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Collaborative photo-identification and monitoring of grey nurse sharks (Carcharias taurus) at key aggregation sites along the eastern coast of Australia

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Before the worldwide decline of the 'globally vulnerable' Carcharias taurus may be addressed, an understanding of its migratory patterns and locations, and/or times when sharks may be vulnerable, is required to identify habitats that are critical to its survival. A collaborative framework for photo-identification and monitoring of C. taurus may greatly assist with conservation management initiatives. Images of C. taurus were sourced from public submissions to the www.spotashark.com (verified 12 February 2009) website and during targeted surveys. A computer-assisted program (I(3)S) was used to match the images of sharks photographically from the database. Research revealed patterns of movement, site utilisation and population structure similar to those in previous tagging studies. With the use of an underwater camera and two laser-scaling devices, 408 individual sharks were identified. Average occupancy times at two locations in New South Wales (NSW), Australia, were 308 days (Fish Rock) and 363 days (Magic Point). Seventeen individuals undertook northward or southward movements, averaging 350 km. The present study showed that a broad-based technique for data acquisition, coupled with rigorous evaluation of photographic identifications can provide support for local research and management programs and may aid with the conservation of the C. taurus species worldwide.
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Collaborative photo-identification and monitoring
of grey nurse sharks (Carcharias taurus) at key
aggregation sites along the eastern coast of Australia
Sean M. Barker
A
,
B
and Jane E. Williamson
A
A
Marine Ecology Group, Department of Biological Sciences, Macquarie University,
Sydney, NSW 2109, Australia.
B
Corresponding author. Email: seanmbarker@hotmail.com
Abstract. Before the worldwide decline of the ‘globally vulnerable’ Carcharias taurus may be addressed, an
understanding of its migratory patterns and locations, and/or times when sharks may be vulnerable, is required to identify
habitats that are critical to its survival. A collaborative framework for photo-identification and monitoring of C. taurus
may greatly assist with conservation management initiatives. Images of C. taurus were sourced from public submissions to
the www.spotashark.com (verified 12 February 2009) website and during targeted surveys. A computer-assisted program
(I
3
S) was used to match the images of sharks photographically from the database. Research revealed patterns of movement,
site utilisation and population structure similar to those in previous tagging studies. With the use of an underwater camera
and two laser-scaling devices, 408 individual sharks were identified. Average occupancy times at two locations in New
South Wales (NSW), Australia, were 308 days (Fish Rock) and 363 days (Magic Point). Seventeen individuals undertook
northward or southward movements, averaging 350 km. The present study showed that a broad-based technique for data
acquisition, coupled with rigorous evaluation of photographic identifications can provide support for local research and
management programs and may aid with the conservation of the C. taurus species worldwide.
Additional keywords: mark resighting, migration, philopatry, photo-ID, photographic database.
Introduction
Shark populations worldwide are in decline. Many species
face extinction because of past and present fishing, coupled with
slow life-history characteristics and low population growth
(Musick 1999; Dulvy et al. 2008). One species under particular
threat is the grey nurse shark, Carcharias taurus, which is
known as the spotted ragged-tooth shark in South Africa and
sand-tiger shark in the USA. C. taurus inhabits tropical and
temperate waters of the North and South Atlantic, Indian
and Western Pacific Oceans (Last and Stevens 2009). Although
once widely distributed, populations in several locations have
been severely depleted (Cavanagh et al. 2003). Contributing
factors include very low rates of reproduction (e.g. just one or
two offspring every 2 years), susceptibility to fishing pressure,
slow increase in population size and extremely limited mixing
among populations (Environment Australia 2002; IUCN 2008).
Because of these population declines, C. taurus is now listed
as Globally Vulnerable on the IUCN Red List of Threatened
Species.
Separate populations of C. taurus occur in coastal waters of
western and eastern Australia. The conservation status of this
species is listed as Critically Endangered and Vulnerable to
Extinction, respectively, along the eastern and western coasts of
Australia under the Environmental Protection and Biodiversity
Act 1999 (Environment Australia 2002). Genomic DNA studies
by Stow et al. (2006) suggested that these two populations are
genetically isolated, inferring that replenishment of the stock is
unlikely to be achieved through increased natural immigration
from populations elsewhere. Moreover, with no immigration
into the eastern population, and without an urgent conservation
plan, extinction may be imminent for C. taurus along the eastern
Australian coast (Stow et al. 2006).
Carcharias taurus has been studied by a variety of methods,
including catch records from fishing and protective beach
meshing (Pepperell 1992; Reid and Krogh 1992), visual surveys
by divers, and by tagging of individual sharks (Parker and
Bucher 2000; Otway et al. 2003; Dicken et al. 2006a). Tagging
has provided essential information for management, including
population size and demography. Initially, C. taurus individuals
were physically tagged with a variety of methods (Otway and
Burke 2004; Dicken et al. 2006a). More recently, the use of
photographic images, in which patterns of unique pigmentation
spots that allow individuals to be identified, have been used to
glean demographic information on this species (van Tienhoven
et al. 2007; Bansemer and Bennett 2009) and their vulnerability
to fishing-related injuries (Bansemer and Bennett 2010).
