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Population Structure, Abundance and Movement of Whale Sharks in the Arabian Gulf and the Gulf of Oman

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Data on the occurrence of whale sharks, Rhincodon typus, in the Arabian Gulf and Gulf of Oman were collected by dedicated boat surveys and via a public-sightings scheme during the period from 2011 to 2014. A total of 422 individual whale sharks were photo-identified from the Arabian Gulf and the northern Gulf of Oman during that period. The majority of sharks (81%, n = 341) were encountered at the Al Shaheen area of Qatar, 90 km off the coast, with the Musandam region of Oman a secondary area of interest. At Al Shaheen, there were significantly more male sharks (n = 171) than females (n = 78; X2 = 17.52, P < 0.05). Mean estimated total length (TL) for sharks was 6.90 m ± 1.24 (median = 7 m; n = 296). Males (7.25 m ± 1.34; median = 8 m, n = 171) were larger than females (6.44 m ±1.09; median = 7 m, n = 78; Mann-Whitney U test, p < 0.01). Of the male sharks assessed for maturity 63% were mature (n = 81), with 50% attaining maturity by 7.29 m and 100% by 9.00 m. Two female sharks of >9 m individuals were visually assessed as pregnant. Connectivity among sharks sighted in Qatari, Omani and UAE waters was confirmed by individual spot pattern matches. A total of 13 identified sharks were re-sighted at locations other than that at which they were first sighted, including movements into and out of the Arabian Gulf through the Strait of Hormuz. Maximum likelihood techniques were used to model an estimated combined population for the Arabian Gulf and Gulf of Oman of 2837 sharks ± 1243.91 S.E. (95% C.I. 1720–6295). The Al Shaheen aggregation is thus the first site described as being dominated by mature males while the free-swimming pregnant females are the first reported from the Indian Ocean.
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RESEARCH ARTICLE
Population Structure, Abundance and
Movement of Whale Sharks in the Arabian
Gulf and the Gulf of Oman
David P. Robinson
1,7
*, Mohammed Y. Jaidah
2
, Steffen Bach
3
, Katie Lee
4
, Rima
W. Jabado
5
, Christoph A. Rohner
6
, Abi March
4
, Simone Caprodossi
7
, Aaron
C. Henderson
8
, James M. Mair
1
, Rupert Ormond
1,9
, Simon J. Pierce
6
1Heriot-Watt University, Edinburgh, United Kingdom, 2Qatar Ministry of Environment, Doha, Qatar,
3Maersk Oil Research and Technology Centre, Doha, Qatar, 4Environment Department, University of
York, York, United Kingdom, 5Gulf Elasmo Project, Dubai, United Arab Emirates, 6Marine Megafauna
Foundation, Truckee, CA, United States of America, 7Sharkwatch Arabia, Dubai, UAE, 8School of Field
Studies, Turks & Caicos Islands, South Caicos, 9Marine Conservation International, Edinburgh, United
Kingdom
*sharkwatcharabia@gmail.com
Abstract
Data on the occurrence of whale sharks, Rhincodon typus, in the Arabian Gulf and Gulf of
Oman were collected by dedicated boat surveys and via a public-sightings scheme during
the period from 2011 to 2014. A total of 422 individual whale sharks were photo-identified
from the Arabian Gulf and the northern Gulf of Oman during that period. The majority of
sharks (81%, n = 341) were encountered at the Al Shaheen area of Qatar, 90 km off the
coast, with the Musandam region of Oman a secondary area of interest. At Al Shaheen,
there were significantly more male sharks (n = 171) than females (n = 78; X2 = 17.52, P <
0.05). Mean estimated total length (TL) for sharks was 6.90 m ±1.24 (median = 7 m; n =
296). Males (7.25 m ±1.34; median = 8 m, n = 171) were larger than females (6.44 m ±1.09;
median = 7 m, n = 78; Mann-Whitney U test, p <0.01). Of the male sharks assessed for
maturity 63% were mature (n = 81), with 50% attaining maturity by 7.29 m and 100% by
9.00 m. Two female sharks of >9 m individuals were visually assessed as pregnant. Con-
nectivity among sharks sighted in Qatari, Omani and UAE waters was confirmed by individ-
ual spot pattern matches. A total of 13 identified sharks were re-sighted at locations other
than that at which they were first sighted, including movements into and out of the Arabian
Gulf through the Strait of Hormuz. Maximum likelihood techniques were used to model an
estimated combined population for the Arabian Gulf and Gulf of Oman of 2837 sharks ±
1243.91 S.E. (95% C.I. 17206295). The Al Shaheen aggregation is thus the first site
described as being dominated by mature males while the free-swimming pregnant females
are the first reported from the Indian Ocean.
PLOS ONE | DOI:10.1371/journal.pone.0158593 June 30, 2016 1/18
a11111
OPEN ACCESS
Citation: Robinson DP, Jaidah MY, Bach S, Lee K,
Jabado RW, Rohner CA, et al. (2016) Population
Structure, Abundance and Movement of Whale
Sharks in the Arabian Gulf and the Gulf of Oman.
PLoS ONE 11(6): e0158593. doi:10.1371/journal.
pone.0158593
Editor: Chaolun Allen Chen, Biodiversity Research
Center, Academia Sinica, TAIWAN
Received: January 22, 2016
Accepted: June 17, 2016
Published: June 30, 2016
Copyright: © 2016 Robinson et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any
medium, provided the original author and source are
credited.
Data Availability Statement: All relevant data are
within the paper and its Supporting Information files.
Funding: Logistics for this project were provided by
the Qatar Ministry of Environment (QMOE) and
Maersk Oil Research and Technology Centre
(MORTC). SB acted as an independent researcher
throughout this study with financial support in the
form of a salary from the MORTC. The MORTC did
not have any additional role in the study design, data
collection and analysis, decision to publish, or
preparation of the manuscript. DPRs work on this
manuscript was supported by two small grants from
Introduction
The whale shark, Rhincodon typus (Smith, 1828), is the worlds largest fish. This species is dis-
tributed throughout tropical and warm temperate seas [1]. The whale shark is one of three fil-
ter-feeding shark species, and preys on a variety of nektonic and planktonic organisms [1,2].
Although significant gaps in our knowledge of its biology still exist [3], the whale shark has
been classified as Vulnerable on the IUCN Red List of Threatened Species [4] due to anthropo-
genic pressures, particularly directed fisheries in south-east and south Asia [510]. A challenge
in conservation assessment to date has been a lack of knowledge on the population ecology of
mature whale sharks.
Whale sharks form feeding aggregations in a number of regions around the world, including
Western Australia [11], Belize [12], northern Mexico [13], the Philippines [14], Djibouti [15],
Mozambique [16], Tanzania [17], the Maldives [18,19], the Seychelles [20,21], Red Sea [22]
and Qatar [23]. All of these aggregation sites, excepting the Red Sea where the sex ratio is 1: 1
[22], are frequented primarily by juvenile males [15,19,2427]. In most areas, the mean total
length (TL) of sharks is between 6 and 8 m [27], the exceptions being Djibouti and the Red Sea
where smaller mean sizes of 3.7 m and 4.0 m respectively have been documented [15,22]. In
male whale sharks, visual assessment of claspers can be used to determine maturity [17,28].
Size at maturity for males varies from approximately 7 m in the Caribbean [29] to 8 m in West-
ern Australia [28,29], with sizes estimated visually, to 9.16 m in Mozambique [27] estimated by
laser photogrammetry [30]. Maturity in free-swimming female whale sharks cannot be assessed
unless pregnancy is visibly indicated by a distended and swollen abdomen [31,32]. The few
places in the world where pregnant female whale sharks have been documented include sites
off the Pacific and northwestern Caribbean Sea coasts of Mexico [13,33], Taiwan [34], and
around the northern Galapagos Islands off Ecuador [32,35]. Based on those limited data, size
at maturity for females is approximately 9 m [36].
