the distribution and abundance of dugong and … · department of environment and natu ral...

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DEPARTMENT OF

ENVIRONMENT AND NATURAL RESOURCES

The distribution and

abundance of Dugong and

other marine megafauna in

the Northern Territory

November 2015

ii

Disclaimer: To the extent permitted by law, NT Government (including its employees)

excludes all liability to any person for any consequences, including but not limited to

all losses, damages, costs, expenses and any other compensation, arising directly or

indirectly from using this publication (in part or in whole) and any information or

material contained in it.

ISBN: 978-1-74-350120-7

Cite report as:

Groom RA, Dunshea GJ, Griffiths AD, and Mackarous K (2017). The distribution and

abundance of Dugong and other marine megafauna in Northern Territory, November 2015.

Department of Environment and Natural Resources, Darwin.

iii

Executive summary

The northern Australian coast is a region of national and international significance for

Dugongs. The distribution and abundance of Dugong along the coast varies and is generally

associated with extensive seagrass and algal habitats. The Northern Territory supports a

moderate population compared with the Torres Strait, which is the largest global population

with approximately 16,000 animals (± 3000) (Sobtzick et al. 2014).

Aerial surveys are an established method of collecting broad scale distribution and

abundance data for Dugongs. We conducted an aerial survey based on the established strip-

width transect methodology (Marsh and Sinclair 1989). The method accounts for biases in

Dugong counts caused by different detection probabilities among those observed (perception

bias) and the fact that not all Dugongs are near the water’s surface, so are unavailable to be

seen (availability bias). Sampling intensity of survey blocks ranged from 5 - 9% of the survey

area. The Dugong aerial survey occurred during October/November 2015 and surveyed over

93,145 km2 of the Northern Territory (up to 5.5 km seaward from the coast) and

Commonwealth’s coastal waters. This represents the first broad scale survey conducted in

20 years and the fifth aerial survey of Dugongs by the Northern Territory government in the

Gulf of Carpentaria.

We sighted a total of 194 Dugongs within transects, including 26 calves (13.4%). Mean group

size was 1.4 and maximum group size was six Dugongs. With perception and availability

correction factors applied, the population estimate was 8,176 (± 958) Dugongs. The Sir

Edward Pellew Island Group and Limmen Bight had the highest population estimates,

consistent with results from 2007 and 2014. The mean density of Dugongs across the survey

area was generally low at 0.13 Dugong km-2 and ranged from 0 – 0.85 Dugong km-2. The

density of Dugong for the Gulf of Carpentaria has not changed significantly across three

survey periods (2007, 2014 and 2015).

In addition to Dugongs, we recorded other marine megafauna during the survey. Marine

turtles were the most abundant animal recorded with 1,854 turtles observed on transect, they

were not identified to species level except for the leatherback turtle, which is distinctly larger

and darker than other species. Dolphins were abundant across the survey area with 1,497

individuals on transect. Five species of dolphin were recorded, although most observations

(1189 individuals) could not be identified to species. Of the dolphins that were identified to

species, the Australian Snubfin Dolphin (Orcaella heinshoni) was the most frequently

observed. Larger cetaceans including the False Killer Whales (Pseudorca crassidens) and

iv

Humpback Whales (Megaptera novangliae) were also recorded on survey, as well as other

marine megafauna including Whale Sharks (Rhincodon typus) and Manta Rays (Manta

birostris/alfredi).

This broad scale survey provides a regional context for the significant marine biodiversity

values of the Northern Territory coast. The high relative abundance of many marine

megafauna species, particularly coastal dolphins and turtles suggest that the Northern

Territory has some of the most important populations in Australia and the Asia-Pacific region.

These data will inform future research priorities for Dugong, marine turtles and cetaceans

and contribute towards improved management of these important species.

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Contents

Executive summary iii

1. Introduction 1

1.1 Background 1

2. Methods 3

2.1 Study area 3

2.2 Study design 4

2.3 Data analysis 6

2.3.1 Dugong population size and density 6

2.3.2 Availability and perception bias 7

2.3.2 Quality assurance 7

3 Results 9

3.1 Environmental Conditions 9

3.2 Dugong sightings 10

3.2.1 Population size estimate trends 10

3.3 Observations of other marine megafauna 14

4 Discussion 17

5 Acknowledgments 20

6 References 21

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List of Figures

Figure 1 a) Flying height of 152 m relative to transect strip width of 200 m. b) Transect

markers fitted to the outside of the plane to demarcate transect width and zones within

transect (Sobtzick et al. 2013b). ............................................................................................. 4

Figure 2 Location of 2015 Dugong aerial survey transects and survey blocks, Northern

Territory. ............................................................................................................................. 8

Figure 3 Comparison of relative densities between Dugongs, inshore dolphins and turtles

observed on aerial survey along the NT coast with detection and perception correction

factors applied to Dugong and perception corrections factors applied to turtles and inshore

dolphins ........................................................................................................................... 16

List of Tables

Table 1 Water visibility categories used in the survey, based on Pollock et al. (2006). ..... 6

Table 2 Categories of environmental conditions (Pollock et al. 2006) and the mean

proportions experienced during the Dugong aerial survey .................................................... 10

Table 3 Model of best fit from both aerial survey planes (all observers different) with

perception probability for each team on each side of the aircraft. ......................................... 11

Table 4 Summary of estimated Dugong abundance (± s.e.) using Pollock et al. (2006)

and Marsh & Sinclair (1989a) methods for the 2015 Dugong aerial survey. .......................... 12

Table 5 Comparison of Dugong population estimates (Pollock et al. 2006) across aerial

survey years in 2007, 2014 and 2015 ................................................................................... 13

Table 6 Estimated densities (km-2) of Dugong in the Gulf of Carpentaria, 2007, 2014 and

2015 based on Pollock et al. (2006) population abundance estimates .................................. 13

Table 7 Megafauna sightings other than Dugongs (dolphins, turtles, manta rays, whale

sharks and humpback whales) during the 2015 aerial survey. The number in parenthesis

represents animals observed outside the strip transect area on survey ................................ 15

Appendices

Appendix 1 Overview of survey flights undertaken in the Gulf of Carpentaria ................. 24

Appendix 1.1 Raw data sightings ...................................................................................... 26

Appendix 1.2 Details of tested models perception correction ............................................. 27

Appendix 2 Geo-referenced survey sightings ................................................................. 28

Appendix 2.1 Dugong sightings ......................................................................................... 29

Appendix 2.2 Dolphin sightings .......................................................................................... 30

Appendix 2.3 Turtle sightings ............................................................................................ 31