Patterns of pigmentation spots show no evidence of signifi-
cant change over years and may therefore be used to identify
C. taurus individuals through time, with minimal disturbance
(Bansemer and Bennett 2008, 2010). In addition to studies on
CSIRO PUBLISHING
www.publish.csiro.au/journals/mfr Marine and Freshwater Research, 2010, 61, 971–979
ÓCSIRO 2010 10.1071/MF09215 1323-1650/10/090971
C. taurus, previous photographic-identification (photo-ID) stu-
dies have successfully acquired information on the abundance,
population structure, site fidelity and long-distance movements
of white sharks (Carcharodon carcharias) (Bonfil et al. 2005;
Domeier and Nasby-Lucas 2007). Survival trends, along with
segregations of sex and sizes, have also been reported in photo-
ID studies on whale sharks (Rhincodon typus) (Graham and
Roberts 2007; Holmberg et al. 2008, 2009).
The use of photo-ID is now being applied to studies involving
the critically endangered population of C. taurus in the eastern
coast of Australia. Recent ‘by eye’ studies were conducted for
C. taurus at Wolf Rock in Queensland, Australia (Bansemer
and Bennett 2008, 2009). A preliminary study was conducted at
Magic Point in NSW by using a computer-assisted program to
re-identify individuals (S. M. Barker, unpubl. data). Although it
may be possible to manage small numbers of photographs and
identify individuals ‘by eye’, the process becomes inefficient
and unreliable when collating data from a large image dataset.
This led to the need for a web-based system for accurate and
efficient cataloguing of images. Arzoumanian et al. (2005) has
successfully achieved such a system for Rhincodon typus, and
this holds promise for application to C. taurus.
Diving to observe C. taurus is an extremely popular pastime
amongst SCUBA divers and is the basis for numerous dive-
charter operations along the eastern coast of Australia (Byron
1984; Hockton 2003). Moreover, a code of conduct for diving
is now in place that regulates interactions between divers and
C. taurus (Environment Australia 2002). A large proportion of
recreational divers are also competent underwater photogra-
phers and recreational divers at a diverse range of sites and
times take many photographs of C. taurus. Thus, there is
potential to use such images in obtaining data on the locations
of individual sharks.
In the present paper, we present an efficient, non-invasive
and cost-effective approach for determining abundance and
resighting rates at occupied sites, and movement patterns for
C. taurus on the eastern coast of Australia. The following three
questions were addressed: (1) can population monitoring for
C. taurus be undertaken using a web- and a computer-assisted
program, with input from the dive community; (2) can
photographic techniques provide more specific information
than community input on the rate of occupation for C. taurus
at aggregation sites (Magic Point) over 4 years; and (3) when
compared with a short-term study of another aggregation site in
the northern waters of NSW (Fish Rock), do the sex and size
classes of C. taurus differ between the two locations?
Materials and methods
Images acquired through public submissions
A website was constructed (www.spotashark.com) that allowed
public photographers to upload their images of C. taurus taken
along the Australian eastern coast. Photographers submitted a
jpg image (o2 MB), along with a data sheet that included the
location of where the image was taken (GPS coordinates or
nearest town), the time and date that the image was taken, water
clarity and visibility (m), and the type of camera used. Several
aggregation sites were focussed on to acquire photographs
(Table 1).
Images acquired through surveys
Additional dives were undertaken at the following three sites:
(1) Magic Point (3385702900S, 15181505100 E), (2) Seal Rocks
(3282705000S, 15283301500 E) and (3) Fish Rock (3085602500S,
15380600500E), which provided supplementary information for
the database (Table 2). This also enabled us to assess the number
of male, female and juvenile (sex unknown) sharks for a mark–
resighting analysis. Images from Magic Point and Fish Rock
were also used for size-class estimates. Images of C. taurus
were recorded on video (Canon Mv200i, Tokyo, Japan, with
Amphibico housing, Canada). Still pictures were taken with a
compact digital still camera (Canon G10, Tokyo, Japan, with
WP-DC28 housing; Olympus C8080, Japan, with PT023
housing).
Image capture was assisted by the use of two laser-scaling
devices attached to the video camera for acquiring length
estimates as per Bansemer and Bennett (2009). Our study
differed, however, by using only the pre-caudal length (PL)
(i.e. tip of snout to pre-caudal fin notch), because tail flexure
may result in inaccurate estimates for the total length (i.e. tip of
Table 1. The total number of images (i.e. minimum number of left flanks) of individual Carcharias taurus taken
for each location during surveys and from images uploaded via the website
Resightings represent the number of left flanks only; an additional eight sharks were resighted using their right flanks at
more than one location
Location Survey total Female Male Unsexed Resightings
1st location 2nd location
Wolf Rock 1 1 0 0
South Solitary Island 16 6 10 1
Fish Rock 212 119 87 6 30 3
Julian Rocks 62 15 45 2 5 2
Seal Rocks and Forster (4 sites) 84 57 24 3 2 4
Broughton Island 9 4 0 5
Magic Point 20 11 5 3 16
Montague Island 4 1 3 0
Total 408 214 174 20 53 9
972 Marine and Freshwater Research S. M. Barker and J. E. Williamson
snout to tip of tail). This application was tested on C. taurus in
captivity (specimens held in Oceanworld, Manly, Sydney).