The use of photographic identification in elasmobranchs is a reliable and non-intrusive
method of identifying individual animals and obtaining information about populations [37].
Taylor [38] took images of whale sharks at Ningaloo Reef, Australia in an attempt to use scars
as a means of identification and found that the colour patterns on the sides of the sharks were
unique and stable over time. It was concluded that these patterns could be used to confirm the
identification of individuals. Norman [39] found that the area behind the fifth gill slit and
above the pectoral fin was well-suited to identify individual whale sharks; two sharks were
identified at Ningaloo over a 12-year period confirming stability for at least this period. Sight-
ings and re-sightings of individuals within a population can be used in maximum likelihood
models to estimate population size and residency [4042]. The Lagged Identification Rate
(LIR) metric, defined by Whitehead [43] as the probability of re-identifying an individual that
was identified some lag time earlier, is a useful modelling approach for opportunistic sighting
data and best-fit LIR models have now been widely applied to estimate population parameters
[36,37,40,42,44].
Before 2010 there were few regional records of whale sharks in the literature from the Ara-
bian Gulf and Gulf of Oman [4551], with the majority reported in the local press. Here we
show that the Al Shaheen area is in fact a globally-significant whale shark hotspot, and the first
to be dominated by mature male whale sharks. Connectivity within the Arabian Gulf and Gulf
of Oman is established, allowing us to provide the first regional population estimate for this
globally threatened species.
Population Ecology of Whale Sharks in Arabia
PLOS ONE | DOI:10.1371/journal.pone.0158593 June 30, 2016 2/18
the Save Our Seas Foundation. SJPs work on this
manuscript was supported by the Shark Foundation
and private donors. The journal publication fees for
this manuscript were provided by the MORTC. All
other funders, apart from collaborators from the
QMOE, had no role in the preparation or decision to
publish the manuscript.
Competing Interests: The commercial affiliation for
this project does not alter the authors' adherence to
PLOS ONE policies on sharing data and materials.
Materials and Methods
Study area
This study incorporates the Arabian Gulf, the Strait of Hormuz and North-West Gulf of Oman
(Fig 1). This area is one of the most economically-important waterways in the world, with
heavy ship traffic related to oil and gas transportation [52]. The Gulf of Oman is up to 2000 m
deep while the Arabian Gulf is shallow throughout, with a maximum depth of just over 90 m
[53]. Marine environmental conditions in the Arabian Gulf are among the most extreme on
the planet [54], exposed to sea surface temperatures regularly exceeding 35°C, and reaching
40°C in some areas in mid-summer, and dropping to below 10°C during winter. The lack of
precipitation and high evaporation rate results in salinity ranging from 2860 ppt. The Gulf of
Oman has more stable and moderate conditions, as the southwest monsoon causes cool-water
upwellings. Surface temperature rarely exceeds 30°C and salinity is normal at around 35 ppt
with little variation over the year [54].
Fig 1. The locations of all study sites and other points of interest for whale sharks in the region, with the Arabian Peninsula (inset).
doi:10.1371/journal.pone.0158593.g001
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Data collection
Data were collected through public submissions of photographs and dedicated field studies.
Seafaring individuals, such as fishers, oil platform workers, leisure divers and tour boat opera-
tors, were encouraged to submit any information on sightings of whale sharks in this region to
the online forum Sharkwatch Arabia (www.sharkwatcharabia.com). Participants were
requested to provide as much information as available about each encounter, including date
and time, location, size, sex and the presence of associated fauna. All encounters were subse-
quently submitted to the Wildbook for Whale Shark database (www.whaleshark.org) to investi-
gate broader-scale connectivity.
Platform-based observations in Al Shaheen
Platform stationed staff working for Maersk Oil provided opportunistic observations of whale
sharks in Al Shaheen. These sightings, often supported by video and photographs, were logged
on a daily basis for four years from 2011 to 2014. The platforms are elevated, with 360° views of
the surrounding waters. All workers were briefed to report sightings and to record the time of
the sighting, along with the estimated number of sharks, to their designated sightings collator.
One person stationed on each of the eight platforms was asked to collate the sightings on a
daily basis from their platform workers and log every time a shark or group of sharks was
observed. Only the maximum number of sharks observed per day in one group from one plat-
form was used in the analysis to eliminate repeat observations and only sharks reported during
daylight hours were used in the final analyses.
Dedicated surveys in Al Shaheen
Permissions for fieldwork and data collection on whale sharks in the Al Shaheen region of
Qatar were given by the Qatar Ministry of Environment. The whale shark has been classified as
Vulnerable on the IUCN Red List of Threatened Species [4] but is not protected in Qatari
waters where the fieldwork was carried out. Data collection was non-invasive using only
photography.
Forty-one at-sea surveys took place during May to September: 7 in 2011, 16 in 2012 and 9 in
each of 2013 and 2014. For safety reasons, no surveys could be conducted when wind speed
exceeded 12 knots. Surveys were carried out from a 10 m vessel, powered by twin 250hp
engines, which took approximately 2 hours to reach the study area, with the start time ranging
from 5 to 9 am. Time in the field ranged from four to six hours. During each survey, a set route
was followed among the eight fixed gas platforms. Whale sharks were detected from sightings
of the first dorsal and or caudal fin breaking the surface of the water.
Upon sighting either an individual or aggregation of sharks, a GPS location and time was
recorded and shark numbers were visually estimated from the boat. A team of 24 researchers
then entered the water, using snorkelling gear and equipped with digital cameras. Researchers
took photographs of the flank area on each shark behind the fifth gill slit and above the pectoral
fin on the left side of the shark for the purposes of individual identification (IDs) [55]. The sex
of each shark was determined by the presence (males) or absence (females) of claspers. Matu-
rity status of male whale sharks was assigned based on the visual inspection of the length and
thickness of claspers using the criteria described in Rohner et al. [27]. Pregnancy in female
sharks was assessed using both estimated TL and the presence of a distinctive swollen abdomen
as described in Acuña-Marrero et al. [32].
Images collected during fieldwork in Al Shaheen were processed for visual matching using
I
3
S software [56] and also submitted directly to the online database Wildbook for Whale Sharks
Population Ecology of Whale Sharks in Arabia
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to look for matches from other observers. The length at which 50% of males reach maturity
(TL50) was calculated using a generalised linear model (GLM) with a binary logit function.
Estimation of total length
Where possible, the size of each animal was estimated, usually by comparison with the boat or
another snorkeler. A subset of sharks was measured using laser photogrammetry. Green lasers
(Sea Turtle Scuba Inc.) were placed 50 cm apart in a custom made steel frame and aligned in
parallel. A camera in an underwater housing was placed between the two lasers in the middle
of the frame. Lasers were calibrated each day before use by measuring the distance between the
projection of the lasers against a marker at varying distances up to 10 m. Images were taken
perpendicular to the sharks and only clear images which displayed the animal in a stretched
position were used. Methods developed by Rohner et al. [27,30] were used to estimate total
length from a measured body proportion. To test the accuracy of researcher length estimates,
laser photogrammetry estimates were compared to the researchersvisual estimates. Laser pho-
togrammetry was performed throughout the 2012 season and 13 sharks had a suitable image
for analysis as well as an independent visual size estimate.
Spatial analyses
All whale shark encounter locations reported through the project were input to ArcGIS 10.2.1.
The kernel density toolwas used to calculate occurrence magnitude per km
2
. To determine
areas of overall and core habitat usage, 50% and 95% Volume Contours (PVC) were produced.
Both kernel density and PVC were produced following the methodology outlined in MacLeod
[57].
Regional population estimates
The residence time for individuals within the study area was investigated by using the move-
mentmodule of SOCPROG 2.4 [58] to calculate the Lagged Identification Rate (LIR), the
probability that an animal identified from an area will be re-sighted again after a variable lag
time [40]. All whale sharks that had been individually identified from within or outside the
Gulf from both dedicated fieldwork and submitted encounters were used for this analysis. All
individuals were linked to a location from January 2010 to December 2014. The lowest value
from quasi-Akaike information criterion (QAIC) values were used to select the best fitting resi-
dence model, accounting for over-dispersion of data [43].
