anguilla anguilla, l) in the shannon estuary, ireland ... - wordpress…€¦ · harrington, 1980)....

Biology and Management of European Eel (Anguilla anguilla, L) in the Shannon Estuary, Ireland __________________________________________________________________________________________

__________________________________________________________________________________________

www.EuropeanEel.com| www.ecofact.ie |www.OldRiverShannon.com Page | 36

Biology and Management of European Eel

(Anguilla anguilla, L) in the Shannon Estuary, Ireland

Chapter 3 Immigration timing and activity rhythms of glass eels in the Shannon estuary in relation to temporal and environmental factors


__________________________________________________________________________________________


Chapter 3 Immigration timing and activity rhythms of glass eels in the

Shannon estuary in relation to temporal and environmental factors

3.1 Introduction

There is a long history of glass eel fishing in European estuaries (Tesh, 1977). Extensive

fisheries exist in countries such as Britain, France, Spain, Portugal and Morocco (Sinah &

Jones, 1975; McCarthy et al, 1996; Moriarty & Dekker, 1997). This fishing activity involves

the use of fixed, boat mounted or hand held nets which are used to ‘filter’ glass eels from

the incoming tides. The glass eels captured are utilized for direct human consumption

(especially in Spain), for restocking inland waters in various parts of Europe or for

aquaculture in Europe as well as through exports to Asian eel farms (Wray, 1996; Moriarty &

Dekker, 1997; Liao, 2001). There is no history of glass eel fishing in Ireland prior to the

current work and other experimental work undertaken by ESB in the 1990’s (Reynolds et al,

1994). Experimental glass eel fishing has recently been undertaken on the River Erne

(Matthews et al, 2002). The objectives of the glass eel fishing proposed for Ireland

(McCarthy et al, 1994a), is to provide material for release into wild eel fisheries for the

purposes of conservation and ranching.

The types of methods utilized for glass eel fishing are outlined in Sinah & Jones (1975), Tesh

(1977), Beumer & Harrington (1980), and McCarthy et al (1994a). The Tela net (Jorge et al,

1989) is widely used in Portugal, especially on the Rio Minho (Weber, 1985) and has recently

been used on the River Erne in Ireland (Matthews et al, 2002). The net utilises floated wing

nets (10m x 2m), anchored at the extremities, which are used to guide glass eels carried on

the incoming tide to a boat from which a fisherman captures them using a dip net (rapetta).

Another net used is the Botirao wing net which is a Portugese design (Jorge et al, 1989). This

net is similar to the glass eel fyke nets used to capture A. rostrata elvers in Maine, USA

(Sheldon, 1974; Dow, 1980; Jessop, 1998). This net consists of two wings each 8m in length

and 2 m in height attached to a narrow cod end. The net is set across a river facing the

incoming tide and glass eels traveling with the tide are channeled into the cod end. Boat

mounted nets are widely used in Spanish and Portuguese estuaries (Lara, 1994; McCarthy et

al, 1996). This method uses boat mounted rectangular nets. Hand netting or dip netting of

glass eels/elvers is widely used. Almost the entire River Severn elver catch is realised by

hundreds of fishermen using dip nets near the tidal limits (White & Knights, 1994; Knights et

al, 2001). This method is also used on the River Nalon in Spain (Lara, 1994; McCarthy et al,

1994a). Dip nets are also widely used for catching A. rostrata elvers in the Scotia-Fundy area

of Canada (Jessop, 1997), and A. australis elvers in New Zealand (Jellyman, 1977; Beumer &


__________________________________________________________________________________________


Harrington, 1980). A typical net design consists of a fine nylon mesh (1-3mm) mounted on a

c.50cm diameter ‘D’ shaped frame and a c.2m handle. Nets are operated from the bank or

from a boat moored close to the shore.

Plate 11 Conical nets on the River Ratty (Owengarney).

During the period 1997-1999, a pilot-scale glass fishery was operated in the Shannon

estuary. In this chapter the results of observations on this experimental fishing are

presented. Glass eel catches are examined in relation to spatial, temporal and

environmental factors and the feasibility of collecting glass eels for subsequent overland

transport and restocking of the Shannon lakes is assessed.