Estimates were obtained by collecting information for (1) young
of the year and sexually immature individualso120 cm PL and
(2) sexually mature adult individuals 4120 cm in PL (Dicken
2006).
Photo-ID (public submissions and surveys)
Photo-ID for C. taurus was undertaken by analysing images of
spot patterns on both the left and right flanks as per Bansemer
and Bennett (2008), except that we utilised both a computer-
assisted program and ‘by eye’ examinations for confirmation
of identity. Image analysis was undertaken with the inter-
active individual identification system (I
3
S, Version 2) (van
Tienhoven et al. 2007) and image processing was undertaken by
three trained operators to confirm any positive resightings of
individuals or to add new individuals to the database. All images
were also confirmed by eye using the unique ‘spot-cloud’ ana-
lysis step and by looking at the unique alignment of spots on the
flanks of each shark. Images were incorporated into the larger
database of photographic images with the location, the indivi-
dual’s sex, photograph code, the number of spots and their
position, and any additional features that facilitated identifica-
tion (e.g. presence of scars and fish hooks). This enabled an
overall estimate of the annual number of photographic images
(i.e. using left flanks only) being submitted to the database
via volunteer photographers and during scientific surveys.
Resightings were also divided up into two separate geographical
areas (i.e. Forster and sites to the north, and Seal Rocks and sites
to the south) to determine whether similar information can be
collected as per previous mark–resighting studies using physical
tags (Otway and Burke 2004). Photographic resightings were
also analysed for potential movement patterns, the authenticity
of photographic resightings and the number of images being
submitted from each location. This allowed an estimate of the
rate at which images were being acquired at different locations.
Image analysis
Images at Magic Point were acquired between January 2006 and
December 2009. Data were extracted from each image on gender
(male, female and sex unknown) and size. The laser-scaling
devices were not introduced to the study until July 2007; hence,
size classes were estimated only from data for July 2007 to
December 2009. All sharks were examined for identifiable fea-
tures that allowed a 4-year mark–recapture (resighting) analysis
and to assess the minimal abundance and subsequent movement
patterns between locations. Chi-square analysis (SPSS, Version
17, Chicago, IL, USA) was used to examine whether the ratio of
females and males differed significantly from 1 : 1 between 2007
and 2008.
Images at Fish Rock between October 2008 and March 2009
were collected in our surveys for a short-term analysis of the
population and to explore any movement patterns of individuals
between sites (Table 2). Data on images taken during the 14
dives at Fish Rock were categorised to estimate the size-class
range for individuals at this location. All sharks were examined
for identifiable features that allowed a short-term mark–
recapture (resighting) analysis for estimating a minimal abun-
dance and for movement patterns between site locations. Images
at Seal Rocks were collected during one research trip in May
2009 to determine sex ratios, abundance, and to explore any
movement patterns of individuals between sites (Table 2).
Results
Carcharias taurus photo library (all sites)
The number of newly identified sharks by photo-ID exponen-
tially increased from 2004 to 2009 (Fig. 1). The number of
previously identified sharks (mark–resightings) also increased,
although only slightly (o10%) (Fig. 1). Four hundred and eight
new individuals were archived to the photographic database,
with images spanning the 13 sites listed as ‘critical habitats’
along the eastern coast of NSW and Queensland (Table 1). Three
hundred and fifteen (77%) images were obtained during
2008 and 2009 from 29 different photographers (Fig. 2). Pro-
portionally more (22 of the 29) photographers were public
volunteers who contributed their images to the website directly.
The remainder (7 of the 29) were professional photographers or
researchers who made a direct contribution to the image data-
base at the time of the study (Fig. 2). Despite the proportional
differences in photographic effort, most of the new sharks
identified (460%) and positive resightings (490%) (i.e.
resightings at the same or different location) were from pro-
fessional photographers and researchers who were directly
0
50
100
150
200
250
2004 2005 2006 2007 2008 2009
No. sharks identified
Fig. 1. Number of new sharks (n¼408) (black bar) added yearly to the
photographic database, along with the number of individuals resighted from
the previous year (white bar).
Table 2. Sites surveyed, with the total number of males and females
of Carcharias taurus and the sex ratio
Survey site
and date
No. of
sharks
No. of
males
No. of
females
Sex
ratio
Magic Point
January 2006 to
December 2009
20 5 11 1.0 : 2.2
Fish Rock
October 2008 32 15 17 1.0 : 1.1
November 2008 13 24 18 1.0 : 0.8
March 2009 40 27 13 1.0 : 0.5
Seal Rocks
May 2009 41 20 21 1.0 : 1.1
Photo-identification and monitoring of grey nurse sharks Marine and Freshwater Research 973
involved in the project. Slightly more (8 of the 13) critical-
habitat sites were covered by public photographers, whereas
fewer (5 of the 13) were covered by photographers directly
related to the project.