The LIR analysis was extended to split the study area into two sub-areas: inside and outside
the Arabian Gulf. The LIR then represented the probability that an individual identified within
one area will be re-sighted in the other area after a specified time lag [40]. A fully-mixed model
was fitted to the data as photo-ID results showed that some movement between areas did occur
(see Results). The model also accounted for movements to a third, hypothetical area (i.e. out-
side the Gulf region). Data were bootstrapped for 100 repetitions to calculate confidence inter-
vals (C.I.) and standard errors (S.E.)
Results
Sightings, photo-identification and seasonality
Three hundred and forty-one individual whale sharks were identified from 980 useable identi-
fication photographs taken during fieldwork in Al Shaheen from 2011 to 2014. Both sex and
size were recorded for 249 individuals. A total of 4350 whale shark sightings were reported
Population Ecology of Whale Sharks in Arabia
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from Al Shaheen between 2011 and 2014 while only one other whale shark was reported out-
side of this area, from a man-made waterway in Doha in 2012.
Excluding Al Shaheen, 81 individual whale sharks were identified from the Arabian Gulf
and Gulf of Oman from records starting in 2004, including the Musandam (n = 46) and Day-
maniyat Islands (n = 27) in Oman, and Fujairah (n = 8) in UAE (Fig 2).
Ninety-five whale shark encounters were reported from the Musandam (Oman). Sightings
were most frequent at Lima Rock (Fig 3) with 64 encounters, followed by Octopus Rock with
19 encounters. Fifty whale shark encounters were reported from the Daymaniyat Islands, with
19 encounters made at the dive sites Junnand Aquariumand nine sharks from Sira.
Forty-three whale shark encounters were reported from UAE waters. The majority of sharks
were seen off Fujairah on the East coast, with 10 sharks encountered at the dive site Martini
Rock followed by six sharks encountered at Dibba Rock. A feeding aggregation of approxi-
mately 10 sharks was reported by fishermen from 35 km off Fujairah in 2012. Twelve sharks
were reported from inside the Arabian Gulf coast of the UAE; all but three of these sharks were
reported from marinas and ports. Two sharks were encountered by members of the public in
2013 off Jumeirah Beach and one shark was reported from the Salman Oil Field about 100 km
offshore of mainland UAE. Body size of sharks ranged between an estimated 4 and 6 m TL.
Most sharks, at all sites, were encountered in the summer months from April to October,
although sharks were encountered year-round outside of Al Shaheen. Sharks seen from
November to March were usually single individuals, while aggregations were common from
Fig 2. Kernel density analysis showing the overall regional occurrence of whale sharks based on submitted sightings.
doi:10.1371/journal.pone.0158593.g002
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April to October. This pattern was consistent with observations from oil platform workers in
Al Shaheen (Fig 4).
Population ecology in Al Shaheen
Sixty individual sharks were identified during 2011, 178 during 2012, 119 during 2013 and 128
during the 2014 season (Table 1). The percentage of re-sighted sharks varied between 13% and
59% with the mean inter-annual re-sight rate being 41%. Re-sights of sharks identified in previ-
ous years were seen in all subsequent field seasons from 2012 to 2014.
There were significantly more male sharks (n = 171) than females (n = 78; X
2
= 17.52,
P<0.05). Whale sharks at Al Shaheen ranged from 4 to 10 m TL (Fig 5), with an overall mean
of 6.9 m ± 1.24 TL (median = 7 m, n = 296). Mean TL for males (7.25 m ± 1.34; median = 8 m,
n = 171) was significantly larger than TL for female sharks (6.44 m ± 1.09; median = 7 m,
n = 78; Mann-Whitney Utest, p <0.01). Mean shark TL from laser photogrammetry was
Fig 3. The frequency of whale shark encounters at different sites around the Musandam Peninsula, together with the 50% and 95%
Percentage Volume Contours (PVC).
doi:10.1371/journal.pone.0158593.g003
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6.80 ± 1.10 m (median 6.74 m, n = 13; range 5.28.2 m). The corresponding mean TL for visual
estimates of the same sharks was 7.00 ± 1.15 m (median = 7 m, n = 13; range 59 m). There
was no significant difference in TL estimates derived from the two different techniques, with
only one individual differing by >1.00 m.
In sharks where both sex and size were recorded, there was a notable male bias (69%,
n = 249). Of the males assessed for maturity, 63% (n = 81) were mature. Males reached matu-
rity between 7 and 9 m TL, with TL
50
at 7.29 m (Chi
2
test: d.f. = 79, Res. Dev = 41.996,
p<0.0001). All sharks were mature at or above 9 m TL. Two 9 m TL females had conspicu-
ously distended and swollen abdomens, suggesting they were pregnant.
Fig 4. Regional whale shark encounters reported to the Sharkwatch Arabia project (Al Shaheen is shown on the right hand Y axis).
doi:10.1371/journal.pone.0158593.g004
Table 1. Inter-annual re-sight rates (%) for individual sharks first identified in Al Shaheen.
Year Identied sharks 2011 re-sights (%) 2012 re-sights (%) 2013 re-sights (%) Total re-sight (%)
2011 60
2012 178 13 13
2013 119 21 38 59
2014 128 15 27 8 50
Mean re-site rate (%) 41
doi:10.1371/journal.pone.0158593.t001
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Regional connectivity and population size
Thirteen sharks were re-sighted at different locations from those at which they were first
recorded (Fig 6). The majority of movements were between Musandam and Al Shaheen. The
longest duration between a re-sighting was four years, for a shark seen in Fujairah in 2010 and
re-sighted at Al Shaheen in 2014. The longest distance travelled was for a shark first identified
in the Daymaniyat Islands (in 2012) and re-sighted in Al Shaheen (2014), an estimated
straight-line journey of 828 km through the Strait of Hormuz.
Modelled LIR gradually declined from 1 to 100 days (Fig 7), suggesting that most sharks left
the area after a short period of residency, but there was a slight increase after approximately
one year, indicating periodic return to the study area by some sharks. Model G was the best fit
to the empirical data based on QAIC (Table 2). In this model scenario, there were an estimated
123.72 sharks ± S.E. 15.3062 (95% C.I. 95.3501152.3211) within Al Shaheen on any given day.
The mean residency time within Al Shaheen was 28.78 days ± S.E. 9.3522 (95% C.I. 8.7195
46.5447), and 62.74 days ± S.E. 16.7609 (95% C.I. 22.660786.947) outside of Al Shaheen.
Lagged Identification Rate was then redefined to model movement between two distinct
sub-areas: (1) Al Shaheen, and (2) the Gulf of Oman including the Strait of Hormuz. The fully
mixed model J (a1 = N) was the best fit (Table 2), although both fully-mixed models had a high
degree of support. These results indicate that sharks regularly move between the Arabian Gulf
Fig 5. Visual size estimates of whale sharks from Al Shaheen.
doi:10.1371/journal.pone.0158593.g005
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and northern Gulf of Oman over inter-annual timescales. The estimated population size for
this entire area was 2837 sharks ± 1243.9 S.E. (95% C. I. 17206295.0).
Sharks were most likely to be re-sighted again within the same area, but there was also a
chance of sighting an individual across all areas (including outside the study region; Table 3).