3.2 Fishing sites

A detailed description of the Shannon estuary and its tributaries is given in chapter 2.

Fishing for glass eels took place on six of the estuarine tributaries. The glass eel fishing sites

are given in figure 3.1. On the northern side of the estuary the main glass eel fishing sites

were located on the lower reaches of the Rivers Rine which is a lower tributary of the River

Fergus, and Owengarney at Bunratty. The River Fergus and Meelick creek were also

occasionally fished. The main glass eel fishing sites on the southern side of the estuary were


__________________________________________________________________________________________


located on the River Maigue and on Ballinacurra creek. Some glass eel fishing was also

carried out on the Rivers Deel and Feale.

3.3 Materials and Methods

During 1997, fishing for glass eels was carried out using conical shaped nets. These nets

were constructed from 1-3 mm nylon mesh and consisted of a removable cod end made

from 1mm mesh. The nets had a diameter of 1.5m and a total length of 2.5m. During 1998,

fishing was carried out using the conical nets described above, and also using 1m diameter

and 2m diameter rectangular shaped nets. These nets were constructed from 3 mm mesh

and consisted of cod ends made from 1mm mesh. The large rectangular nets had a total

length of 3.5m, while the smaller nets had a total length of 2.5m. Experimental nets with

larger mesh sizes were also used occasionally. During 1999, fishing was carried out with the

nets used in 1997 and 1998 and also with ‘Maine’ type glass eel fyke nets. The Maine fyke

nets used were 16m wide, 2 m deep and constructed from 3mm mesh. During the study

(and in 1996) experimental fishing was also carried out with dip nets and boat mounted nets

to obtain samples of glass eels (see figure 5).

Plate 12 Dip netting for elvers on the River Inagh at Ennistymon Falls. The vast majority of elvers captured at this site were captured this way.


__________________________________________________________________________________________


Figure 3.1 Glass eel fishing locations utilsed in the current study. Additonal areas where

experiemental netting was undertaken, or were visited and considered suitable for such

work, are also indicated.


__________________________________________________________________________________________


Plate 13 Conical nets on Ballinacurra Creek, Limerick City.

Plate 14 Experimental use of a Tela net on the Rine estuary, Co Clare.


__________________________________________________________________________________________


Plate 15 Example of one of the conical nets used during the current study.

Plate 16 Conical nets with different mesh sizes were evaluated duing the current investigations.


__________________________________________________________________________________________


Plate 17 Boat mounted glass eel net on the River Shannon (Long Shore).

Plate 18 Another boat mounted glass eel design being tried on the River Maigue Estuary. The conical and rectangular shaped nets were set from a bridge or boat and fyke nets were set from a bridge or boat and were anchored to fixed points on the riverbank, or to a bridge.


__________________________________________________________________________________________


All nets were set so that they faced into the incoming tide, and thus filtered the water

flowing inshore/upstream on the tide. The nets were generally set 3-4 hours before high

water time (HWT) and usually emptied once per hour. Fishing was generally carried out

until 40 minutes before the HWT. Glass eels within the catch were separated from by-catch

and debris using a series of graders of decreasing mesh size through which the glass eel

actively descended to a storage container. The catch of glass eels in each net was estimated,

and the total catch from each fishing session was weighed on an electric balance. The

numbers of fish captured as bycatch were also recorded, and all bycatch was released after

enumeration (see chapter 9). Samples of glass eels were retained for laboratory

examination (see chapter 5).

During 1997, glass eels were transported upstream in Styrofoam boxes within hours of

capture and scatter stocked in the littoral zones of Lough Derg. During 1998 and 1999, eels

were transported to holding tanks based at Ardnacrusha generating station for subsequent

transport upstream within 48 hours in a custom made tank.

3.4 Results

In table 3.1, summary results for the three study years are presented. During the 1996/97

season, a total of 424 hours fishing by the three (two-person) fishing crews over 152

sessions resulted in the capture of 480kg of glass eels at four principal estuarine sites (R.