Fifty per cent of the images were added to the database within
the first 12 months of the launch of the website and during our
2008 and 2009 research trips to Seal Rocks and Fish Rock
(Fig. 1, Tables 1, 2). Of the 408 sharks, 62 (15%) were positively
resighted a second time after their initial documentation; of
these, 53 individuals (85%) were resighted at the same location
(Fig. 3). Thirty-three (62%) were female, 16 were male (30%)
and four were unsexed juvenile sharks (8%). Thirty-five (66%)
from this cohort (35 of the 53) were resighted at their place of
initial documentation in northern waters of NSW (i.e. Forster
and areas to the north) whereas 18 (34%) were resighted in the
southern waters of NSW (i.e. Seal Rocks and areas to the South).
Eighteen females and eight males in the northern coastal waters
were resighted within a year of their original documentation.
Ten females, one male and four unsexed juveniles in the south-
ern coastal waters were resighted within a year of their original
documentation (Fig. 3).
Site-to-site movement patterns (all images)
Seventeen individuals showed distinct patterns of movement
among aggregation sites of distances4200 km (Fig. 4). Twelve
individuals were resighted at a second location within a year
of their initial documentation. Eleven individuals (10 females,
1 male) were photographically resighted at sites north of their
initial documentation (Fig. 4). At least one female appeared
to have travelled north during winter. FL-327 was photo-
graphically resighted at Julian Rocks on 11 September 2009,
,200 km north of its original documentation at Fish Rock on
3 August 2009.
Nine (9 of the 17) movement patterns showed four individuals
travellingfrom north to south and five individuals travelling from
aggregation sites in the south to the northern sites over distances
ranging from 200 to 1150 km (Fig. 5). The average site-to-site
movement pattern was ,350 km. One individual first documen-
ted on 21 October 2005 at Julian Rocks (Byron Bay) was
resighted on 4 July 2008 at South Solitary Island and then at
Montague Island, ,1150 km further south on 15 March 2009.
Validating the pre-caudal length of Carcharias taurus
in captivity
Laser-scaling devices proved to be an accurate method for
estimating the length of sharks in captivity. Repeated mea-
surements of the pre-caudal lengths (PL) of individual captive
C. taurus indicated an acceptable level of precision. As with I
3
S,
the photographic perspective of the image affected the accuracy
of the measurements. There was an increase in the size of
standard errors with increasing distortion of the images mea-
sured (Table 3, Barker, unpubl. data). This was not likely to be
an issue in categorical size estimations of wild populations.
However, the image distortion of the two reference points of the
laser beams, owing to the body angle change of individual
sharks, waso3% (Table 3).
Size class-estimate study: Magic Point and Fish Rock
The size and sex of 20 individuals were obtained from Magic
Point during 18 separate dives between July 2007 and March
0
5
10
15
20
25
2004 2005 2006 2007 2008 2009
No. photographers
Fig. 2. Number of contributing photographers as an estimate of the yearly
effort, including public photographers (black bar), and professional and/or
research photographer contribution (white bar).
0
5
10
15
20
25
30
1 year 1–2 years 2–3 years 3 years 1 year 1–2 years 2–3 years 3 years
North South
No. shark resightings
Fig. 3. Number of male (black bar), female (white bar) and unknown (grey
bar) sharks photographically resighted a second time at their original site.
The x-axis separates the sharks into northern and southern sites (see text),
with subdivisions ofo1 year, 1–2 years, 2–3 years and43 years from their
initial documentation.
0
1
2
3
4
5
6
7
8
9
1 year 1–2 years 2 years 1 year 1–2 years 2 years
North South
No. shark movements
Fig. 4. Number of movements of male (black bar), female (white bar)
and unknown (grey bar) sharks photographically resighted more than once
at a different location. The x-axis is as in Fig. 3. Note: in total, there are
19 movements for 17 sharks as two sharks showed multiple site-to-site
movements.
974 Marine and Freshwater Research S. M. Barker and J. E. Williamson
2009, and a further 22 individuals were measured during 14
separate dives at Fish Rock between October 2008 and March
2009 (Table 2, Fig. 6). Proportionally more sexually immature
females and unsexed juvenile sharks in the size-class range of
60–120 cm (PL) were found at Magic Point (93%), whereas a
significant proportion of sexually mature males and females in
the size-class range of 4120 cm (PL) were found at Fish Rock
(74%) from October 2008 to March 2009 (74%) (Fig. 6,
Table 2). Further, there was a significant and positive bias
towards sexually immature sharks at Magic Point and a strong
positive bias towards larger and sexually mature sharks at Fish
Rock (x
1
2
¼15.506, Po0.001).
Mark–resighting study
Twenty (5%) of the new 408 sharks were identified at Magic
Point during our 2006–2009 studies. Sixteen (80%) individuals
were photographically resighted a second time, ,30 days later.
Twelve (60%) of these individuals were photographically
resighted a third time, ,90 days from their initial documenta-
tion. The minimum number of resighted individuals, using
both flanks sides, occurred during the sixth and seventh field
visit (,7 months from their initial documentation). The time
between resighting events for Magic Point, therefore, ranged
from 28 days to 2.5 years, averaging 363 days (95% CI ¼
226–501 days). The ratio of female and male sharks, however,
did not differ significantly during the study period 2006–2008
(x
1
2
¼2.250, P¼0.134). The mature adult male and female
sharks disappeared from the site in September 2008. However,
three unsexed juvenile sharks remained at this site for ,66% of
the time during July 2008 and June 2009. A larger mature male
that had not been previously identified (4200 PL) appeared
in May 2009, and three sexually mature females (4120 PL)
were sighted soon after. One of the females was subsequently
resighted 493 days after its last sighting on the 7 June 2009. This
individual had also been identified previously at Magic Point
on 3 March 2008. A second male shark was also subsequently
resighted on 28 June 2009, ,3 years after its last sighting.