Appendix 2.4 Sea snake and manta ray sightings ............................................................. 32

Introduction

1

1. Introduction

Background

The Dugong, Dugong dugon, is a species of high cultural and conservation significance in

Australia and many other coastal regions globally. It is listed as Vulnerable to extinction by

the International Union for Conservation (IUCN 2015), a Migratory/Marine species under the

Commonwealth’s Environment Protection and Biodiversity Conservation Act and as Near

Threatened under the Northern Territory’s Territory Parks and Wildlife Conservation Act. The

Dugong has an extensive range spanning some 40 countries in Africa, the middle east, Asia

and Oceania including tropical and subtropical coastal waters, between about 26° and 27°

north and south of the equator (Nishiwaki and Marsh 1985). Dugong numbers are generally

declining throughout most of their range (Marsh et al. 2002) where they are subject to a

range of natural and human-related pressures, including entanglement in shark and fishing

nets (e.g. mesh and gill nets) (Marsh et al. 2005), marine debris (Perrin et al. 1996, Coates

2002, Marsh et al. 2002), loss and degradation of seagrass habitat (Preen and Marsh 1995,

Haynes et al. 2005, Marsh et al. 2011), unsustainable hunting (Marsh et al. 2002) and

collisions with boats (Groom et al. 2004, Marsh et al. 2004, Grech and Marsh 2008, Hughes

et al. 2008).

Monitoring population trends provides insight into population health and potential significant

impacts. Surveying for Dugongs in coastal waters of northern Australia is difficult due to

heterogeneous environmental conditions and Dugong diving behaviour, in particular

detection of Dugongs is limited by highly turbid or deep water (Pollock et al. 2006). Aerial

surveys using standardised techniques developed by Marsh and Sinclair (1989) have

provided much of the information used to manage Dugongs in Australia. The aerial survey

data analysis was improved by Pollock et al. (2006) to account for the spatial heterogeneity

in Dugong availability bias (related to environmental conditions on survey).

In the Northern Territory, Dugong aerial surveys have previously been undertaken by the

Northern Territory government and collaborators in the Gulf of Carpentaria in 1984-1985,

1997,2007 and 2014 (Bayliss and Freeland 1989, Parks and Wildlife Service 2003, Marsh et

al. 2008, Groom et al. 2015). These surveys have indicated that a significant Dugong

population exists in the Northern Territory with recent population estimates over an area

comparable to previous surveys ranging from 4786 (±1101) – 5784 (±767) Dugongs (Marsh

et al. 2008, Groom et al. 2015). The relative density of Dugong within Limmen Bight to Sir

Edward Pellew Islands and Blue Mud Bay ranks within the top eight Dugong density areas in

Introduction

2

Australia (Environment 2016). In the western region of the Northern Territory the distribution

and abundance of Dugongs is smaller and more variable than the Gulf of Carpentaria (Parks

and Wildlife Service 2003). More recently, Dugong aerial surveys have been undertaken over

the Darwin-Bynoe Harbour region as part of an INPEX-led Ichthys LNG Project Nearshore

Environmental Monitoring Plan (Cardno 2015b). This program used methods comparable to

those in other Dugong aerial surveys with a greater intensity to try to detect change in

Dugong populations.

The primary objective of this Dugong aerial survey was to improve current knowledge of

Dugong abundance and distribution in Northern Territory coastal waters and compare this

with historical survey efforts.

Methods

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2. Methods

2.1 Study area

The aerial survey was conducted along the entire Northern Territory coastline which is

extensive (10,953 km) and shallow (<70 m), with the boundaries defined by the Gulf of

Carpentaria (139° longitude east) in Queensland and the Joseph Bonaparte Gulf in the west

(129° east longitude). The region is dynamic and characterised by monsoonal seasonality in

temperature, salinity, rainfall and wind regimes with the wet season occurring during

December to April and the dry season outside of this period. Tidal regimes in the survey area

vary from micro (tidal range: ~2 m) in the western Gulf of Carpentaria to macro in the

Bonaparte Gulf and Darwin (tidal range: ~8 m) (Duke 2006) with low wave energy allowing

for seagrass and mangrove communities to establish. The majority of the seagrass

communities occur along open coastlines that are characterised by depth-zoned species

distributions (intertidal and sub tidal) (Poiner et al. 1987), and are considered important

feeding areas for Dugongs (Parks and Wildlife Service 2003). Seagrass species in northern

Australia are known to be highly dynamic with respect to seasonality and annual variability

(Rasheed et al. 2008, Cardno 2015a). Northern Australia seagrasses are usually at

maximum biomass in the dry season and lowest in the wet season (Unsworth et al 2010).

Such seasonality is similar to patterns observed throughout the Indo-Pacific (Erfteheijer and

Herman 1994; McKenzie 1994; Rasheed 2004, Unsworth et al 2010).

Within the survey area, two distinct regions were sampled: a turbid near shore zone to a

depth of 15 - 20 m and deeper waters separated from the coastal zone by a boundary current

(Wolanski and Ridd 1990). The Dugong aerial survey was undertaken between October 9

and November 3, 2015. This maximises survey efficiency due to the low-wind period

(October-November) and being prior to heavy monsoonal rains that increase turbidity and

sea state conditions (Godfred-Spenning and Reason 2002). The total area surveyed was

approximately 93,145 km2 along the Northern Territory coastline with a sampling intensity

(the proportion of each block within transect strip-widths) between blocks ranging from 5 - 9

% (Figure 2). The survey area was generally over water less than 45 m deep and mostly less

than 15 m deep. The deepest waters in the survey area were approximately 60 – 88 m

occurring between the Cobourg Peninsula mainland and the eastern coast of Melville Island.

The dates that transects were flown are presented in Appendix 1.

Methods

4

2.2 Study design

Aerial survey methods were based on previously described strip-width dugong aerial survey

methods (Marsh & Sinclair 1989b; Pollock et. al. 2006). Two, six-seater, high-wing twin-

engine Partenavia 68B’s were flown along predetermined transects (5 km apart), at an

altitude of approximately 152 m (Figure 1a and 1b) and as close as possible to 100 knots

groundspeed. One plane started at the extreme east of the survey area (Northern Territory

border with Queensland) and the other at the extreme west (Northern Territory border with

Western Australia), meeting around the middle of the northern coastline at the completion of

the survey (east of Maningrida). The survey team in each aircraft consisted of a dual

observer team on each side of the aircraft and a team leader positioned next to the pilot. All

fauna observers were experienced in field identification of the marine mammal species in

northern Australia, with at least one member of each dual observer team (the mid-seat

observer) experienced (>45 - 200 hours on effort) in marine megafauna aerial surveys,

and/or dugong surveys, in northern Australia. All survey participants undertook a training

workshop and trial survey flight before the commencement of the survey.