Rine, R. Owengarney, R. Maigue and Ballinacurra creek). An additional 127kg was captured

using conical nets over three consecutive days in the tailrace of Ardnacrusha generation

station and is not included in the analyses. During 1998 a total of 416 hours fishing over 136

sessions resulted in the capture of 396Kgs of glass eels. During 1999 a total of 347 hours

fishing over 125 sessions resulted in the capture of 468Kgs of glass eels. Four fishing crews,

who fished the four principal estuarine sites, and Meelick Creek, obtained the catches

during the 1998 and 1999 seasons.

Details of the net utilisation for each year by month are given in table 3.2. A summary of

fishing session length (time) is presented in table 3.3. A summary of glass eel catches and

CPUE (g/net/hour) for the three study years is presented in figure 3.2. A monthly summary

of glass eel catch and CPUE is given for each year in tables 3.4 and 3.5 respectively. Only a

small number (n=3) of test fishing sessions were carried out during December. Significant

numbers of fishing sessions were carried out during the months of January, February, March

and April each year. The mean number of hours per fishing session during January-April

each year was in the range of 3-5.2 hours per session.


__________________________________________________________________________________________


In figures 3.3, 3.4 and 3.5 the relationships between total glass eel catch, fishing effort (no.

of crew fishing sessions per 24hr period) and mean daily CPUE are presented. It can be seen

that the total catch of glass eels per day increased with the number of fishing sessions per

day for 1997, 1998 and 1998 data. The total catch was also seen to increase with the total

number of fishing hours per day for each year. A positive relationship between CPUE and

daily total glass eel catch for each of the study years was identified. There was a positive

relationship between the number of fishing sessions per day and total daily fishing time for

each year reflecting the relatively consistent extent of individual sessions. Increased mean

daily CPUE was also related to both the increased total number of crew fishing sessions and

fishing hours per day for 1997 and 1998. The relationships between catch/CPUE and effort

can be explained to some extent by the fact that fishing crews fished with significantly

greater effort during times of increased glass eel activity. The relationships are different

during 1999 due to the use of fyke nets in this year.

The relative monthly distribution of glass eel catch for each study year is given in figure 3.6.

In figure 3.7, the percentage contribution of glass eel catch from each fishing location is

presented. The fishing effort (net nights) at each location is also shown in figure 3.7. The

spatial variation in CPUE for each season is given in figure 3.8. A summary of glass eel catch,

CPUE and summary values of selected environmental parameters for each study month are

given in table 3.6. The variation in daily catches and CPUE (g/net/hr) for the north and south

estuarine sites for 1997, 1998 and 1999 are illustrated in figures 3.9, 3.10 and 3.11

respectively. The variation in selected environmental parameters (tidal height, water

temperature at Parteen, max air temperatures, rainfall, wind speed and discharge from

Ardnacrusha and Parteen) is also shown for the same seasons. The extent of the fishing

season (January – April) and the general relationships of fishing activity/yield and of CPUE to

the semidiurnal tidal cycles are evident. The end of the glass eel fishing in April coincided

with the upstream migration of elvers, as evidenced by catches in the traps at Ardnacrusha

and other sites. River temperatures exceeded 10oC at this stage. In figure 3.12, the

relationship between glass eel CPUE and selected environmental parameters is shown.

A Principal Components Analysis (PCA) of environmental data for each year was undertaken.

The first three PCA axes accounted for 67.1%, 64.7% and 67.3% of the variance during the

years 1997, 1997 and 1999 respectively. Component derived for 1997, 1998 and 1999 data

are plotted in figures 3.13, 3.14 and 3.15 respectively. The relationship of glass eel CPUE to

the environmental conditions described by the PCA axes was investigated for each season

by means of Spearman Rank correlation analyses.


__________________________________________________________________________________________


Plate 19 Glass eels from the Shannon estuary.