In all, 212 sharks were photographically identified during
field visits to South West Rocks (Table 1). Images of at least
Table 3. Mean (61 s.e.) pre-caudal lengths (PL) for six individuals of
various sizes, sexes (F, female; M, male) and purported ages in captivity
Numbers in parentheses represent the number of times the individual was
photographed
Shark # Year of capture Sex PL (cm)
S1 (3) F 196 4
S2 (6) 1984 F 216 3
S3 (7) 1995 F 225 5
S4 (7) 1985 M 226 6
S5 (4) 1994 M 227 2
S6 (6) 1985 M 239 7
0
2
4
6
8
10
12
14
60–120 cm 120 cm 60–120 cm 120 cm
Magic Point Fish Rock
No. sharks
Fig. 6. Size-class estimates based on the pre-caudal lengths (cm) for male
(black bars), female (white bars) and unsexed juvenile (grey bars) Carchar-
ias taurus individuals during July 2007 to December 2009 at Magic Point
(n¼20) and October 2008 to March 2009 at Fish Rock Cave (n¼22).
The x-axis separates the sharks into unsexed juvenile (i.e. 60–120 PL) and
sexually mature (i.e.4120PL) individuals.
South Solitary
Island
150°E 155°E
30°S
35°S
N
0 100 200 km
Fish Rock
Seal Rocks
Magic Point
South West Rocks
Forster
Sydney
Montague
Island
South Solitary Island to Fish Rock
Magic Point to Seal Rocks
Fish Rock to Seal Rocks
Magic Point to Fish Rock
South Solitary Island to Montague Island
Pinnacle to Fish Rock
Seal Rocks to Fish Rock
Magic Point to the Barge
Fig. 5. Movement patterns of the nine observed Carcharias taurus
individuals. Note: there are two separate movement patterns for Fish Rock
to Seal Rocks.
Photo-identification and monitoring of grey nurse sharks Marine and Freshwater Research 975
30 ‘unique’ left flanks resulted in 9% (20 of 212) being
photographically resighted through I
3
S at least 30 days from
initial documentation. At least 4% (8 of 212) of these indivi-
duals were resighted a third time (90 days from the initial
documentation). A minimum number of two individuals were
resighted a third time after the initial acquisition. The max-
imum number of resights with all other sharks located at Fish
Rock occurred in our field visit in November 2008, with
13 (6%) sharks identified in the database of 212 sharks at Fish
Rock (Tables 1 and 2, respectively). Thus, the time between
resighting events for sharks at Fish Rock ranged from 1 day to
4 years, with an average time of 308 days (95% CI ¼239–
382 days). At least one well-documented Magic Point indivi-
dual was recorded at Fish Rock, 3 months after its last sighting
at Magic Point. The number of males was significantly higher
than the number of females at Fish Rock (x
1
2
¼4.900,
P¼0.027).
Discussion
Carcharias taurus photo library
Our study is the first to use an internet-based method for soli-
citing, acquiring and storing images for a retrospective analysis
on the critically endangered C. taurus population in the eastern
coast of Australia. Further, this is the first time that I
3
S has been
used on a large scale to catalogue C. taurus in Australian waters.
A similar study involving the public has been conducted,
except that spot-analysis estimates were performed manually
(Bansemer and Bennett 2008). The combination of a resourceful
internet site and a computerised database for storing our images
enabled the present study to mark–recapture individuals pho-
tographically, and to observe their rate of site occupancy and
movement patterns. Similar internet sites for the whale shark,
R. typus, have been active for several years (Arzoumanian
et al. 2005). Aided by a computerised spot-recognition system,
Arzoumanian et al. (2005) followed the fate of individuals
through time and space.
Photographic identification with an underwater still camera
and two laser-scaling devices proved to be a successful non-
invasive alternative to the traditional tagging method. Similar
photographic methods have been used on the whale shark,
R. typus (Arzoumanian et al. 2005), and the grey nurse shark,
C. taurus, in Queensland, Australia (Bansemer and Bennett
2008) and in South Africa (van Tienhoven et al. 2007). In view
of the decision by the Commonwealth Government of Australia
to ban permanent tagging for C. taurus (Department of the
Environment and Heritage 2003; Bansemer and Bennett 2008),
a photo-ID study combined withan inexpensive approach (laser-
scaling devices) to obtain size-class estimates may help fulfill the
requirement of estimating potential growth and recovery set out
in the 2002 recovery plan for C. taurus along the eastern coast
of Australia (Environment Australia 2002). Although no other
computer-assisted programs were considered in the present
study, our analyses suggest that spot-recognition programs such
as I
3
S may help sort through a largecollection of images acquired
through community participation and the internet.