Discussion
The Al Shaheen field off Qatar, where large aggregations of whale sharks occur in the boreal
summer months, is the main hotspot for whale shark occurrence within the Arabian Gulf and
region. Sharks aggregate here to feed on tuna spawn produced by mackerel tuna, Euthynnus
affinis (Cantor, 1849) [23]. Over 340 whale sharks have been identified from this site since
2011, indicating that the area is a globally-significant feeding area. The majority of whale
sharks encountered in Al Shaheen were mature males, and this is the first global site where a
significant proportion of mature male individuals have been reported. The second identified
regional hotspot was in the Musandam region of Oman. There were high levels of observed
Fig 6. Movements of 13 whale sharks identified from their spot pattern and re-sighted at different locations from those at which they were first
recorded.
doi:10.1371/journal.pone.0158593.g006
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and modelled connectivity between the Arabian Gulf and Gulf of Oman, but no connectivity
with other Indian Ocean sites was noted. This high connectivity allowed the application of
maximum likelihood population models, providing the first regional population estimate for
the species: 2837 sharks ± 1243.9 S.E., albeit with broad 95% confidence intervals of 1720 to
6295 sharks.
Fig 7. The probability of re-identifying an individual whale shark over time (LIR; mean ±S.D.) within Al Shaheen compared to the best-
fitting movement model.
doi:10.1371/journal.pone.0158593.g007
Table 2. Model parameters and comparison for the Lagged Identification Rate of whale sharks.
Model Model descriptions for Al Shaheen ΔQAIC
A Closed 17050
Ba1=N 6334
C Emigration/mortality 6325
D Closed: emigration+reimmigration 6323
E a1 = N; a2 = Mean residence 6325
F Emigration+reimmigration+mortality 6317
G a1 = N; a2 = res time in; a3 = res time out 6270
H a1 = N; a2 = res time in; a3 = res time out; a4 = mortality 6298
Model descriptions for LIR between sub-areas
I Fully Mixed (a1) 0.0054
J Fully Mixed (a1 = N) 0
K MigrationFull Interchange (a1 = diffusion rate from area 1 to area 2; a2 = 1/N) 2.0054
L a1 = N; a2 = mean residence time in area 1 2
N = Population
doi:10.1371/journal.pone.0158593.t002
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Regional occurrence and abundance
Overall, the majority of sharks were encountered in the warm boreal summer months between
April and October. No whale shark feeding activity, from anywhere in the study region, has
been documented from outside these summer months. The seasonal nature of the aggregation
off Qatar matches the seasonal occurrence of whale shark sightings elsewhere around the
region. The Strait of Hormuz, situated north of the Musandam region of Oman, is the sole con-
nection between the Arabian Gulf and the Gulf of Oman. These are productive areas with high
coral coverage, fish numbers, and diversity. Within the Musandam region, most encounters
were recorded from around Lima Rock, particularly from the south side (Fig 3). Lima Rock is
accessible by speedboat for day-tripperdivers from the town of Dibba, increasing diver num-
bers compared to less accessible sites further north that are only reachable by two or three-day
dive trips. Lima Rock is surrounded by an area of comparatively deeper water, and has strong
upwelling currents that may make this area more attractive to whale sharks. The Daymaniyat
Islands are a popular weekend diving destination for UAE and Oman residents. Whale sharks
were most frequently observed on the dive sites on the outside of the island chain, where feed-
ing has also been observed. Although there have been few records of surface feeding within the
Gulf of Oman, studies from elsewhere have shown that sub-surface zooplankton can constitute
a significant proportion of whale shark diet [2]. It is possible that feeding activities are underre-
ported. One shark was encountered at Octopus Rock in the Musandam and then re-sighted
exactly a year later in the same local area. Sharks encountered in Omani waters were re-sighted
one, two, three and four years apart and between different local hotspots showing a level of
affinity to the area. The majority of recreational diving off Oman occurs during the local week-
end, Fridays and Saturdays. Most encounters reported from the Musandam and Daymaniyats
occurred on weekends, suggesting that more sharks would be encountered if diving activity
increased over weekdays.
Within UAE waters, the majority of encounters with whale sharks took place at popular
coastal dive sites on the East coast. Similar to the Musandam, whale sharks have not been
reported to feed here or be resident to the area. However, one feeding aggregation was reported
from 35 km offshore. With a single exception, encounters from the west coast of the UAE were
from marinas or ports along the coast. Several dive companies frequently dive the waters of the
west coast of the UAE, but no whale sharks have been reported to date. One shark was reported
from a UAE platform worker from the Salman Oil Field. Similarly, whale sharks were rarely
reported from Qatari waters outside of Al Shaheen, with only one observation from mainland
Qatar. These sighting data, viewed as a whole, indicate that whale sharks rarely use the shal-
lower coastal waters around the Gulf. As regional whale shark encounters were mainly
recorded from popular recreational areas, data reported may be skewed towards these areas
and underestimate occurrence along this coastline.
The rapid increase in reports and successful identification of sharks by seafaring individuals
from 2004 to 2013 can be attributed to increased diving activity, an increase in the number of
underwater cameras in use, and improving awareness of how to report and submit an
Table 3. The probability of a shark originally identified from an area being re-sighted within a different area.
To Area
Al Shaheen Gulf of Oman Outside
Al Shaheen 0.9093 0.0338 0.0569
From Area: Gulf of Oman 0.1118 0.8856 0.0025
Outside 0.1364 0.2625 0.6011
doi:10.1371/journal.pone.0158593.t003
Population Ecology of Whale Sharks in Arabia
PLOS ONE | DOI:10.1371/journal.pone.0158593 June 30, 2016 12 / 18
encounter. The Sharkwatch Arabia project focused on engaging divers and dive centers
through social media such as Facebook and Twitter. These sites allow for easy sharing of
images and information. As social media presence and online networking tools grow, so will
the reach and success of citizen science initiatives such as Sharkwatch Arabia.
Population ecology in Al Shaheen, Qatar
The majority of regional encounters took place in the Al Shaheen area of Qatar, a major aggre-
gation site for whale sharks [23]. Three hundred and forty-one sharks were identified here
between 2011 and 2014, with a modelled estimate of 124 sharks present each day over the
aggregation season. Sharks varied in size from 4 to 10 m TL, with laser photogrammetric mea-
surements comparable to visual length estimates. The aggregation is male dominated, with a
median length of 8 m. Sharks below 5 m or in excess of 9 m length were rarely encountered. A
high proportion of adult sharks, particularly males, was documented. This is unique amongst
the predictable feeding aggregations examined to date, which are typically biased towards juve-
nile males [17,19,2426,59]. The 7.29 m TL
50
for male maturity was similar to the 7 m estimate
from Quintana Roo in Mexico [36], lower than that reported from Western Australia (8 m,
[28]) and Mozambique (9. 16 m, [27]). Although Indian Ocean-level genetic population struc-
ture has not been shown in whale shark [60], these differences in maturity suggest that limited
broad-scale mixing may occur.
Two 9 m TL female sharks were visually assessed to be pregnant. To date suspected preg-
nant females have only been reported from Galapagos, the Gulf of California and the Gulf of
Mexico [3133], while one confirmed pregnant female was examined in Taiwan [6]. To our
knowledge, Al Shaheen is the only site in the Indian Ocean basin where pregnant female sharks
have been observed. Size of maturity has also been estimated to be around 9 m in the Eastern
Pacific [32,36]. There have been several reports of neonatal sharks off the Balochistan coast in
Pakistan and the discovery of a 58.6cm free swimming neonate suggests that whale sharks do
give birth in the Northern Indian Ocean [61].
Modelled re-sighting data estimated a mean residency time of 29 days within Al Shaheen.
This was similar to the results of mark-recapture modelling of whale sharks from Ningaloo
Reef, where mean residency was estimated to be 33 days [62], and the estimated 2433 days of
sharks off Quintana Roo in Mexico [33]. These latter two sites, which are noted feeding areas
for whale sharks [63,64], contrast with the relatively short estimated 12 day residency at Utila,
where sharks are feeding opportunistically on baitfish [42]. A high degree of inter-annual site
fidelity was also evident amongst Al Shaheen sharks, with 41% of sharks seen in more than one
year, even with the limited number of sampling trips that were possible. This re-sighting rate
was higher than most other whale shark aggregation sites, e.g. 22% in Utila, Honduras [42],
13% in Holbox, Mexico [29], 23% in Djibouti and 28% in Seychelles [26], and 35% in Western
Australia [62].