Analyses of 1997 data concluded that mean daily CPUE recorded at all sites combined was

significantly inversely correlated with component 1 (p=0.019, R2 =-0.31). Results also

showed that the CPUE recorded at the sites located at the northern side of the estuary had

a highly significant inverse association (p=0.01, R2 =-0.36) with component 1, reflecting the

relationship to day number, water and air temperatures and wind direction, and axis 3

(p=0.0002, R2=-0.52), reflecting the role of tidal height. The mean daily CPUE recorded at the

sites on the southern side of the estuary was significantly correlated (p=0.02, R2 =-0.32) with

axis 3 only. The influence of these environmental factors on catches and CPUE is also

suggested by the variation in individual parameters as illustrated in figure 3.9. A sudden rise

in air temperatures in January, with the River Shannon water temperatures rising to more

than 5oC coincided with an initial peak in CPUE at some sites (figure 3.9). Prevailing

southerly winds during the sampling period may have resulted in its generally higher CPUE

values and catches at the fishing sites on the North side of the Shannon estuary. Especially

strong winds in February seem to have a similar effect.

Analyses of 1998 data showed that mean daily CPUE recorded was highly significantly

negatively associated with component 2 (p=0.005, R2 =-0.36). This reflected the relationship

between CPUE and air temperature and wind speed. The same association was found for


__________________________________________________________________________________________


both north and south sites when analysed separately (p=0001, R2 =-0.5 and p=0.03, R2 =-

0.37 respectively).

The CPUE data recorded during 1999 was highly significantly correlated with component 1

(p=0.03, R2 =0.31). This reflected the relationship between CPUE, day number, water

temperature and air temperature. The CPUE data recorded was also found to have a highly

significant negative association with component 2 (p=0.001, R2 =-0.45). This mainly reflected

the relationship between CPUE and wind speed. CPUE data from the north sites alone was

found to be similarly associated with component 1 (p=0.01, R2 =0.36), and component 2

(p=0005, R2 =-0.41). CPUE data from the south sites was also associated with component 1

(p=0.02, R2 =0.35), and Component 2 (p=0005, R2 =-0.53). The CPUE data for fyke nets alone

was not correlated with any components from the PCA. When the fyke net data was

excluded from the analyses it was found that CPUE was correlated with component 2 only

(p=0.01) for combined data and with component 2 and component 3 for CPUE data from the

north sites (p=0.008, R2 =-0.47 and p=0.01, R-sq.=0.43 respectively). Component 3 primarily

reflected the role of wind direction. The CPUE data for the south sites (fyke net data

excluded) was correlated with component 2 only (p=0.02, -0.431).

Fishing crews noted that significant greater catches were made when fishing took place

during hours of darkness. The diel pattern of glass eel activity as indicated by CPUE recorded

is given in figure 3.16 for each year and figure 3.17 for each month. The highest CPUE values

were recorded in the morning tides, and were generally seen to decline in later tides. In

January and February, higher maximum CPUE values were recorded in the afternoon tides.

These tides were generally under darkness during these months. During March, the highest

CPUE values were recorded during the morning tides and CPUE was seen to decline for later

tides. In April a similar trend for morning tide catches is apparent. The high CPUE for

afternoon tides reflected significant catches made on evening tides with fyke nets on the

upper Owenogarney River during 1999. In figure 3.18, the glass eel catch in nets spaced

across the rivers Rine and Owenogarney are presented. On the River Rine significantly

greater catches were made in the nets located on the right side of the river. Tidal flows were

greater on this side of the river. A similar pattern was recorded on the River Owenogarney

where tidal flows and catches were greater on the left side of the river.

In figure 3.19, the CPUE of rectangular nets with large and small mesh sizes are compared.

During the three fishing sessions, the CPUE of the smaller mesh (3mm) was significantly

higher than that of the larger mesh size (9mm). In table 3.7, the mean glass eel CPUE for 7

different gear types used during 1999 are compared. In figure 3.20 the mean glass eel CPUE

(+/- 95% C.I.) are also illustrated. The fyke net CPUE was significantly higher that all other


__________________________________________________________________________________________


net types used. Numerous small fish were recorded as bycatch during the experimental

glass eel fishing (see chapter 9). The morphometrics and development stages of glass eels

captured are discussed in chapter 5.