Collaborative projects involving monthly surveys and com-
munity help may identify individual C. taurus through time. In
the future, I
3
S (or other similar programs) may be built into the
website www.spotashark.com to share images amongst aca-
demic and conservation communities as per www.whaleshark.
org (Holmberg et al. 2008). However, trained operators to
ensure proper orientation and alignment of spots should check
images.
Programs such as I
3
S were specifically developed for
C. taurus, and although other spot-recognition programs are
available (e.g. Arzoumanian et al. 2005), I
3
S worked well for the
purpose of our study. Perhaps a dual spot-recognition approach
as suggested by Holmberg et al. (2009) for R. typus could
be conducted in future studies of C. taurus, should horizontal
or vertical angle errors occur. In our study, when either the
horizontal or vertical angle exceeded the limits of I
3
S, we
disregarded the image. Additionally, all images were confirmed
by eye to reduce any discrepant results. Photographs of even
moderate quality can also be used, provided the arrangement
of spots and their relative position to one another are initially
accurately recorded in the spotting-out procedure (van
Tienhoven et al. 2007; S. M. Barker, unpubl. data).
Although photographs were extremely useful for identifica-
tion with the I
3
S program, the accuracy of this system relied on a
shark being perpendicular to the focal axis, with no lateral body
flexion (Bansemer and Bennett 2009). Moreover, because of the
opportunistic nature of our web-based study, sampling did not
enable careful calibration of the sampling effort and may have
underestimated the true population (Holmberg et al. 2008).
Some tourist divers in our study appeared to be overwhelmed
by the experience of diving with these sharks and did not always
record the number of male, female and juvenile sharks. Thus, we
were able to acquire more images per dive and record the shark’s
gender during our surveys. Graham and Roberts (2007) also
reported problems with public involvement during their mark–
resight study where mistakes were often made by tourists
forgetting to record the tag number. Moreover, there is also
room for duplicating counts in previous volunteer-based visual
census surveys conducted for C. taurus in Australia, although a
thorough photo-ID analysis of the sharks’ pigmentation spots
could remove this variable in future population estimates.
Although the reliability and accuracy of the estimates may be
greatly increased by acquiring just the pre-caudal length, this
method has drawbacks because it reduces the ability to accu-
rately age individuals within a population. Age estimates based
on the total length of C. taurus are now well studied using von
Bertalanffy growth curves (Goldman et al. 2006). Accurate
total-length estimates require the use of stereophotography.
Such systems are expensive and require two correctly aligned
cameras. Moreover, they guarantee an accuracy of only 5%
(Klimley and Brown 1983). The laser-scaling method used in
this current study provides an alternative and cost-effective
approach for baseline demographic-population analyses using
size-class categories.
Philopatric findings
Twenty identifiable C. taurus individuals repeatedly visited the
Magic Point site during the 4 years of the present study. There
was a high rate of site fidelity for individuals between the first
and subsequent visits to the Magic Point site. Although female
sharks were more abundant during 2007, the exact number
of sharks was difficult to quantify because of the inability to
976 Marine and Freshwater Research S. M. Barker and J. E. Williamson
appropriately sex juveniles whose claspers were not clearly
distinguished. However, one female and one male shark did
return to this site after being absent for ,10 months and
43 years, respectively, suggesting that some individuals may
return periodically. Therefore, future studies using the photo-ID
and size-estimate approach demonstrated here may also high-
light the periodicity at this site of larger males and females or
may incorporate visual signs of mating (i.e. mating scars).
Future studies should also consider any offspring that may
subsequently appear at the site, through genetic analyses.
Bansemer and Bennett (2009) determined the reproduction
periodicity and localised movement patterns of C. taurus at
Wolf Rock in south-eastern Queensland by using identification
(by eye) of the pigmentation spots and other distinguishing
marks such as fish hooks and tail nicks. Interestingly, mature
females of C. taurus remained close to a rock formation and
were segregated from the rest of the C. taurus population.
Similar long-term habitat and movement patterns have been
observed for C. taurus in southern African waters, by using the
same I
3
S as used in the current study (van Tienhoven et al.
2007).
Site-to-site movement patterns
Carcharias taurus undergoes coastal seasonal migration asso-
ciated with the reproductive cycle (Cliff 1989; Gilmore 1993;
Pollard et al. 1996) and governed by water temperature (Parker
and Bucher 2000; Compagno 2001). Dicken et al. (2006b)
postulated that juvenile C. taurus individuals in South African
waters remain in geographically distinct nursery areas for the
first 4–5 years of life before joining the subadult and adult
populations. During our study at Magic Point, at least three
young juvenile sharks displayed similar site attachment. Two
individuals remained at this site for ,12 months and one indi-
vidual remained on site for ,3 years. Mature female and male
sharks also displayed site attachment at all sites studied. In some
instances, individuals were initially documented before being
caught by fishermen and then subsequently resighted during
photographic analysis, with large protruding fishing lures or
hooks. Photo-ID methods, therefore, may help gauge an
appropriate level of protection of C. taurus, especially since
some individuals remain at particular habitats for more than
1 year, thus increasing the likelihood of being caught by fishing
gear while anglers target less vulnerable species at nearby
locations (Bansemer and Bennett 2009, 2010). Their philopatry
for specific sites may lead to such impacts being more sig-
nificant for the critically endangered population in the eastern
coast of Australia.
