern North Sea. Journal of the Marine Biological Association of the United Kingdom. 85. 1199-1200

IUCN (2007) IUCN Red list of Threatened Species: Dipturus batis. [online]

International Union for Conservation of Nature and Natural Resources. Available from: http://www.iucnredlist.org/search/details.php/39397/all viewed on 15/05/2008

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Jennings, S., Reynolds, J.D and Mills, S.C. (1998) Life history correlates of ersponses to fisheries exploitation. Proc. Royal Society of London. 265. 333-339

Jennings, S., Greenstreet, S.P.R. and Reynolds, J.D. (1999) Structural change in an

exploited fish community: a consequence of differential fishing effects on species with contrasting life histories. Journal of Animal Ecology. 68. 617-627

Koop, J.S., (2005) Reproduction of captive Raja spp. in the Dolfinarium Harderwijk.

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leopard shark, Triakis semifasciuta. Environmental Biology of Ifhses. 35. 187-203

Licandeo, R., Cerna, F. and Céspedes, R. (2007) Age, growth and reproduction of the

roughskin skate, Dipturus trachyderma, from the southeastern Pacific. Ices Journal of Marine Science 64 141-148

Little, W. (1995) Common Skate and Tope: First Results of Glasgow Museum’s

Tagging study. Glasgow Naturalist. 22, part 5.455-466 Little, W. (1997) Common Skate in the Sound of Mull. Glaucus. Vernal/Summer. 42-

43 Little, W. (1998) Tope and Skate Tagging off West Scotland: Part 2. Glaucus.

Vernal/Summer. 36-38 Little, W. (date unknown) Tope & Common Skate tagging off the West of Scotland - an

update. Available from SAH: http://www.catchalot.co.uk/tagging/tskate.htm viewed on 09/07/2008

Lyle, J. M. (1983) Food and feeding habits of the lesser spotted dogfish, Scyliorhinus

canicula (L.), in Isle of Man waters. Journal of Fish Biology. 23, issue 6. 725-737 MarLIN, The Marine Biological Association. Direct link to Common skate (Dipturus

batis) http://www.marlin.ac.uk/species/Dipturusbatis.htm. viewed on 10/05/2008 Matta, E.M and Gunderson, D.R. (2007) Age, growth, maturity and mortality of the

Alaska skate, Bathyraja parmifera, ni the eastern Bering Sea. Environmental Biology of Fishes. 80. 309-323

McCallum, Hamish. (2000) Population Parameters: Estimation for Ecological Models.

Blackwell Publishing. 87-95

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Oddone, M.C. and Vooren, C.M. (2005) Reproductive behaviour of Atlantoraja cyclophora (Regan 1903)(Elasmobrnchii: Rajidae) off southern Brazil. ICES Journal of Marine Science. 62. 1095-1103

Payne, A., Cotter, J. and Potter, T. (2008) Management of elasmobranch fisheries in the

North Atlantic. Advances in Fisheries Science. Blackwell Publishing, Oxford. Philippart, C. J. M. (1998) Long-term impact of bottom fisheries on several by-catch

species of demersal fish and benthic invertebrates in the south-eastern North Sea. ICES Journal of Marine Science. 55. 342-352

Rogers, S.I and Ellis, J.R. (2000) Changes in the demersal fish assemblages of British

coastal waters during the 20th century. ICES Journal of Marine Science. 57. 866-881 Sharktrust, (2004) Common Skate code of conduct. Available from Glasgow Museums. Stevens, J.D., Bonfil, R., Dulvy, N.K and Walker, P.A. (2000) The effects of fishing on

sharks, rays and chimeras (chondricthyans), and the implications for marine ecosystems. ICES Journal of Marine Science. 57. 476-494

Stephen, A.C. (1929) Large skate from the Firth of Clyde. Scottish Naturalist. 175. 94 Sutcliffe, R. (1994) Twenty years of tagging common skate and tope off the west coast

of Scotland. Shark, Skate and Ray Workshop. London. 15-16 February. 14-16 Tanaka, S., Cailliet, G.M. and Yudin, K.G. (1990) Difference in growth of the Blue