Surface water temperatures within the Arabian Gulf frequently exceed 35°C [53]. Water
temperatures from zero to 10 m depth at Al Shaheen, where the sharks are feeding for extended
periods of time, can exceed 33°C during the hottest months [23]. Berumen et al. [22] recorded
temperatures ranging from 8°C to 34°C from satellite-tagged sharks in the Red Sea. Whale
shark distribution appears to be limited by <21°C surface temperatures [65,66], but their
upper tolerance is not yet known. The presence of large numbers of whale sharks surface feed-
ing within the Arabian Gulf during the warm summer months for extended periods of time
shows that whale sharks are able to tolerate temperatures in excess of 30°C for hours at a time.
There is evidence for behavioural thermoregulation influencing whale shark dive patterns in
cooler temperatures [67], and they may seek to cool their internal temperature in warmer
Population Ecology of Whale Sharks in Arabia
PLOS ONE | DOI:10.1371/journal.pone.0158593 June 30, 2016 13 / 18
waters [68]. Al Shaheen is likely to be the aggregation site where whale shark experience the
warmest water temperature, so a more detailed evaluation of their coping strategies may pro-
vide insight into their long-term response to a warming ocean climate.
Regional connectivity and population size
Within the Indian Ocean, limited international connectivity has previously been observed
[17,59,62], which has precluded the estimation of population size at a regional scale. In the
present study, whale sharks were recorded and re-sighted in all significant regional hot spots,
suggesting that individual sharks are moving freely between the Arabian Gulf and Gulf of
Oman. The largest number of shared individuals was noted between Musandam and Al Shah-
een, the two areas with the greatest number of recorded encounters (Fig 2). Model selection
identified a fully-mixed model as the most likely scenario. Regional population size was esti-
mated at 2837 ± 1243.9 S.E. The broad 95% confidence intervals bounding this estimate, 1720
to 6295 sharks, indicate that this should be treated as a rough approximation. However, as the
first broad-scale population estimate that has been obtained for whale shark, it is an important
step towards understanding relative abundance of this species and developing management
strategies for the conservation of this globally threatened species.
Conclusions
Research in Al Shaheen began comparatively recently, in 2011, and has already documented
high abundance, long residency time and philopatric behaviour to the site. The reason for the
unique adult male bias observed in the Al Shaheen feeding aggregation has not been identified,
but the sex and size-based segregation inherent in whale shark aggregations globally makes this
an interesting topic to investigate. The high connectivity, and relatively small regional popula-
tion size makes the quantification and mitigation of human threats, and potential management
measures, a priority within the region.
Supporting Information
S1 Fig. An image of a large female whale shark and researcher taken in Al Shaheen.
(TIF)
S2 Fig. An image of the remote and mountainous Musandam Governorate of Oman.
(TIF)
S3 Fig. An aerial image of a whale shark aggregation in the Al Shaheen area showing typical
density of feeding sharks and variation in size.
(TIF)
S1 Data. Whale shark encounter reports in the Arabian Gulf and Gulf of Oman up to and
including 2014.
(XLS)
Acknowledgments
We thank everyone involved in the Qatar Whale Shark Research Project, as well as the staff at
the Qatar Ministry of Environment, Maersk Oil Research and Technology Centre (MORTC),
Qatar and the Qatar Coast Guard for providing the platform to carry out field research in
Qatar. We thank Ali Abdulrahmen from the MORTC for his support and help throughout the
fieldwork; the Maersk Oil platform workers for their continued and dedicated support with
Population Ecology of Whale Sharks in Arabia
PLOS ONE | DOI:10.1371/journal.pone.0158593 June 30, 2016 14 / 18
data collection, especially Soren Stig for his continued enthusiasm. We thank Jonathan Ali
Khan and Warren Baverstock for their support and help with the Sharkwatch Arabia initiative
and thanks also go to every contributing individual and dive center for submitting images or
reporting their whale shark encounters, in particular the Emirates Diving Association. We also
thank Jennifer Schmidt for helpful comments on the manuscript and the Save Our Seas Foun-
dation for their support. Chistoph Rohner and Simon Pierces contribution to this project were
supported by two private trusts, the Shark Foundation, Aqua-Firma, and Waterlust.
Figures throughout this manuscript were created using ArcGIS1software by Esri, please
visit www.esri.com. We acknowledge the use of free vector and raster map data sourced from
www.naturalearthdata.com.
This research has made use of data and software tools provided by Wildbook for Whale
Sharks, an online mark-recapture database operated by the non-profit scientific organization
Wild Me.
Author Contributions
Conceived and designed the experiments: DR MJ SB KL RJ CAR AM SC AH JM RO SP. Per-
formed the experiments: DR MJ SB KL RJ CAR AM SC AH JM RO SP. Analyzed the data: DR
MJ SB KL RJ CAR AM SC AH JM RO SP. Contributed reagents/materials/analysis tools: DR
MJ SB KL RJ CAR AM SC AH JM RO SP. Wrote the paper: DR MJ SB KL RJ CAR AM SC AH
JM RO SP.
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PLOS ONE | DOI:10.1371/journal.pone.0158593 June 30, 2016 18 / 18
... Adult-dominated sites remain a rarity, namely at Darwin's Arch in the Galapagos Islands (ECU), Gorda Banks in Baja California (MEX), at an offshore aggregation in Qatar (QAT), and at St Helena Island (GBR) in the South Atlantic, and at a newly identified area in the mid-equatorial Atlantic off Brazil ( Ram?rezMac?as et al., 2012b;Acu?a-Marrero et al., 2014;Clingham et al., 2016;Macena and Hazin, 2016;Robinson et al., 2016). Whale sharks at Donsol are uncharacteristically larger than those found elsewhere in the Philippines (e.g., 5.2 m mean total length in the Bohol Sea, Authors, unpub. ...
... However, it is similar to the size at maturity from the Gulf of Mexico and Qatar, where 50% maturity in males was estimated at 7.0 m and 7.3 m, respectively ( Ram?rez-Mac?as et al., 2012b;Robinson et al., 2016). In the present study we used visual estimates, which have inherent errors (Rohner et al., 2011;Sequeira et al., 2016), and thus our results are indicative more than absolute. ...
... This is considerably longer than that reported elsewhere. Using the same methodology, whale sharks at Panaon Island, Southern Leyte spent c. 27 days in the area ( Araujo et al., 2016), and at an offshore aggregation in Qatar, whale sharks spent c. 29 days there (Robinson et al., 2016). In Ningaloo Reef, Western Australia, Holmberg et al. (2009) estimated whale sharks spent 33 days in the area using different methods. ...
Article
Full-text available
Donsol in the Philippines is the longest running community-based whale shark (Rhincodon typus) ecotourism site in Southeast Asia, with peak visitation in 2012 of over 27,000 tourists. In order to understand this aggregation and the importance of the area to whale sharks, dedicated photographic identification (photo-ID) research began in 2007. In-water photo-ID surveys were conducted from tourism boats, weather and operator permitting, from December to June between 2007 and 2016. Visual matches of the unique spot patterns of each individual shark were validated by the pattern-recognition software Interactive Individual Identification System (I3S), and on the online database Wildbook for Whale Sharks (www.whaleshark.org). A total of 1,985 photo-ID trips over 895 survey days resulted in 6,786 encounters with R. typus. Combined with encounters from both dedicated research and citizen science dating back to 1998, 479 individual whale sharks were identified, making up 44% of the known whale shark population in the Philippines (n = 1,095). Of these, photographs of the pelvic region confirmed the sex for 158 males and 22 females. Visual size estimates ranged from 2 to 10 m (mean ± SD = 6.5 ± 1.6 m). Maturity in males (LT50) was estimated at 6.8 ± 0.2 m total length, with 53% of males considered mature. Annually, the total number of individuals sighted varied between 15 and 185 (mean ± SD = 104 ± 55.53), with a recruitment of 3–90 new individuals yearly (mean ± SD = 46.8 ± 36.29). Modeled residency using maximum likelihood methods suggested whale sharks spent 49.8 ± S.E. 14.5 [95% CI (32.3–78.6)] days in Donsol each season, with 47.1–60.8 whale sharks at any one time during the season. Twenty individuals were recorded through photo-ID at other sites across the Philippines. The extended residency of whale sharks at Donsol, paired with the presence of sexually mature animals and the economic value of the tourism industry, highlights the importance of Donsol for this endangered species.