Figure 1 a) Flying height of 152 m relative to transect strip width of 200 m. b) Transect

markers fitted to the outside of the plane to demarcate transect width and zones within

transect (Sobtzick et al. 2013b).

Dual observer teams on both sides of the aircraft scanned the water surface in a 200 m wide

strip, marked on either side using fiberglass rods attached to a thin artificial wing strut. Within

the transect, four 50 m transect zones were marked by colored tape (from the closest to the

most far away from the plane: the low, medium, high and very high zones), so the observers

could record the approximate lateral location of sightings in relation to the plane, which aids

in identifying recapture sightings within dual observer teams (i.e. distance zones sensu

Pollock et al 2006). Fauna observation data were recorded orally on separate audio tracks

for each team member (TASCAM DR-680), when the sighting was perpendicular to the

Methods

5

observer in relation to the transect (where possible). The team leader and observers all wore

high-noise environment headsets consisting of earphones and microphones (Beyer Dynamic,

DT 797 LTD) and all fauna observers were acoustically isolated but could communicate with

the team leader (using a custom signal splitter from the TASCAM DR-680). Observers in dual

teams were also visually separated by a curtain between mid- and rear-seats.

At the start of each flight, two handheld GPS units were activated, which stored the time and

aircraft position every second throughout the survey flight. Prior to commencement of the first

transect for each flight, the audio recording device was started at an exact time on the GPS

and the recording starting time noted, such that the time of an observation recording could be

referenced by the GPS position of the aircraft when the observation was made. During each

transect, observers scanned within the 200 m strip and recorded observations of the

following ‘datatypes’: dugongs, dolphins, turtles, stingrays, manta rays, whales and Whale

Sharks. For each observation, the datatype, species (if known/applicable), transect zone,

group size, number at the surface, the number of calves in dugong and dolphin groups

(Defined as <2/3 the size of adults and swimming in close proximity) and the ‘water clarity’

category was recorded. Water clarity categories were defined as per Pollock et al. (2006) and

Sobtzich et al. (2015) where category 1 = Clear water, bottom clearly visible; 2 = Variable

water quality, bottom visible but unclear; 3 = Clear water, bottom not visible; 4 = Turbid

water, bottom not visible. The level of confidence associated with the datatype and species

of all sightings was also recorded (certain, probable or guess), with the default being a

certain sighting, such that observers only needed to indicate a confidence level if they were

uncertain. The mid-seat observer also recorded the glare category (0 = none of field of view

in the transect affected, 1 = 1-25% of field of view in the transect affected, 2 = 25-50%

affected, 3 = >50% affected) at the start of the transect and updated it if there was any

change. All surveys were conducted in passing mode (i.e. continual flying along transect),

however if observers noted a group size of around 10 dugongs, the plane broke off the

transect and tried to circle the sighting area in an effort to gain an accurate count of the entire

group (this occurred once, outside of the transect).

The team leader recorded broad environmental conditions in the region at the start of each

survey flight gathered from air-traffic control and observation (cloud cover and height, wind

speed and direction, visibility) (visibility scale reference, Table 1). All environmental data was

entered inflight via a small laptop and custom database with single key short-cuts facilitating

fast data entry. Records were time-stamped according to the computer clock, which was

synchronized with GPS time. For each transect the start and end time was recorded and at

the start the cloud cover (in octas), Beaufort sea state (BSS) and starboard-side water clarity

Methods

6

were recorded, as well as glare on each side of the aircraft (from the mid-seat observers),

with subsequent update of these fields throughout the transect when they changed.

Following each survey, dual observer teams jointly viewed and listened to the both audio files

from each observer simultaneously using audio signal visualization software (Audacity 2.1.2,

www.audacityteam.org). All sighting data from both observers (excepting single-observer

sting ray sightings) was entered into a custom database. Recapture sightings were identified

by their proximity in observation time (within 6 seconds, usually <= 2 s) and the position in

the transect (the same, or adjoining distance zones). Recapture sightings were linked within

the database by a field referencing the paired record. This design of data management

facilitated easy recognition of single platform- and dual- (i.e., recapture) sightings, while also

allowing analysis of discrepancies in data collection within and between dual observer teams.

Following completion of the survey, all data sources (survey metadata, time-stamped GPS

data, time-stamped environmental data and time-stamped fauna observations) were

imported into a custom database and cross-referenced.

Table 1 Water visibility categories used in the survey, based on Pollock et al. (2006).

Visibility category Visibility of sea floor Water quality

1 Clearly visible Clear

2 Visible but unclear Opaque

3 Not visible Clear

4 Not visible Opaque

Where calves were present, these were recorded for Dugong and dolphins and were defined

as being less than two-thirds of the size of the cow and swimming in relatively closer

proximity to an adult. All animals were recorded when flying along transects including those

that were not within the demarcated transect strip. Outside of this strip, animals were called

‘inside’ (below) or ‘outside’ (above) the transect.

2.3 Data analysis

2.3.1 Dugong population size and density

Survey blocks were demarcated based on historical survey blocks or the broad scale

distribution of sightings. The data from each survey block were analysed to estimate Dugong

population size and density, following the methods developed by Marsh & Sinclair (1989) and

Pollock et al. (2006). These methods aim to correct for perception bias (animals visible in the

survey transect but missed by observers) and availability bias (animals not available to

Methods

7

observers because of water visibility/turbidity). All population estimates are provided with one

standard error (± s.e.).

2.3.2 Availability and perception bias

Availability Correction Factors (ACFs) were calculated following Marsh & Sinclair (1989). For

Dugongs, the proportion of sightings at the surface was compared to the proportion of

animals at the surface in Moreton Bay, Queensland where, due to the shallow water depth, it

was assumed that all Dugongs were visible. ACFs for Dugongs for the Marsh & Sinclair

(1989) method apply to the entire survey area. Perception correction factors (PCF) were

derived from a mark-recapture analysis of Dugong group sightings using the two-platform

setup of observers in the plane. For the Marsh & Sinclair (1989) method, a Peterson-mark-

recapture model was used to calculate PCFs (for details see Marsh & Sinclair (1989)). For

the Pollock et al.(2006) method perception bias was estimated by calculating the probability

of detection (Pd) by analysis of individual Dugong recapture sightings among and within

observer teams on the same aircraft. This consisted of comparing four different closed

capture models being tested in the program MARK (White and Burnham 1999): (1) all

observers the same; (2) front and back seat observers the same; (3) port and starboard

observers the same and (4) all observers different, with the best fitting model chosen to

calculate the Pd. Support for the models was assessed using the Akaike Information Criterion

corrected for small sample sizes (AICc). Models for PCFs containing environmental variables

were also tested though were not found to explain the differences in perception bias as well

as “Differences in Observers”, see tables in Appendix 1.2. PCF were also applied to inshore

dolphin and marine turtle observations.