Shark, Prionace Glauca: Technique or Population. Elasmobranchs as Living Resources: Advance in Biology, Ecology and Systematics and the Status of the Fisheries. U.S Department of Commerce, Washington. 90. 177-188

Walker, P.A. and Heessen, H.J.L. (1996) Long-term changes in ray populations in the

North Sea. ICES Journal of Marine Science. 53. 1085-1093 Walker, P., Howlett, G. and Millner R. (1997) Distribution, movement and stock

structure of three ray species in the North Sea and eastern English Channel. ICES Journal of Marine Science. 54. 797-808

Walker, P.A. and Hislop, J.R.G. (1998) Sensitive skates or resilient rays? Spatial and

temporal shifts in ray species composition in the central and north-western North Sea between 1930 and the present day. ICES Journal of Marine Science. 55. 392-402

77

Walker, P.A. (1999) Fleeting Images Dynamics of North Sea Ray Populations. PhD Thesis.

ZeeInZicht Digitale Encyclopedie. Direct Link:

http://www.zeeinzicht.nl/vleet/index.php?item=blue+skate. Viewed on 15/06/2008

78

Appendix 1. Annual Growth patterns of individual fish

Figure A.1: Individual growth curves for females showing annual growth patterns.

Figure A.2: individual growth curves for males showing annual growth patterns

79

Appendix 2: Growth rate versus Weight Scatter Plots

Figure A.3: Scatter plot for weight against growth rate for males using smoothed data, showing logarithmic trend line and equation.

Figure A.4: Scatter plot for weight against growth rate for males using unsmoothed data, showing logarithmic trend line and equation.

y = -0.0127Ln(x) + 0.0644

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0 10 20 30 40 50 60 70

Weight (kg)

Gro

wth

rate

(kg

per d

ay)

y = -0.0688Ln(x) + 0.306

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0 10 20 30 40 50 60 70

80

Figure A.5: Scatter plot for weight against growth rate for females using smoothed data, showing logarithmic trend line and equation.

Figure A.6: Scatter plot for weight against growth rate for females using unsmoothed data, showing logarithmic trend line and equation.

y = -0.0492Ln(x) + 0.2888

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60 80 100 120

Weight(kg)

Grw

oth

rate

(kg

per d

ay)

y = -0.0214Ln(x) + 0.1313

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0 20 40 60 80 100

Weight (kg)

Gro

wth

rate

(kg

per d

ay)

81

Appendix 3: Spearmans rank results for Weight against Depth

Table A.1: Spearmans rank results for all males and females in study area. Significant association between weight and depth for males

Depth Weight Spearman's rho Depth M Correlation Coefficient 1.000 .179(**)

Sig. (2-tailed) . .000 N 448 448

Weight M

Correlation Coefficient .179(**) 1.000 Sig. (2-tailed) .000 . N 448 468

Table A.2: Spearmans rank results for males and females within each are, A, B and C.

Depth Weight Spearman's rho Depth F Correlation Coefficient 1.000 -.065

Sig. (2-tailed) . .061 N 829 827

Weight F

Correlation Coefficient -.065 1.000 Sig. (2-tailed) .061 . N 827 830

82

Appendix 4: Distance travelled by each fish, male and female

Table A.3: Total distance travelled by each fish re-caught more than 3 times. Median values shown at the bottom.

Sex Female Male

Distance travelled (metres)

134093 8530 0 5525 0 4458

5663 0 0 9383 7859 8543 100 0 150 8733 0 298 0 8561 0 7787 0 5768 16346 224990 509 10744 752 1555 1904 1295 250 895 2178 10243 8757 3079 11012 11046 4178 7458 4217 4765 5744 5527 9947 317 8062 150 200 150 243330 100 100 250 4887 1415 740 100 32104 800 8501 100 253020 0 Median 6613 746

83

Appendix 5: Total number of Fish caught each year within the study area.