Strong philopatric and movement behaviour for C. taurus
was observed during the present study. However, the exact
timing of sharks either leaving or arriving at these sites during
photographic resighting remains unclear. This aspect would
require multiple years to collect sufficient data relating to
temperature, sex and size classes of male, female and young-
of-the-year juvenile sharks. Otway et al. (2004) suggested that
the female and juvenile sharks are more sensitive to impaired
population growth and recovery because of their preference for
shallow water and are more at risk to fishing mortality as much
of the commercial and recreational fishing is located in inshore
waters. Removing beach-meshing nets during certain months
from sites where C. taurus has been photographed may reduce
the by-catch previously seen in NSW (Reid and Krogh 1992).
Size-class estimate, mark–resighting and sex-ratio studies
Otway and Burke (2004) found that C. taurus occurred in dis-
tinct size and sex classes, separated by the Seal Rocks, NSW,
location. Proportionally more juveniles and adult males occur-
red north of Forster, whereas more juveniles and adult females
were found south of Seal Rocks. Sexually mature females and
juveniles were generally found in shallower (inshore) waters,
whereas males tended to spend time in deeper waters (Otway
and Burke 2004). Further, male and female sharks moved north
over autumn and winter. Similar male : female ratios were found
in the current study for Magic Point and Fish Rock. For example,
proportionally more female and juvenile sharks were observed
south of Seal Rocks at Magic Point, at depths of ,16 m, whereas
proportionally larger males and females were found at Fish
Rock Cave. Further, there were also proportionally more males
in deeper waters of ,25 m at Fish Rock in March 2009. We
noted northward movements over winter, with at least four
female sharks moving from Magic Point to Seal Rocks and
Fish Rock in 2008. The time of leaving Magic Point was well
documented because of monthly surveys of this site. However,
the time of arrival at their second destination in winter at the
northern sites could not be accurately determined. Additionally,
although some individuals moved south during our study, owing
to the rather small sample size of our study, we are unable to
draw any conclusions about movement patterns for this popu-
lation or the timing of any movements because of the ‘time at
liberty’ between photographic recaptures for these sharks.
Increasing the effort through more frequent visits at the northern
sites may help distinguish the time of immigration and emi-
gration of sharks at these sites. Coupled with photographic
identification of animals caught as ‘by-catch’ (i.e. beach-
meshing, recreational and commercial fishing), a more accurate
estimate of the critically endangered eastern coast population
may unfold. If photographic recapture events are undertaken
within 4 weeks (preferably sooner) of the first encounter, the
exact placement of the particular individual in terms of their
seasonal use of a site can often be well documented. Subsequent
movement events can then be predicted on the basis of the
lifecycle of the particular individual in question.
Recommendations and future work
Community-based management through ecotourism and a
photographic website to obtain C. taurus images may prove to
be an efficient and cost-effective way of acquiring additional
information and building understanding. This knowledge may
assist with conservation and therefore the long-term survival of
the species worldwide. However, community-based projects
mainly rely on amateur divers and/or photographers to submit
their images over the web, which may undermine the true
abundance of C. taurus at their aggregation sites. This issue may
be addressed by utilising the internet site more effectively, with
a more structured approach giving access to academic and
community-based projects. Our research group will continue to
focus on monthly surveys at Magic Point and we are hoping
to dovetail with other academic projects to conduct surveys at
Photo-identification and monitoring of grey nurse sharks Marine and Freshwater Research 977
other known sites to increase the survey output. A more struc-
tured approach could clarify the local population dynamics,
migration routes, site fidelity and critical breeding and nursery
areas for C. taurus.
Acknowledgements
We thank Pro Dive Manly, Cardno Ecology Laboratory Pty Ltd, Eco Divers,
contributing photographers and dive teams and members of the Marine
Ecology Group at Macquarie University for their invaluable assistance in the
field. Thanks also go to Vic Peddemors and Marcus Lincoln Smith for advice
that greatly improved the quality of our research, and to Andrew Boulton and
two anonymous referees who provided invaluable feedback on this manu-
script. This research was funded by NSW DPI, a FRC Research Award, an
AMSA student Award and Macquarie University. Special thanks go to Peter
Simpson for his financial assistance with setting up the www.spotashark.
com website. This research was conducted in accordance with the Animal
Ethics Committee Research Office at Macquarie University (Reference
number: 2006/008) and complies with current occupational diving laws
in Australia. Thanks also go to all the contributing photographers involved
in the Spot-A-Shark project, with special thanks to Peter and Kevin Hitchins
(South West Rocks Dive Centre), Robin Nagy, Anita Roche, Peter
McGee and Jeremy Weinman, whose images were used in this publication
(see Accessory Publication to this paper). Thanks also go to Mark Gray,
Paul Krattiger, Nicci Kershler, Nigel Coombes, Don Silcock, Silke
Stuckenbrock, Dave Thomas, Jayne Jenkins, Lynda Clarke, the Underwater
Research Group (URG), Eco Divers and the Ryde Underwater Club. We
would like to acknowledge Phil Bowman who started an earlier manual spot
recognition project to monitor the eastern population of the Australian grey
nurse shark in 1987.