2.3.2 Quality assurance

Data for the aerials surveys were managed and stored on a central computer in the field to

avoid corruption or accidental over-write. Audio data collected during each aerial survey was

backed up and error checked by the Survey Leader before undertaking any analysis or

presentation in a GIS mapping package. All aerial survey transcription files were quality

checked and any comments noted in a customised database at the end of each day. Final

survey data was stored in a customized Microsoft Access database containing all

observation, environmental and GPS data from each day of survey. Before data compilation

and manipulation, all data entries were checked by a second person for transcription errors.

Any amendments or changes to databases were applied where necessary and documented

in a quality control log.

8

Figure 2 Location of 2015 Dugong aerial survey transects and survey blocks, Northern Territory.

9

3 Results

3.1 Environmental Conditions

The weather during the survey period was characteristic of the ‘build up’ period where it was

generally calm with occasional high winds and patchy rain. Higher winds were experienced in

offshore waters adjacent to Groote Eylandt and the Wessel Islands. Glare varied throughout

the survey relative to the direction of flight and sea state, and while glare was recorded

throughout the survey, it is not required for statistical analysis. The proportion of each block

at each combination of Beaufort sea state and visibility were determined for each survey

block as required by the Pollock et al. (2006) analysis. A large proportion of the survey was

conducted in conditions with relatively poor visibility (high turbidity)(Table 2). The

environmental conditions varied slightly between the east and west planes for the duration of

the survey. The eastern plane experienced Beaufort sea state (BSS) <=2 for ≈47% of

cumulative transect distance, BSS 3 for ≈42% and BSS 4 for ≈11%, whereas the western

plane had calmer conditions (BSS <=2 for ≈77% of cumulative transect distance, BSS 3 for

≈21% and BSS >=4 for ≈2%). Most waters surveyed were turbid, where the bottom could not

be discerned (i.e., water clarity ‘4’, ≈79% and ≈89% of cumulative transect distance for east

and west planes, respectively) with <1% of waters for both planes consisting of clear water

with the bottom clearly visible (i.e., water clarity ‘1’). Short sections of some transects were

flown in a Beaufort 4 sea state in the east and Beaufort 5 in the west - generally transects

further offshore, and these were considered sufficiently short and unlikely to be Dugong

habitat, to not warrant repeating. The maximum-recorded wind speed on survey was 15

knots. Cloud cover and glare were also recorded on survey but are not required for the

population analysis.

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Table 2 Categories of environmental conditions (Pollock et al. 2006) and the mean proportions

experienced during the Dugong aerial survey

Category Turbidity

Beaufort Sea

State

Length Surveyed

(km) Mean Proportion

1 1 <=2 127.1 0.02

2 2 <=2 390.8 0.03

3 3 <=2 1105.1 0.06

4 4 <=2 7562.2 0.51

5 1 >2 8.4 0

6 2 >2 126.5 0.01

7 3 >2 625.3 0.04

8 4 >2 4905.1 0.33

3.2 Dugong sightings

A total of 151 Dugong groups, consisting of 229 individuals (including 26 calves) was sighted

on the Northern Territory survey (in and out of transect). The mean group size for the survey

area was 1.4 (SD = 0.7) Dugongs. The largest herd observed comprised eight individuals

(not within transect). Unless otherwise stated, the sightings reported here include those

animals observed within the transect strip.

3.2.1 Population size estimate trends

Based on the closed-capture models, the model of best fit in each aircraft was “all observers

different”. This model indicated that double-observer teams had combined detection

probabilities of 0.63- 0.95. The individual observer and observer team probability estimates

are provided in Table 3.

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Table 3 Model of best fit from both aerial survey planes (all observers different) with perception

probability for each team on each side of the aircraft.

Team and Position Pd Estimates (± s.e.) For

Individual Observers

Pd estimates for each

observer team

WEST

Port Front 0.85 (± 0.06) 0.95

Port Rear 0.65 (± 0.08)

Starboard Front 0.64 (± 0.10) 0.79

Starboard Rear 0.41 (± 0.08)

EAST

Port Front 0.45 (± 0.09) 0.63

Port Rear 0.33 (± 0.07)

Starboard Front 0.82 (± 0.07) 0.92

Starboard Rear 0.54 (± 0.07)

Based on the Pollock et al. (2006) method, the Dugong population estimate for the 2015

survey was 8,176 (± 958) with a precision (s.e./m.%) of 11.7%. The Marsh & Sinclair (1989)

method gave an estimate of 16,007 (± 2465) for the survey area (Table 4). The difference in

abundance between the two analytical methods is significant due to the Marsh & Sinclair

(1989) method referring to “at the surface” observations being approximately 16.7% of

available Dugongs. This availability correction factor was determined in an environment

(Moreton Bay, Queensland) that generally has much lower turbidity than the Northern

Territory coast, this has the effect of multiplying sightings by approximately 4 times as the

proportion of Dugongs observed at the surface is greater in the Northern Territory.

The Dugong abundance estimate for Block 7 was not reliable as there was only one sighting

and an estimate was not determined for block 9 as there were no sightings (western side of

Groote Eylandt).

Three Dugong aerial surveys of the Gulf of Carpentaria have occurred since 2007 over

broadly similar spatial scales and sampling intensities (Table 5). Population estimates have

not differed significantly between these years (Marsh et al. 2008, Groom et al. 2015). Dugong

density estimates (km-2) (Table 6) were used to better account for differences in survey area

and sampling intensity. The density estimates between years have also not changed

significantly.

12

The raw data on sighting conditions, Dugong sightings, group sizes and perception correction

factors are listed in Appendix 1.1. Appendix 1.2 presents maps of the geo-referenced

Dugong sightings.

Table 4 Summary of estimated Dugong abundance (± s.e.) using Pollock et al. (2006) and

Marsh & Sinclair (1989a) methods for the 2015 Dugong aerial survey.

Block Area (km2) Sampling

Intensity

(%)

Dugong

Density

km-2

Mean Group

Size (SD)1

Pollock

Est (± s.e.)

Marsh and

Sinclair Est (±

s.e.)