3.5 Discussion

The general patterns of glass eel activity recorded in the Shannon are similar to those

reported previously for this and other Irish locations (McGovern & McCarthy, 1992;

Reynolds et al 1994). Glass eels have been recorded arriving on Irish shores from November

onwards (McGovern & McCarthy, 1992) and this also appears to be the case in the Shannon

estuary. Donnelly et al (1996) reported that the main movement of glass eels in the

Shannon estuary occurs at the beginning of April. However, the current three year study

found that the main movement occurred during the months of February and March.

Plate 20 Conical nets on the tributary of the Shannon estuary, April 1997.

The immigration of glass eels into river systems is understood to be a two-stage process

(Tesh, 1977, Gascuel, 1986, White & Knights, 1997a). Firstly, glass eels migrate upstream

into the estuary proper by using the tidal transport mechanisms (McCleave & Klecker, 1982)

with the abundances following a symmetrical curve. This pattern is due to the passage of a

wave of migrants. The estuarine hydrology then leads to a natural trapping of migrants in

the upper oligohaline zone with the abundances following an asymmetrical curve describing


__________________________________________________________________________________________


the progressive accumulation of eels. Secondly, when a suitable water temperature

threshold is reached, the glass eels swim actively upstream. The current chapter deals with

the first stage described above as all fishing took place below the tidal head.

The abundance of glass eels at the tidal head is determined by both the initial extent of

recruitment to the estuary from the Sargasso Sea, and the subsequent survival of glass eels

in the estuary. Glass eels require the occurrence of suitable environmental conditions to

reach the tidal head. Low initial recruitment to the estuary and/or unfavorable

environmental conditions in the estuary during this migration phase will result in poor

recruitment to the tidal head. The anticipatory ecophysiology of glass eels may make them

particularly vulnerable to abnormal environmental conditions (Fontaine, 1985). The

potential impact of local climatic and environmental variations has direct consequences for

both glass eel harvesting, and the determination of recruitment abundance indices.

Investigations of the effects of environmental factors on glass eels activity in the Shannon

estuary, as evidenced by their catchability in the pilot scale glass eel fishery, suggest that

tidal height, temperature, discharge and to a lesser extent factors such as wind influence

the swimming behavior and vertical distribution of glass eels in the water column. Wind is

thought to affect glass eel catches through its influence on water movements and tidal

height (McCleave & Klecker, 1982). River discharge can also affect tidal movements. Sheldon

& McCleave (1985) reported that glass eel abundance was greater on flood tides than on

ebb tides and at all stages of the tide glass eel density at the surface was greater near shore

than in mid-channel and was greater at a more landward station than at a more seaward

station. In the current study fishing took place in relatively small estuaries and glass eel

abundances were higher in areas of greater tidal flow. The highest glass eel CPUEs were

obtained in areas closer to the tidal head (i.e. the Upper Owenogarney estuary).

The role of tidal currents and temperature on glass eel and elver activity has been widely

reported (Deelder, 1970, Tesch, 1977, Sheldon & McCleave, 1985; Desaunay et al, 1987;

Ciccotti et al 1995). Egginton & Johnston (1984) studied the respiratory responses of

European elvers to acute and chronic temperature change. Their results indicated that

elvers are only capable of limited adjustments in metabolic rate, which may account for

their activity patterns at different temperatures. Glass eels/elvers are virtually unable to

make any progress against flows greater than 50cm s-1 (McCleave, 1980, Barbin, & Krueger,

1994). This means that they would not be able to ascend many estuaries during ebb tides.