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Photo-identification and monitoring of grey nurse sharks Marine and Freshwater Research 979
... Seasonality in SMP catches was related to species-specific migratory patterns along the east coast of Australia using the 8 months of fishing effort. The influence of physical characteristics of netted areas influencing shark captures in the nets were assessed using: (i) proportion of each beach that is covered by a net (net:beach) (%); (ii) percentage of rocky reef cover within a 1 km 2 quadrat centred on net location, downloaded from the NSW Government database (NSW Government, 2019); and (iii) distance to nearest known grey nurse shark aggregation site (Figure 1) to assess whether the species would be more vulnerable to capture in the nets deployed near to where these aggregations occur (Barker and Williamson, 2010). The environmental influence of sea surface temperature (SST; • C) was also tested (Wintner and Kerwath, 2018). ...
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... These non-invasive methods can be applied to other aggregation sites where individuals can be reliably distinguished and reidentified over time (Marshall and Pierce 2012). Species, such as the white shark (Carcharodon carcharias; Domeier and Nasby-Lucas 2007), sand tiger shark (Carcharias taurus; Van Tienhoven et al. 2007;Barker and Williamson 2010), bull shark (Carcharhinus leucas; Brunnschweiler and Baensch 2011) and zebra shark (Stegostoma fasciatum; Dudgeon et al. 2008), that aggregate in particular locations or have small home ranges, are suitable for photo identification and, potentially, growth studies using these methods. Aggregation sites where individual whale sharks are re-sighted annually (Norman et al. 2017) could also benefit from our methodology. ...
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Whale sharks (Rhincodon typus) are an endangered species whose growth and reproductive biology are poorly understood. Given their conservation concern, estimating growth parameters, as traditionally derived from vertebral samples of dead animals, is challenging. We used a non-invasive approach to investigate growth parameters of whale sharks frequenting the South Ari Atoll, Maldives, by analysing repeat measurements of free-swimming sharks over a 10-year period. Total lengths of the sharks were estimated by three measurement methods. Visual estimates underestimated the sizes of large sharks, whereas laser and tape measurements yielded results that were similar to one another. The Maldives aggregation consisted of primarily male (91%) juvenile (total length = 3.16-8.00 m) sharks and sharks new to the area were significantly smaller than were returning sharks, which suggests that this site may constitute a secondary nursery ground. Estimates of von Bertalanffy (VBG) growth parameters for combined sexes (L∞ = 19.6 m, k = 0.021 year⁻¹) were calculated from 186 encounters with 44 sharks. For males, VBG parameters (L∞ = 18.1 m, k = 0.023 year⁻¹) were calculated from 177 encounters with 40 sharks and correspond to a male age at maturity of ∼25 years and longevity of ∼130 years. Differences between these estimates and those from other studies underscore the need for regional studies.
... As with other shark species where photo-ID catalogues exist (e.g. white shark Carcharadon carcharias, whale shark Rhincodon typus), with repeat deployments this information could potentially be used to track individuals throughout their range [56][57][58] . With adequate coverage and seasonality to In a comparative context, it is also revealing to examine our Greenland shark video survey results from unexploited regions to estimates of dominant shark local density and biomass from intensively sampled, pristine tropical areas. ...
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Baited remote underwater video cameras were deployed in the Eastern Canadian Arctic, for the purpose of estimating local densities of the long-lived Greenland shark within five deep-water, data-poor regions of interest for fisheries development and marine conservation in Nunavut, Canada. A total of 31 camera deployments occurred between July-September in 2015 and 2016 during joint exploratory fishing and scientific cruises. Greenland sharks appeared at 80% of deployments. A total of 142 individuals were identified and no individuals were observed in more than one deployment. Estimates of Greenland shark abundance and biomass were calculated from averaged times of first arrival, video-derived swimming speed and length data, and local current speed estimates. Density estimates varied 1-15 fold among regions; being highest in warmer (>0 °C), deeper areas and lowest in shallow, sub-zero temperature regions. These baited camera results illustrate the ubiquity of this elusive species and suggest that Nunavut's Lancaster Sound eco-zone may be of particular importance for Greenland shark, a potentially vulnerable Arctic species.
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The capture of recently inseminated or pregnant specimens of Carcharias taurus, Isurus paucus, I. oxyrinchus, Alopias superciliosus and A. vulpinus has allowed new information to be obtained on the reproductive biology of these species. Oophagy and embryonic cannibalism (adelphophagy) have been documented in C. taurus, but only oophagy in other lamnoid species. The occurrence of up to nine embryos of similar size per uterus in Isurus and no indication of functional erect teeth in embryos leaves considerable doubt that embryophagy occurs in this genus. Considerable data has been collected on Carcharias taurus which allows a lamnoid reproductive model to be developed and tested, in spite of the obvious differences between the reproductive biology of this species and other lamnoids. Gonad structure, ovarian development, fertilization, early embryonic differentiation, embryonic nutrition and parturition, in C. taurus and other lamnoids differs significantly from other elasmobranchs.
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