WEST_1 4769.49 0.07 0.07 1.43 (0.79) 317 (±130) 512 (±255)

WEST_2 3725.03 0.07 0.09 1.67 (1.63) 322 (±119) 520 (±226)

WEST_3 5044.91 0.07 0.02 1.33 (0.58) 122 (±39) 274 (±151)

WEST_4 3800.15 0.07 0.11 1.36 (0.92) 421 (±131) 899 (±510)

WEST_5 5950.91 0.08 0.11 1.28 (0.46) 644 (±164) 1484 (±559)

WEST_6 5848.27 0.08 0.02 1.00 (0.00) 96 (±29) 270 (±192)

WEST_7 2319.55 0.08 0.08 1.38 (0.74) 186 (±88) 503 (±305)

WEST_8 3158.57 0.08 0.04 1.17 (0.41) 130 (±47) 352 (±270)

WEST_9 3745.67 0.09 0.09 2.17 (2.40) 326 (±113) 457 (±209)

MID 7184.98 0.05 0.01 1.00 (0.00) 84 (±25) 253 (±176)

EAST_1 1842.13 0.05 0.25 1.09 (0.30) 460 (±137) 1472 (±768)

EAST_2 2830.65 0.05 0.85 1.77 (1.11) 2404 (±785) 4005 (±1555)

EAST_3 3076.93 0.05 0.37 1.18 (0.39) 1129 (±352) 2564 (±1244)

EAST_4 3052.14 0.05 0.10 3.00 (1.41) 303 (±106) 303 (±293)

EAST_5 4413.05 0.05 0.17 2.00 (1.29) 745 (±196) 1056 (±410)

EAST_6 2460.31 0.05 0.14 1.20 (0.45) 344 (±90) 797 (±488)

EAST_7 3009.94 0 NA - -

EAST_9 3055.38 0 NA - -

NE-Block 23857.34 0.06 0.01 1.5 (0.71) 143 (±35) 285 (±202)

TOTAL 93,145 8,176 (±958) 16,007 (±2,465)

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Table 5 Comparison of Dugong population estimates (Pollock et al. 2006) across aerial

survey years in 2007, 2014 and 2015

Table 6 Estimated densities (km-2

) of Dugong in the Gulf of Carpentaria, 2007, 2014 and

2015 based on Pollock et al. (2006) population abundance estimates

A T-test was calculated on the difference between mean Dugong density from 2007 and

2015 aerial surveys. The change in dugong density in the Gulf of Carpentaria between these

years was not significant according to Welch's t-test, t(0.60) = 9.4 p = 0.56.

Year 2007 2014 2015

Block Area

(km2)

Sampling

Intensity

Pop. Est.

(± s.e.)

Area

(km2)

Sampling

Intensity

Pop. Est.

(± s.e.)

Area

(km2)

Sampling

Intensity

Pop. Est.

(± s.e.)

2 2046 0.11 1702

(± 936) 2921 0.08

1379

(± 241) 2831 0.05

4005

(±1555)

3 1631 0.09 612

(± 281) 2814 0.08

1722

(± 501) 3077 0.05

2564

(±1244)

4 1937 0.09 555

(± 251) 3045 0.08

739

(± 184) 3052 0.05

303

(±293)

5 3734 0.09 994

(± 372) 4586 0.08

648

(± 270) 4413 0.05

1056

(±410)

6 2021 0.09 534

(± 165) 2198 0.08

598

(± 345) 2460 0.05

797

(±488)

7 2435 0.10 389

(± 174) 2852 0.08

698

(± 233) 3010 0.05 0

Total 13804 4786

(± 1101) 18416

5784

(± 767) 18843

4925

(± 893)

Block 2007 2014 2015

2 0.83 0.47 0.85

3 0.38 0.61 0.37

4 0.29 0.24 0.10

5 0.27 0.14 0.17

6 0.27 0.27 0.14

7 0.16 0.24 0

Mean Dugong density (km2) 0.37 0.33 0.27

14

3.3 Observations of other marine megafauna

In addition to Dugong sightings, we recorded a range of marine megafauna (Table 7). Marine

turtles were the most abundant animal recorded with 1,857 turtles observed on transect. With

exception of the Leatherback (which is distinctively larger and darker than other species),

turtles were not identified to species. Five species of dolphin were recorded though most

(approximately 50%) were not identified to species. Rates of dolphin species identification

were different between survey planes because of observer experience. Dolphins were

abundant across the survey range with 1,393 individuals on transect. Of the dolphins that

could be identified, the Snubfin Dolphin (Orcaella heinsohni) was the most prevalent marine

mammal species encountered with 243 individuals observed (in and out of transect

combined). We recorded 32 False Killer Whales (Pseudorca crassidens), and three

Humpback Whales (off transect). Other animals observed on transect included 93 Manta

Rays and three Whale Sharks.

Density estimates of observed Dugongs, turtles and inshore dolphins were compared across

the NT coast (Figure 3). Availability and detection correction factors were applied to Dugong

sightings and detection only, whereas (perception bias) correction factors were applied to

dolphin (certain and probable sightings) and turtle observations. False Killer Whales and

Dwarf Spinner Dolphins were excluded from the density estimates as they were generally

found in deeper water, in larger groups making them incomparable. The two highest density

areas for turtles were the same as those for Dugongs - the Sir Edward Pellew Island Group

and Limmen Bight. Inshore dolphins were found to be most dense in blocks west of Groote

Eylandt.

15

Table 7 Megafauna sightings other than Dugongs (dolphins, turtles, manta rays, whale sharks

and humpback whales) during the 2015 aerial survey. The number in parenthesis represents

animals observed outside the strip transect area on survey

Animal/Species # of

Groups

# of

Individuals

# of

Calves

%

Calves

Avg.