Figure A.7: Number of fish caught discounting recaptures within the same year within the study area

0

20

40

60

80

100

120

1977

1979

1981

1983

1985

1987

1989

1991

1993

1995

1997

1999

2001

2003

2005

2007

Year

Num

ber c

augh

t

84

Appendix 6: Tables used for population estimates.

Table A.4: Showing information needed for the Schnabel model to estimate population assuming a sampling period of one day with no same day recaptures included.

YEAR Ct Rt Ut Mt CtMt

Numbers using (CtMt)/Rt

1973 0 0 0 0 0 0.00 1974 0 0 0 0 0 0.00 1975 1 0 1 0 0 0.00 1976 0 0 0 1 0 0.00 1977 28 1 27 1 28 0.00 1978 23 4 19 28 644 134.40 1979 28 3 25 47 1316 248.50 1980 12 2 10 72 864 285.20 1981 9 2 7 82 738 299.17 1982 15 3 12 89 1335 328.33 1983 15 4 11 101 1515 338.95 1984 16 3 13 112 1792 374.18 1985 46 6 40 125 5750 499.36 1986 8 7 1 165 1320 437.20 1987 24 8 16 166 3984 448.51 1988 26 5 21 182 4732 500.38 1989 48 5 43 203 9744 637.02 1990 38 4 34 246 9348 756.32 1991 16 6 10 280 4480 755.40 1992 30 3 27 290 8700 852.88 1993 65 15 50 317 20605 949.32 1994 45 19 26 367 16515 934.10 1995 70 24 46 393 27510 975.16 1996 54 29 25 439 23706 945.27 1997 38 7 31 464 17632 1014.11 1998 64 9 55 495 31680 1147.56 1999 62 12 50 550 34100 1259.88 2000 58 13 45 600 34800 1354.84 2001 55 15 40 645 35475 1427.33 2002 101 19 82 685 69185 1611.83 2003 100 26 74 767 76700 1748.81 2004 91 37 54 841 76531 1789.45 2005 109 39 70 895 97555 1873.59 2006 71 27 44 965 68515 1923.81 2007 41 14 27 1009 41369 1962.72 2008 7 4 3 1036 7252 1961.12

Population estimate using all recaptures except same day catches.

85

Table A.5: Showing information needed for the Schnabel model to estimate population assuming a sampling period of one year

YEAR Ct Rt Ut Mt CtMt

Numbers using (CtMt)/Rt

1973 0 0 0 0 0 0.00 1974 0 0 0 0 0 0.00 1975 1 0 1 0 0 0.00 1976 0 0 0 1 0 0.00 1977 27 0 27 1 27 0.00 1978 21 2 19 28 588 307.50 1979 25 0 25 47 1175 895.00 1980 12 2 10 72 864 663.50 1981 9 2 7 82 738 565.33 1982 14 2 12 89 1246 579.75 1983 14 3 11 101 1414 550.18 1984 16 3 13 112 1792 560.29 1985 44 4 40 125 5500 741.33 1986 8 7 1 165 1320 586.56 1987 24 8 16 166 3984 565.09 1988 25 4 21 182 4550 626.97 1989 46 3 43 203 9338 813.40 1990 38 4 34 246 9348 951.91 1991 16 6 10 280 4480 927.28 1992 30 3 27 290 8700 1038.94 1993 53 3 50 317 16801 1283.30 1994 32 6 26 367 11744 1348.53 1995 51 5 46 393 20043 1547.04 1996 33 8 25 439 14487 1575.19 1997 34 3 31 464 15776 1716.86 1998 59 4 55 495 29205 1989.27 1999 59 9 50 550 32450 2149.12 2000 48 3 45 600 28800 2386.91 2001 46 6 40 645 29670 2540.40 2002 92 10 82 685 63020 2882.36 2003 84 10 74 767 64428 3179.07 2004 80 26 54 841 67280 3073.75 2005 88 18 70 895 78760 3216.63 2006 59 15 44 965 56935 3265.16 2007 34 7 27 1009 34306 3326.72 2008 5 2 3 1036 5180 3318.88

Population estimate using an assumed annual sampling period

86

Table A.6: Showing information needed for the Schnabel model to estimate population assuming a sampling period of 30 days