... However, the population structure (number of sharks at a given length) at most individual coastal constellations is similar to a bell curve (i.e. Rowat et al. 2011;Araujo et al. 2014Araujo et al. , 2017Robinson et al. 2016), implying that larger sharks of both sexes are, in fact, leaving these sites. The question, then, is where are they going, and why? ...
... As noted previously, few sharks exceeding 9 m in length are seen in coastal waters. Interestingly, length and sex data from some constellations, such as Qatar (Robinson et al. 2016) and ...
... For example, it appears that mature male blue sharks Prionace glauca will routinely try to mate with smaller immature females (Calich and Campana 2015). Mature males represent 63% of the identified whale shark population in Qatar (Robinson et al. 2016). Tagged adult males at Al ...
... The online database 'Manta Matcher' Wildbook (see text footnote 1) ) was used to collate data (date, time, location, and identifying photographs of manta rays) from the public and researchers (Germanov and Marshall, 2014) (Supplementary Figure 2). Citizen science was an important part of this process and has been demonstrated to provide basic information when data are limited (e.g., Jaine et al., 2012;Rohner et al., 2013;Couturier et al., 2014;Germanov and Marshall, 2014;Robinson et al., 2016;Norman et al., 2017;Pierce et al., 2018) and engage local communities in marine stewardship (Miller-Rushing et al., 2012). Approximately 40 observers were trained to take photos of manta ray ventral spot patterns, record sex and maturity data, estimate size, and to note relevant behaviors (discussed below), as well as the maximum number of boats present at the sites. ...
... A modified maximum likelihood approach was used to compare manta ray re-sighting data against residency models, implemented in the program SOCPROG 2.8 (Whitehead, 2009). These statistical models were previously used for manta rays in Hawaii by Deakos et al. (2011) and in several more recent studies on whale sharks Rhincodon typus ( Fox et al., 2013;Robinson et al., 2016;McCoy et al., 2018) and cetaceans (e.g., Chabanne et al., 2017). This approach determines the spatial and temporal distribution of sampling effort using the re-identification data itself, making this approach suitable for opportunistic and presence-only sighting data ( Fox et al., 2013). ...
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Manta rays (Mobula spp.) are highly valued in nature-based tourism globally. In Indonesia, although manta rays are protected, critical information is lacking on their habitat use, population dynamics and movements. We investigate the population structure and residency patterns of reef manta rays (Mobula alfredi) in the Nusa Penida Marine Protected Area (MPA). From photo-identification data logged by citizen scientists and trained observers (mantamatcher.org), we identified 624 reef manta rays from 5,913 sightings (January 2012–April 2018) based on their unique ventral coloration patterns. Year-round records were collected from two shallow (<20 m) reefs – Manta Bay (MB; n = 3,029 sightings) and Manta Point (MP; n = 3,058) – that are used frequently by tourism operators. Maximum likelihood techniques and a Markov movement analysis were used to model residency patterns and movement between these sites within the MPA. Manta rays at MB were predominantly male (64%, n = 261 individuals), with immature males (14%, n = 59) being sighted most frequently (39%, n = 1,170). In contrast, few immature individuals were sighted at MP (6%, n = 28), and they were sighted on few occasions (2%, n = 45), while mature female manta rays comprised 26% (n = 127) of the MP community and were the most frequently sighted (48%, n = 1,413). Lagged identification rates indicated high site fidelity at each location. However, 44% (n = 278) of individuals moved between the two sites and cumulative discovery curves showed a continued recruitment of individuals over the 6 years of the study. In addition, the behaviors displayed by the manta rays differed markedly between the two sites: MB appears to be a foraging ground, especially for juveniles, and potentially a nursery, while MP is used mainly for cleaning and courtship, indicating a social and reproductive site. Reproductive behavior coincided with the peak annual sightings in May. To prevent disturbance to this threatened species by tourism, regulations for the number of boats and interactions, especially during key reproductive times should be considered. Further, strict fishing regulation in the area is recommended as fishing gear entanglement was identified as a threat to this population. Keywords: Mobula alfredi, citizen science, photo-identification, population structure, animal behavior, site fidelity, nursery, tourism management
... Clasper morphology was used to distinguish juvenile from adult males: claspers are short, soft, and smooth in sexually immature males, but quickly grow and calcify during maturation (Norman & Stevens, 2007). Females were categorized as either juvenile or adult based on their estimated lengths and external indications of pregnancy (i.e., distended abdomen; Ramírez-Macías, Vázquez-Haikin & Vázquez-Juárez, 2012;Acuña Marrero et al., 2014;Robinson et al., 2016). While we acknowledge the difficulty of confirming pregnancy in adult female sharks, the visual assessments made for the present study were based on extensive field observations of adult sharks, both pregnant and non-pregnant, by the first author ( Ramírez-Macías, Vázquez-Haikin & Vázquez-Juárez, 2012). ...
... Given the number of researchers surveying coastal areas around the world, the possibility that large whale sharks are commonly using this habitat seems remote (Hueter, Tyminski & De la Parra, 2013). Dedicated surveys in some regions (RamírezMacías, Vázquez-Haikin & Vázquez-Juárez, 2012;Ketchum, Galván-Magaña & Klimley, 2013), and observations from others (Afonso, McGinty & Machete, 2014;Robinson et al., 2016), indicate that mature whale sharks primarily inhabit offshore habitats. While the use of inshore habitats by juvenile sharks has been associated with zooplankton biomass in Bahía de La Paz and Bahía de Los Angeles (Hacohen-Domené, Galván-Magaña & Ketchum-Mejia, 2006;Nelson & Eckert, 2007;Ketchum, Galván-Magaña & Klimley, 2013), the seasonal biomass of zooplankton was not strongly correlated with adult shark presence in the outer Bahía de La Paz (Ketchum, Galván-Magaña & Klimley, 2013). ...
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Eight whale sharks tagged with pop-up satellite archival tags off the Gulf of California, Mexico, were tracked for periods of 14–134 days. Five of these sharks were adults, with four females visually assessed to be pregnant. At least for the periods they were tracked, juveniles remained in the Gulf of California while adults moved offshore into the eastern Pacific Ocean. We propose that parturition occurs in these offshore waters. Excluding two juveniles that remained in the shallow tagging area for the duration of tracking, all sharks spent 65 ± 20.7% (SD) of their time near the surface, even over deep water, often in association with frontal zones characterized by cool-water upwelling. While these six sharks all made dives into the meso- or bathypelagic zones, with two sharks reaching the maximum depth recordable by the tags (1285.8 m), time spent at these depths represented a small proportion of the overall tracks. Most deep dives (72.7%) took place during the day, particularly during the early morning and late afternoon. Pronounced habitat differences by ontogenetic stage suggest that adult whale sharks are less likely to frequent coastal waters after the onset of maturity.
... Their movement may be affected by oceanographic conditions, such as temperature, current, and chlorophyll-a concentration [2,4,6,7]. Juvenile whale sharks measure less than 7 m [8][9][10][11][12], females less than 9 m [13][14][15]. Several studies have shown that juvenile whale sharks tend to reside in coastal habitats [12,[16][17][18] whereas adults tend to be in pelagic habitats [19,20]. ...