Group

Size

Australian Snubfin Dolphin

Orcaella heinsohni 45 (5) 206 (37) 10 4.6 3.4

Dwarf Spinner Dolphin

Stenella longirostris

rosieventris

10 146 (2) 1 0.7 12.8

Bottlenose Dolphin

Tursiops aduncus 18 77 (1) 5 6.1 4

Australian Humpback Dolphin

Sousa sahulensis 10 (2) 18 (7) 3 14.3 1.8

False Killer Whale

Pseudorca crassidens 2 32 1 3.0 17.5

Unidentified dolphin spp. 383 (60) 1018 (171) 46 (5) 4.3 2.7

Humpback Whale

Megaptera novaengliae (2) (3) 0 0 1.5

Marine turtles 1674 (154) 1854 (157) - - 1.1

Leatherback Turtle 2 (1) 2 (1) - - 1

Whale Shark

Rhincodon typus 3 (1) 3 (1) - - 1

Manta Ray 78 (32) 93 (48) - - 1.2

16

Figure 3 Comparison of relative densities between Dugongs, inshore dolphins and turtles observed on aerial survey along the NT coast with

detection and perception correction factors applied to Dugong and perception corrections factors applied to turtles and inshore dolphins

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Ind

ivid

ua

ls p

er

km

2

Turtles

Inshore dolphins

Dugongs

17

4 Discussion

Dugong population abundance in 2015 for the NT survey area was estimated to be 8,176 (±

958) Dugongs using the Pollock et al. (2006) method. This analytical method improves the

survey precision by 3.7% compared with the Marsh & Sinclair method (Marsh and Sinclair

1989) which does not account for heterogeneity of environmental variables known to affect

Dugong availability to observers. The Pollock et al. (2006) method is considered superior

because the assumptions are more reasonable and more easily met in the field. The method

attempts to correct for availability bias (animals not available to observers because of water

turbidity), and perception bias (animals visible in the survey transect but missed by

observers). The Pollock et al. (2006) method, unlike Marsh and Sinclair (1989) uses

environment-specific correction factors for availability bias, addressing the spatial

heterogeneity in sighting conditions within each survey whereas the Marsh and Sinclair

(1989) method averages these conditions within surveys and only corrects for differences in

availability bias between surveys (Marsh et al. 2008). In addition, observers find it difficult to

determine whether the Dugongs sighted are at the surface (or not), as required by the Marsh

and Sinclair (1989) method.

Despite efforts to account for difficulties with detection, the availability bias remains a

considerable yet consistent source of error. It is likely that dive behaviour of Dugongs is

influenced by water clarity, tidal conditions and water depth, and that the dive behaviour of

individual Dugongs within a herd is not independent (Meager et al. 2013). It has recently

been argued that systematic variation in dive profiles could lead to biased aerial survey

estimates in areas with heterogeneous bathymetry (Hagihara et al. 2014). The resultant

effect of this inaccuracy of density estimates is that only a large change in abundance can be

detected, and that small declines (or increases) over time are likely to be missed (Taylor et

al. 2007). These uncertainties decrease survey precision and hence the statistical power to

detect changed in Dugong distribution and abundance.

The Dugong population abundance estimate is considered to be highly conservative as only

Dugong sightings categorised as ‘certain’, within transect markers, were used to develop the

population abundance estimates. The turbidity of Northern Territory coastal waters is a

significant limiting variable for aerial surveys in these waters. Dugong diving and surfacing

behaviour in this environment has not been investigated to date though would likely inform

improved population estimates, as has been achieved for Dugong population estimates in

Queensland (Hagihara et al. 2014).

18

Direct comparison of all previous NT Dugong aerial surveys is not possible as survey

methods differ. However, Dugong aerial surveys have been undertaken in the Gulf of

Carpentaria (GoC) using comparable methods and analysis since 2007. The GoC provides

an important reference for the NT’s Dugong population as this region comprises ~60% of

Dugong found in the Northern Territory. Since the implementation of standardised surveys,

there has been no significant difference in population density estimates detected for this

region with density ranging from 0.27 (2015) – 0.37 (2007) Dugong per km2. Furthermore,

time series analysis by Marsh et al. (2008) indicated that no significant change in the GoC

Dugong population had occurred between surveys in 1994 and 2007 suggesting the Dugong

population in the GoC has likely been stable for over 20 years.

Dugong presence is generally related to the presence of seagrass; this dependency infers a

relationship between Dugong population dynamics and seagrass availability (Meager et al.

2013, Cardno 2015b). Spatially explicit local declines in Dugong density and reproductive

potential have been attributed to major seagrass loss events (Preen and Marsh 1995, Gales

et al. 2004, Marsh and Kwan 2008, Meager and Limpus 2014). Seagrass presence is

generally constrained to shallow coastal environments as their growth is limited by light

availability, more so than algae species (Dennison 1987, Orth et al. 2006). Seagrasses have

a low tolerance to prolonged low-light conditions (Longstaff et al. 1999) and elevated

temperature (Collier et al. 2011, McKenzie et al. 2012, Collier and Waycott 2014, Pedersen

et al. 2016). This implies that processes contributing to these environmental changes, such

as severe weather events, disturbance of sediments, eutrophication and flooding may affect

Dugong populations and other species directly dependent upon seagrass ecosystem health.

Recent observations of extensive mangrove dieback in Limmen Bight correlate with extreme

warming and climate events in the region (Wild 2016). The extent of this dieback is

unprecedented and raises concerns for the seagrass habitat in the region and the

consequential impacts on local Dugong and turtle populations. Therefore, it will be important

to monitor the health of seagrass in this region.

With the exception of Darwin Harbour, seagrass habitats within the NT are not monitored.

Surveys by Poiner et al.(1987) and Roelofs et al. (2005) recorded seagrass presence

throughout the GoC, westward to Van Diemen Gulf. Within these regions, seagrass and

Dugong presence broadly correlate. If seagrass is also impacted by the extreme climate

events, localised density declines of Dugong are likely (Meager and Limpus 2014). However,

establishing a clear link between weather-related disturbance and Dugong abundance is

confounded by large-scale movements undertaken by Dugongs from impacted areas (Preen

19

and Marsh 1995, Holley et al. 2006, Sobtzick et al. 2013a). Deutsch et al. (2003) suggest

large-scale movements by Sirenians have evolved in response to seasonal changes in

environmental variables such as temperature, water levels, salinity and variability of forage.

The Sir Edward Pellew Islands and Limmen Bight region (survey Blocks 2 and 3) has

consistently had the highest Dugong density estimates recorded from aerial surveys (Marsh

et al. 2008, Groom et al. 2015). This survey confirms the significance of these blocks for

Dugongs within the GoC and justifies concern for the potential impacts to Dugong and turtles

in the GoC because of seagrass dieback. Impacts to the Dugong population may manifest as

movements to un-affected areas, animals presenting with reduced body fat, increased

mortality and reduced reproductive capacity.