YEAR Ct Rt Ut Mt CtMt

Numbers using (CtMt)/Rt

1973 0 0 0 0 0 0.00 1974 0 0 0 0 0 0.00 1975 1 0 1 0 0 0.00 1976 0 0 0 1 0 0.00 1977 27 0 27 1 27 0.00 1978 23 4 19 28 644 167.75 1979 26 1 25 47 1222 378.60 1980 12 2 10 72 864 393.86 1981 9 2 7 82 738 388.33 1982 15 3 12 89 1335 402.50 1983 14 3 11 101 1414 416.27 1984 16 3 13 112 1792 446.44 1985 44 4 40 125 5500 615.27 1986 8 7 1 165 1320 512.28 1987 24 8 16 166 3984 509.19 1988 25 4 21 182 4550 570.49 1989 46 3 43 203 9338 743.82 1990 38 4 34 246 9348 876.58 1991 16 6 10 280 4480 862.15 1992 30 3 27 290 8700 969.40 1993 60 10 50 317 19020 1108.60 1994 40 14 26 367 14680 1098.22 1995 64 18 46 393 25152 1152.61 1996 40 15 25 439 17560 1154.98 1997 37 6 31 464 17168 1240.30 1998 61 6 55 495 30195 1420.88 1999 62 12 50 550 34100 1544.43 2000 52 7 45 600 31200 1685.04 2001 49 9 40 645 31605 1791.79 2002 97 15 82 685 66445 2025.92 2003 95 21 74 767 72865 2185.51 2004 87 33 54 841 73167 2190.19 2005 100 30 70 895 89500 2284.24 2006 69 25 44 965 66585 2318.34 2007 41 14 27 1009 41369 2348.86 2008 7 4 3 1036 7252 2341.62

Population estimate using assumed 30 day sampling period

87

Table A.7: Showing information needed for the Schnabel model to estimate population for each area A, B and C assuming a 1 day sampling period to get a lower overestimate. (no mortality so all estimates are over estimates).

YEAR A B C Total 1973 1974 1975 1976 1977 1978 8.8 8.8 1979 19.441176 19.441176 1980 41.054054 41.054054 1981 47.775 47.775 1982 58.431818 58.431818 1983 70.3125 70.3125 1984 58.928571 58.928571 1985 73.597826 73.597826 1986 74.173469 74.173469 1987 83.436364 83.436364 1988 96.85 96.85 1989 127.21212 127.21212 1990 161.72993 161.72993 1991 173.51049 173.51049 1992 204.09459 204.09459 1993 220.68831 220.68831 1994 72.16 223.56129 295.72129 1995 97.626374 4 101.62637 1996 107.52137 9.5675676 117.08894 1997 110.85124 28.5 139.35124 1998 117.14173 44.857143 161.99888 1999 135.00758 69.925926 204.9335 2000 148.88387 84.835294 233.71917 2001 155.36686 106.58511 261.95197 2002 168.76796 118.52475 287.29271 2003 174.215 144.95556 319.17056 2004 191.14938 149.8169 340.96628 2005 201.51341 159.79874 361.31215 2006 212.53737 166.21687 378.75423 2007 216.50842 169.41667 385.92508

88

Appendix 7: Log sheet used for data collection

Figure A.8: Log sheet used to collect data for the project. Data collected was similar to data collected by the Glasgow Museum Tagging Programme, but with the addition of start and finish time.

89

Appendix 8: Questionnaires used to collect economic data

Figure A.11: Questionnaire aimed at anglers onboard charter vessels to collect economic data

90

Figure A.10: Questionnaire aimed at skippers of charter boat businesses to collect economic data

91

Appendix 9: Weight charts for D. batis

Table A.8: Chart to convert Length and wingspan (inches) into Weight (lbs) for males, accurate to 5%. Adapted from SAH, created by Bill Little.

92

Table A.9: Chart to convert Length and wingspan (inches) into Weight (lbs) for males, accurate to 5%. Adapted from SAH, created by Bill Little.