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The whale shark ( Rhincodon typus ) is an endangered species with a declining global population. The South Ari Atoll Marine Protected Area (SAMPA), Maldives, is one of few locations globally where year-long residency of individuals occurs. This SAMPA aggregation appears to consist almost exclusively of immature males. Due to its year-round residency, this local aggregation is subjected to a high degree of tourism pressure. This ecotourism contributes to the high level of interest and protection offered to whale sharks by the local community. Unfortunately, if regulations are not followed or enforced, tourism can bring with it major stressors, such as accidental injuries. We used POPAN capture-mark-recapture models and lagged identification rate analysis to assess the effect of major injuries on whale shark residency within SAMPA. Injuries may be obtained outside SAMPA. We found individuals with major injuries had a higher apparent survival in the area than those without. Lagged identification rates also demonstrated that sharks with major injuries are more likely to return to the area. We suggest that major injuries result in sharks prolonging their time in the developmental habitat. These findings have implications for individual fitness and the population viability of this endangered species. We propose targeted conservation strategies be considered to protect sharks from further injury. Based on the presented spatio-temporal distributions of sharks, and current local knowledge of sighting patterns, speed limit zones and propeller-exclusion zones should be implemented and enforced. If carried out alongside tourist education, these measures will contribute to the protection of whale sharks within SAMPA and beyond. Furthermore, our results can aid research direction, alongside regulation and enforcement development, at similar sites worldwide.
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Recent advances in tracking technologies and analytical approaches allow for deeper insights into the movement ecology of wide-ranging fishes. The whale shark Rhincodon typus is an endangered, highly migratory species with a wide, albeit patchy, distribution through tropical oceans. Aerial surveys along the southern Mozambican coast, conducted over a 5-year period, documented the highest densities of whale sharks to occur within a ~200 km long stretch of the Inhambane Province, with a pronounced hotspot adjacent to Praia do Tofo. We tagged 15 juvenile whale sharks with SPOT5 satellite tags off Praia do Tofo and tracked them for 1–87 days (mean = 26 days) as they dispersed from this area. Sharks travelled between 10 and 2,737 km (mean = 738 km) at a mean horizontal speed of 29 ± 30.7 SD km day ⁻¹ . While several individuals left shelf waters and travelled across international boundaries, most sharks stayed in Mozambican coastal waters over the tracking period. We tested for whale shark habitat preferences, using sea surface temperature, chlorophyll- a concentration and water depth as variables, by computing 100 random model tracks for each real shark based on their empirical movement characteristics. Whale sharks spent significantly more time in cooler, shallower water with higher chlorophyll- a concentrations than model sharks, suggesting that feeding in productive coastal waters is an important driver of their movements. Our results show that, while whale sharks are capable of long-distance oceanic movements, they can spend a disproportionate amount of time in specific areas. The increasing use of large-mesh gill nets in this coastal hotspot for whale sharks is a clear threat to regional populations of this iconic species.
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Background The whale shark ( Rhincodon typus ) is known to aggregate in a number of coastal locations globally, however what causes these aggregations to form where they do is largely unknown. This study examines whether bathymetry is an important driver of coastal aggregation locations for R. typus through bathymetry’s effect on primary productivity and prey availability. This is a global study taking into account all coastal areas within R. typus’ range. Methods R. typus aggregation locations were identified through an extensive literature review. Global bathymetric data were compared at R. typus aggregation locations and a large random selection of non-aggregation areas. Generalised linear models were used to assess which bathymetric characteristic had the biggest influence on aggregation presence. Results Aggregation sites were significantly shallower than non-aggregation sites and in closer proximity to deep water (the mesopelagic zone) by two orders of magnitude. Slope at aggregation sites was significantly steeper than non-aggregation sites. These three bathymetric variables were shown to have the biggest association with aggregation sites, with up to 88% of deviation explained by the GLMs. Discussion The three key bathymetric characteristics similar at the aggregation sites are known to induce upwelling events, increase primary productivity and consequently attract numerous other filter feeding species. The location of aggregation sites in these key areas can be attributed to this increased prey availability, thought to be the main reason R. typus aggregations occur, extensively outlined in the literature. The proximity of aggregations to shallow areas such as reefs could also be an important factor why whale sharks thermoregulate after deep dives to feed. These findings increase our understanding of whale shark behaviour and may help guide the identification and conservation of further aggregation sites.
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The whale shark (Rhincodon typus) is a wide-ranging, filter-feeding species typically observed at or near the surface. This shark's sub-surface habits and behaviors have only begun to be revealed in recent years through the use of archival and satellite tagging technology. We attached pop-up satellite archival transmitting tags to 35 whale sharks in the southeastern Gulf of Mexico off the Yucatan Peninsula from 2003-2012 and three tags to whale sharks in the northeastern Gulf off Florida in 2010, to examine these sharks' long-term movement patterns and gain insight into the underlying factors influencing their vertical habitat selection. Archived data were received from 31 tags deployed on sharks of both sexes with total lengths of 5.5-9 m. Nine of these tags were physically recovered facilitating a detailed long-term view into the sharks' vertical movements. Whale sharks feeding inshore on fish eggs off the northeast Yucatan Peninsula demonstrated reverse diel vertical migration, with extended periods of surface swimming beginning at sunrise followed by an abrupt change in the mid-afternoon to regular vertical oscillations, a pattern that continued overnight. When in oceanic waters, sharks spent about 95% of their time within epipelagic depths (500 m) that largely occurred during daytime or twilight hours (max. depth recorded 1,928 m), had V-shaped depth-time profiles, and comprised more rapid descents (0.68 m sec-1) than ascents (0.50 m sec-1). Nearly half of these extreme dives had descent profiles with brief but conspicuous changes in vertical direction at a mean depth of 475 m. We hypothesize these stutter steps represent foraging events within the deep scattering layer, however, the extreme dives may have additional functions. Overall, our results demonstrate complex and dynamic patterns of habitat utilization for R. typus that appear to be in response to changing biotic and abiotic conditions influencing the distribution and abundance of their prey.
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In coastal waters of several locations globally, whale sharks (Rhincodon typus) form seasonal aggregations, most of which largely comprise juvenile males of 4-8 m length. Evaluation of the period that individuals stay within these size-and age-specific groupings will clarify our understanding of the transition between life-stages in this species and how this might affect their long-term conservation. Long-term photo-identification studies in Seychelles and Djibouti provided data to evaluate this. The Seychelles aggregation had 443 individuals averaging 5.8 m identified between 2001 and 2009; however, the Djibouti aggregation comprised smaller individuals of 3.7 m mean length with 297 individuals identified between 2003 and 2010. In Seychelles, 27% of individuals identified in 2001 were seen again in 2009, while in Djibouti none of the whale sharks identified in 2003 were seen in 2010, although 13% from 2004 were. This suggests that membership periods in the Djibouti aggregation are shorter than in the other juvenile aggregations, such as in Seychelles. Continued photo-identification monitoring of other Indian Ocean aggregations might in time show the next location of these young sharks' life-cycle and thereby allow development of informed national and regional management plans.
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Whale sharks Rhincodon typus are globally threatened, but a lack of biological and demographic information hampers an accurate assessment of their vulnerability to further decline or capacity to recover. We used laser photogrammetry at two aggregation sites to obtain more accurate size estimates of free-swimming whale sharks compared to visual estimates, allowing improved estimates of biological parameters. Individual whale sharks ranged from 432–917 cm total length (TL) (mean ± SD = 673 ± 118.8 cm, N = 122) in southern Mozambique and from 420–990 cm TL (mean ± SD = 641 ± 133 cm, N = 46) in Tanzania. By combining measurements of stranded individuals with photogrammetry measurements of free-swimming sharks, we calculated length at 50% maturity for males in Mozambique at 916 cm TL. Repeat measurements of individual whale sharks measured over periods from 347–1,068 days yielded implausible growth rates, suggesting that the growth increment over this period was not large enough to be detected using laser photogrammetry, and that the method is best applied to estimating growth rates over longer (decadal) time periods. The sex ratio of both populations was biased towards males (74% in Mozambique, 89% in Tanzania), the majority of which were immature (98% in Mozambique, 94% in Tanzania). The population structure for these two aggregations was similar to most other documented whale shark aggregations around the world. Information on small (<400 cm) whale sharks, mature individuals, and females in this region is lacking, but necessary to inform conservation initiatives for this globally threatened species.