Dugongs are particularly vulnerable to mortality as adults (Marsh et al. 2011) given their life

history characteristics of long lifespans (greater than 70 years), long gestation (12–14

months), single offspring, long intervals between births (more than 2.5 years), prolonged

periods until sexual maturity (6–17 years), and high and temporally stable adult survival

(Marsh et al. 1984). Adult survival is the most important determinant of population growth

(Marsh et al. 2011). The maximum rate of population increase under optimum conditions

when natural mortality is low is approximately 5 per cent per year. The maximum sustainable

mortality rate of adult females from anthropogenic activities (e.g. fishing interactions, hunting,

boat strike) is approximately 1 or 2 per cent (Marsh et al. 1997, Heinsohn et al. 2004, Marsh

et al. 2004, Marsh et al. 2015) and lower when food supplies are low (Marsh et al. 2002;

Marsh & Kwan 2008). Pressures that cause Dugong mortality are therefore of potential

concern if such pressures occur over a wide geographic area, even if the magnitude of the

pressures is uncertain (Marsh et al. 2015).

The survey recorded Dugong calves to be within a healthy and comparable range. The calf

count from this survey was 13.4%; greater than the range of mean estimates previously

observed for the GoC and Northern Great Barrier Reef region of 5.3% and 11.4%

respectively (Marsh et al. 2008, Groom et al. 2015). The mean Dugong group size for the

survey (in transect) was 1.4 (SD = 0.7), comparable to the 2007 mean (1.4).

The survey team’s experience on Dugong aerial surveys ranged from highly experienced to

limited. Nonetheless, the probability of observers sighting Dugongs given they were available

for detection was high for the survey teams. The Dugong population estimate from this

survey is a standardised under-estimate given recent research findings that investigated

Dugong surfacing and diving behaviour. Hagihara et al. (2014) used Dugong telemetry data

acquired under varying environmental conditions and determined the availability bias for

20

Dugongs is greater in deeper waters. This aerial survey data have not been modelled with

coastal bathymetry but application of the Hagihara et al. (2014) method would further

improve Dugong population estimates for this area.

Dugong aerial surveys allow for standardised collection of spatial and temporal data to

determine abundance, density and distribution of Dugongs and other marine megafauna.

Aerial surveys are appropriate for the collection of broad-scale data to meet management

objectives that require an understanding of ecological responses and population trends. The

data acquired from these surveys will be used to guide future research and management.

5 Acknowledgments

The research was funded by the INPEX-led Ichthys LNG Project. We wish to thank the

Thammarrurr, Tiwi, li-Anthawirriyarra, Garig Gunak Barlu, Anindilyakwa, Mardbalk and

Crocodile Islands coastal ranger groups for their assistance with survey logistics.

Appreciation and thanks to the aerial survey team who worked well in the heat and confined

space of the survey plane. We are grateful to the DLRM staff that assisted in administration,

survey design, gear preparation and data management. We thank James Cook University for

their technical guidance and provision of transect markers. The Dugong aerial survey was

authorised by the Northern Land Council under Permit ID: 58740.

21

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24

Appendix 1 Overview of survey flights undertaken in the Gulf of Carpentaria

Date Survey Transects Completed

9/10/2015 17_East1, 15_East1, 13_East1, 11_East1, 9_East1, 7_East1, 5_East1, 3_East2,

727_West1, 725_West1, 709_West1, 711_West1, 713_West1, 715_West1, 717_West1

719_West1


745_West1, 747_West1, 749_West1, 751_West1, 753_West1, 755_West1,

757_West1, 759_West1, 761_West1, 763_West1, 765_West1, 767_West2


37_East3, 35_East3, 33_East3, 769_West2, 771_West2, 495_West2, 497_West2,

499_West2, 501_West2, 503_West2

12/10/2015 No surveys due to day off

13/10/2015 75_ East4, 73_ East4, 71_ East4, 99_ East5, 97_ East5, 95_ East5, 505_West2,


523_West2, 525_West2, 527_West2, 529_West3, 531_West3

14/10/2015 69_East4, 67_East4, 65_East4, 63_East4, 61_East4, 59_East4, 533_West3,


519_West3, 521_West3

15/10/2015 57_ East4, 55_ East4, 113_ East5, 943_West4, 935_West4, 921_West4, 913_West4,


835_West5, 827_West5, 821_West5, 817_West5, 813_West5

16/10/2015 111_East5, 101_East5, 105_ East5, 107_ East5, 109_ East5, 823_West5, 809_West5,

1164O_West5, 1163_West5, 1147W_West3, 1159W_West3 951_West4, 957_West4,

963W_West4, 976W_West3

17/10/2015 963E_West4, 1147E_West3, 1159E_West3, 976E_West3, 981_West3, 1001_West3,

1011_West3, 1025_West3, 1035_West3

No surveys in East

18/10/2015 91_East5&9, 89_East5&9, 103_East5, 93_East5, 121_East6, 119_East6

No surveys in West

19/10/2015 147_East7, 115_East6, 117_East6, 145_East7, 143_East7, 141_East7, 139_East7,

123_East9, 1045_West6, 1157_West6, 1153_West6, 1155_West6, 1013_West6,

1003_West6, 991_West3&6

20/10/2015 87_East9, 125_East9, 127_East9, 129_East9, 131_East9, 77_East9, 79_East9,

81_East9, 83_East9, 85_East9, 1179_West7, 1180O_West6, 1181_West6,

1182O_West6, 1027_West6, 1037_West6, 1049_West6, 1047_West6, 1061_West6

21/10/2015 541_EastNE, 543_EastNE, 545_EastNE, 547_EastNE, 549_EastNE, 551_EastNE,