Sheldon & McCleave (1985) reported that glass eel abundances in the water column quickly

increase at the start of a flood tide. McGovern and McCarthy (1992) noted that elver ascent

in the River Corrib system occurred when temperatures were greater than 11oC and it is

generally accepted that temperatures in this range are required for active upstream


__________________________________________________________________________________________


migration. In the present study glass eel catches, though low, were obtained when water

temperatures were in the 3-4oC range. It is clear that the role of temperature is less

important in the initiation of tidally assisted transport than the initiation of active upstream

migration. The extent of the glass eel fishing season in Ireland may ultimately be decided by

climatic determinants of water temperature. In the present study the declining CPUE values

observed in the latter half of April coincided with the rise of water temperatures to more

than 10oC, and the peak catches of elvers at traps located near the tidal head (see chapter

4). Although environmental conditions in the estuarine fishing sites were favorable for glass

eel at that time, the downward catches and poor CPUE’s recorded in the latter half of April

each year clearly reflected the fact that the bulk of the glass eel population had, by that

time, moved into freshwater.

The variability of glass eel activity and catchability, as seen in respect of diel and tidal/semi-

lunar cycles, may involve endogenous rhythms that could be affected by factors not

considered in the present study. However, the relative predictability of their movements is

of considerable significance to commercial fishermen. The favorable geographical location

of Ireland in respect of transatlantic eel larval drift has not has been generally recognised.

However, as evidenced by the high densities of eels in western Irish river systems e.g. the

River Dunkellin (Callaghan and McCarthy, 1992) good natural recruitment has been a

feature of some Irish eel populations despite the downward trend recorded throughout

Europe. The results of the pilot scale glass eel fishery indicate that it is currently feasible to

obtain the juvenile eel stocking requirements necessary for long term conservation and

development of eel stocks of the system from the Shannon region. The future of Irish eel

fisheries and the eel aquaculture industry will be dependent on adequate supplies of

juvenile eels. Development of glass eel fishing and elver trapping in Ireland is currently

restricted by legislative obstacles, and the lack of an overall integrated national policy.

Despite the downturn, it is clear that significant stocks of glass eels continue to occur in Irish

and other European estuaries. The current study confirmed this for the Shannon Region

during 1997, 1998 and 1999 and Mathews et al (2001) reported that substantial quantities

of glass eels occurred in the estuary of the Erne during 1999 and 2000. The glass eel CPUE

values recorded in the Shannon and the annual catches per fisherman compare generally

with those recorded in Spanish, Portuguese and British glass eel fisheries (Moriarty &

Dekker, 1997) and provide support for proposals recently made in respect of the

development of the Irish eel industry (McCarthy et al, 1994a). The current study has shown

the potential of methods such as the Maine Fyke net, and to a lesser extent, the conical net,

for use in the Shannon estuary area. Fyke nets are most suitable to the upper estuarine

areas, and worked particularly well on the Owenogarney estuary. However, many of the

experimental methods used in the current study (i.e. large mesh rectangular nets) are not


__________________________________________________________________________________________


recommended for future commercial fisheries. Tela nets, which were not available to the

current study, are likely to be highly effective in the Shannon estuary. The current study was

focused mainly on sites convenient to Limerick City. Significant numbers of sites suitable for

glass eel fishing are present in the Shannon estuary, however only a small number of these

sites could be fished during the current study. The location of sites investigated during the

current study and considered to be physical suitable for glass eel fishing are detailed in

figure 3.21.

Plate 21 Conical nets on Ballinacurra Creek, Limerick City.

In the current study an annual catch of 400-500kg was maintained over the three years

using three fishing crews fishing operating a relatively small number of nets at a limited

number of sites. Increased fishing effort with optimal netting methods (Tela and Maine Fyke

nets) would be expected to provide significantly increased catches. Although it is difficult to

extrapolate from the results obtained from the current study, it is clear that optimal

harvesting of glass eels on the Shannon and in other Irish estuaries could yield nationally

important catches with significant part-time employment for fishermen. On growing of such

quantities of juvenile eels in wild fisheries or aquaculture systems, together with value

added processing could be of economic significance.


__________________________________________________________________________________________


anguilla anguilla, l) in the shannon estuary, ireland ... - wordpress…€¦ · harrington, 1980)....

Documents