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Large planktivores require high-density prey patches to make feeding energetically viable. This is a major challenge for species living in tropical and subtropical seas, such as whale sharks Rhincodon typus. Here, we characterize zooplankton biomass, size structure and taxonomic composition from whale shark feeding events and background samples at Mafia Island, Tanzania. The majority of whale sharks were feeding (73%, 380 of 524 observations), with the most common behaviour being active surface feeding (87%). We used 20 samples collected from immediately adjacent to feeding sharks and an additional 202 background samples for comparison to show that plankton biomass was ∼10 times higher in patches where whale sharks were feeding (25 vs. 2.6 mg m−3). Taxonomic analyses of samples showed that the large sergestid Lucifer hanseni (∼10 mm) dominated while sharks were feeding, accounting for ∼50% of identified items, while copepods (<2 mm) dominated background samples. The size structure was skewed towards larger animals representative of L.hanseni in feeding samples. Thus, whale sharks at Mafia Island target patches of dense, large, zooplankton dominated by sergestids. Large planktivores, such as whale sharks, which generally inhabit warm oligotrophic waters, aggregate in areas where they can feed on dense prey to obtain sufficient energy.
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We used photo-identification data collected from 2003 through 2009 to estimate population structure, site fidelity, abundance, and movements of this species along the west coast of the Gulf of California to make recommendations for effective conservation and management. Of 251 whale sharks identified from 1784 photographs, 129 sharks were identified in Bahía de Los Ángeles and 125 in Bahía de La Paz. Only juveniles (mostly small) were found in these 2 bays. At Isla Espíritu Santo, we identified adult females; at Gorda Banks we identified 15 pregnant females. High re-sighting rates within and across years provided evidence of site fidelity among juvenile sharks in the 2 bays. Though the juveniles were not permanent residents, they used the areas regularly from year to year. A proportion of the juveniles spent days to a month or more in the coastal waters of the 2 bays before leaving, and periods of over a month outside the study areas before entering the bays again. Additionally, 26 juveniles migrated between Bahía de Los Ángeles and Bahía de La Paz. Pregnant females aggregated for a few days in oceanic waters at Isla Espíritu Santo and Gorda Banks, but no re-sightings occurred between years. The presence of pregnant females and small juveniles (2 m) suggests the presence of a nursery near the 2 far offshore areas. These 4 localities are important for conservation of this endangered species.
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The life history of the whale shark (Rhincodon typus), including its reproductive ecology, still remains largely unknown. Here, we present results from the first whale shark population study around Darwin Island, Galapagos Marine Reserve. Following a diversified approach we characterized seasonal occurrence, population structure and size, and described habitat use of whale sharks based on fine scale movements around the island. Whale shark presence at Darwin Island was negatively correlated with Sea Surface Temperature (SST), with highest abundance corresponding to a cool season between July and December over six years of monitoring. From 2011 to 2013 we photo-identified 82 whale sharks ranging from 4 to 13.1 m Total Length (TL). Size distribution was bimodal, with a great majority (91.5%) of adult female individuals averaging 11.35 m±0.12 m (TL±SE), all but one showing signs of a potential pregnancy. Population dynamics models for apparently pregnant sharks estimated the presence of 3.76±0.90 (mean ± SE) sharks in the study area per day with an individual residence time of 2.09±0.51 (mean ± SE) days. Movement patterns analysis of four apparently pregnant individuals tracked with acoustic tags at Darwin Island revealed an intense use of Darwin's Arch, where no feeding or specific behavior has been recorded, together with periodic excursions around the island's vicinity. Sharks showed a preference for intermediate depths (20-30 m) with occasional dives mostly to mid-water, remaining the majority of their time at water temperatures between 24-25°C. All of our results point to Darwin Island as an important stopover in a migration, possibly with reproductive purposes, rather than an aggregation site. Current studies carried out in this area to investigate regional scale movement patterns may provide essential information about possible pupping grounds for this enigmatic species.
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This study represents the first description of whale sharks, Rhincodon typus, occurring at a provisioning site in Oslob, Cebu, Philippines. Frequent observations of sharks are often difficult, even at tourism sites, giving rise to provisioning activities to attract them. The present study provides repeated longitudinal data at a site where daily provisioning activities took place, and whale sharks were present every day. A total of 158 individual whale sharks were photographically identified between Mar 2012 and Dec 2013, with 129 males (82%), 19 females (12%) and 10 (6%) of undetermined sex. Mean estimated total length was 5.5 m (±1.3 m S.D.). Twenty individuals were measured with laser photogrammetry to validate researchers' estimated sizes, yielding a good correlation (r (2) = 0.83). Fifty-four (34%) individuals were observed being hand-fed by local fishermen (provisioned), through in-water behavioural observations. Maximum likelihood methods were used to model mean residency time of 44.9 days (±20.6 days S.E.) for provisioned R. typus contrasting with 22.4 days (±8.9 days S.E.) for non-provisioned individuals. Propeller scars were observed in 47% of the animals. A mean of 12.7 (±4.3 S.D.) R. typus were present in the survey area daily, with a maximum of 26 individuals (Aug 10 2013) and a minimum of 2 (Dec 6 2012). Twelve (8%) individuals were seen on at least 50% of survey days (n = 621), with a maximum residency of 572 days for one individual (P-396). Twenty four individuals were photographically identified across regional hotsposts, highlighting the species' migratory nature and distribution. Extended residency and differences in lagged identification rates suggest behavioural modification on provisioned individuals, underlying the necessity for proper management of this tourism activity.
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Simple criteria to determine detachment point of towed satellite tags provide first evidence of return migrations of whale sharks (Rhincodon typus) at the Galapagos Islands, Ecuador Abstract Background: Attachment of towed, floating satellite tags to large marine organisms has provided scientists with a wealth of information on the movements of these species. These tags generally are not programmed to detach at a particular time, yet are often prone to detachment by natural means after only a few days or weeks. It is important to be able to distinguish between the tracks provided by a detached, floating tag, and one that is attached to the subject. To this end, we placed three SPOT-5 and one SPLASH tag on large female whale sharks at Darwin Island (Galapagos Islands), and compared their tracks with those of two floating SPOT-5 tags released at the same site. We present a set of criteria to determine whether a towed satellite tag encased in a float is still attached to the study organism. Results: None of the whale sharks remained in the vicinity of the island. Three of the tracks lasted 31 to 48 days, yet one shark was tracked for 167 days. This was the first recorded bona fide homing migration of a whale shark, travelling 1,650 km west then returning to Darwin four months later. Two other sharks also returned to Darwin. We found that at the time of detachment, a clear change in the daily timing and quality of transmissions became evident. This, in conjunction with daily depth and temperature summaries, and knowledge of currents and the biology of the subject, can be used to justify endpoints on tracks that continue to accrue positions as the tag floats with the currents. Conclusions: The data provided by floating tags is sufficiently distinct to be able to determine a detachment date. After detachment, daily transmissions are received in the first hours of the day after midnight (Coordinated Universal Time), the location quality of the transmissions is consistently high, and temperature or depth summaries are consistent with surface records. Prior knowledge of subject behavior and general ocean circulation patterns in the region reinforces the ability to determine detachment date. In some cases, after a prolonged period of more than three or four weeks, the detection pattern may change, yet caution should be exercised in assuming that the tag is, after all, still attached.
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Three well-documented accounts of whale sharks, Rhiniodon typus, in Kuwait's coastal waters represent the first report of this species in the area since 1968.