1178O_West7, 1177_West7, 1176O_West7, 1175_West7, 1174O_West7,

1173_West7, 1172O_West7, 1171_West7, 929_West7, 845_West9, 859_West9,

871_West9, 881_West9, 881A_West9, 891_West9, 901_West9


1125_EastNE, 1123_EastNE, 1069_West6, 1059_West6, 1046O_West6,

1036O_West6, 1026O_West6, 1015_West6, 1004E_West6, 993_West6

25

23/10/2015 133_EastNE, 135_East7, 137_East7, 559_EastNE, 561_EastNE, 563_EastNE,

565_EastNE, 567_EastNE, 569_EastNE, 571_EastNE, 573_EastNE, 575_EastNE,

577_EastNE, 579_EastNE, 581_EastNE, 583_EastNE, 1144O_West4, 1143_West4,

937E_West4, 937W_West9, 925_West4&9, 915_West4&9, 909_West4&9,

900O_West4, 890O_West5, 880O_West5, 870O_West5, 856O_West5, 1A_West5,

2A_West5, 844O_West5, 1133_West5, 1135_West5, 819_West5, 815_West5,

1167_West5


1165_West5, 1166O_West5, 807_West5, 811_West5, 814O_West5, 818O_West5,

1134Z_West5, 831_West5,

25/10/2015 1145_West4, 1146E_West4, 1147E_West6, 972E_West6, 976E_West6,

1150E_West6, 1151_West6, 1152E_West6

No surveys in East

26-28/10/2015 No surveys due to plane maintenance

29/10/2015 1121_EastNE, 1119_EastNE, 1115_EastNE, 1111_EastNE, 1105_EastNE,

1109_EastNE, 1113_EastNE, 1117_EastNE, 1100_EastNE

No surveys in West


609_EastNE, 611_EastNE, 613_EastNE, 615_EastNE, 617_EastNE, 619_EastNE,

621_EastNE, 623_EastNE, 625_EastNE, 1232N_West8&9, 1193_West9,

1194N_West9, 846N_West9, 846Z_West9, 861_West9, 861Z_West9, 873_West9,

873Z_West9, 883_West9, 883Z_West9, 893_West9, 893Z_West9, 903_West9,

1299_West9, 1299Z_West9, 824_West9, 1194O_West9, BP1_West9, 862N_West9,

825_West8, 833_West8, 849_West8, 865_West8, 886O_West9, 875E_West9

875_West8, 887_West8, 887E_West9, 895E_West9, 895_West8, 905_West8,

905_West9


1215_EastNE, 1213_EastNE, 1211_EastNE, 905Z_West9, 911E_West9, 911_West8,

917_West8, 933_West8, 941_West8, 949E_West8, 949_Mid, 955_Mid, 961_Mid,

967_Mid, 1184E_Mid, 1185_Mid


1223_EastNE, 1185W_Mid, 1186N_Mid, 986N_Mid, 996N_Mid, 1000N_Mid, 1019_Mid,

987_Mid, 997_Mid, 1007_Mid


1075_EastNE, 1079_EastNE, 1021_Mid, 1028N_Mid, 1038N_Mid, 1050N_Mid,

1201_Mid, 1201W_Mid, 1207_Mid, 1203_Mid

3/11/2015 1221_EastNE, 1203_EastNE, 1205_EastNE, 1083_EastNE, 1063_Mid, 1207W_Mid,

1205E_Mid, 1077_Mid, 1081_Mid, 1081W_Mid, 1085_Mid

*Survey block number follows the underscore. i.e. 57_East4 is transect 57 in Block East4

26

Appendix 1.1 Dugong raw data sightings

Team/Plane Block Block Area

(Km2) Transects

Transects with Sightings in

transect Total

Sightings Total

Individuals #

Calves Sightings in

Transect Individuals in Transect

Group size mean

Group Size SD

Group Size

median At

surface Below

Surface

East East_1 1842.13 8 4 10 12 1 11 12 1.09 0.30 1 8 4

East East_2 2830.65 9 7 33 62 11 27 48 1.77 1.11 1 42 20

East East_3 3076.93 11 6 20 26 4 16 22 1.18 0.39 1 12 14

East East_4 3052.14 11 1 2 6 3 2 6 3.00 1.41 3 2 0

East East_5 4413.05 13 6 7 14 2 7 14 2.00 1.29 1 9 5

East East_6 2460.31 4 2 5 6 1 5 6 1.20 0.45 1 3 3

East East_7 3009.94 8 0 1 1 0 0 0 1.00 NA 1 0 NA

East East_9 3055.38 13 0 0 0 0 0 0 NA NA NA NA NA

East NW block 23857.34 83 2 2 3 1 2 3 1.5 0.71 1.50 3 0 Totals and

overall statistics

EAST 47597.87 160 28 80 130 23 70 111 1.64 1.14 1 79 47

West West_1 4769.49 17 4 7 10 1 5 8 1.43 0.79 1 8 2

West West_2 3725.03 18 5 6 10 0 6 10 1.67 1.63 1 7 3

West West_3 5044.91 23 6 3 4 0 3 4 1.33 0.58 1 2 2

West West_4 3800.15 23 5 11 15 0 10 13 1.36 0.92 1 10 5

West West_5 5950.91 31 10 18 23 1 16 20 1.28 0.46 1 17 6

West West_6 5848.27 29 2 3 3 0 3 3 1.00 0.00 1 1 2

West West_7 2319.55 10 3 8 11 1 6 7 1.38 0.74 1 7 4

West West_8 3158.57 14 2 6 7 0 4 4 1.17 0.41 1 6 1

West West_9 3745.67 37 5 6 13 0 6 12 2.17 2.40 1 10 3

West MID 7184.98 30 2 3 3 0 2 2 1.00 0.00 1 2 1

Totals and overall

statistics WEST 45547.53 232 44 71 99 3 61 83 1.39 0.99 1 70 29

GRAND TOTALS 93145 392 72 151 229 26 131 194

27

Appendix 1.2 Details of tested models perception correction (modelled with glare and turbidity)

Model AICc Delta AICc AICc Weight Model Likelihood #Parameters

AllObservs-Different-NO-ENVeffects 169.406 0 0.53434 1 4

All:Only-TURB effect:Observs-Different 170.8938 1.4878 0.25395 0.4753 8

All:Only-GLARE effect:Observs-Different 171.9319 2.5259 0.15112 0.2828 8

All-Turb:Glare:Observs-Different 173.7597 4.3537 0.06059 0.1134 12

Results from selected model and perception probability for each team (all observers different, no environmental effects)

Selected Model

Parameter Estimate Standard Error Lower CI Upper CI

WEST TEAM

1:p – port front observers 0.857143 0.06613 0.6755155 0.9453332

2:p – port rear observers 0.648649 0.0784828 0.4845404 0.7838198

3:p - starboard front observers 0.636364 0.1025593 0.4233439 0.8066352

4:p - starboard rear observers 0.411765 0.0844035 0.261214 0.5808634

EAST TEAM

1:p – port front observers 0.448276 0.0923495 0.2810031 0.6281324 0.448276

2:p – port rear observers 0.333333 0.0754851 0.2044149 0.4931584 0.333333

3:p - starboard front observers 0.818182 0.0671408 0.6500957 0.9159612 0.818182

4:p - starboard rear observers 0.54 0.070484 0.4023191 0.6718352 0.54

28

Perception and availability correction factors

Perception Correction Factor (C.V.)

Port Starboard Availability Correction Factor (C.V.)

EAST TEAM

1.525 (0.147) 1.137 (0.050) 3.754 (0.122)

WEST TEAM

1.083 (0.037) 1.218 (0.075) 4.234 (0.121)

29

Appendix 2.1 – Dugong sightings

30

Appendix 2.2 – Dolphin sightings

31

Appendix 2.3 – Turtle sightings

32

Appendix 2.4 – Humpback whale, Whale shark and Manta ray sightings

the distribution and abundance of dugong and … · department of environment and natu ral...

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