Non-invasive methods characterise the world’s largest tiger shark aggregation in Fuvahmulah, Maldives

Abstract

Tiger sharks are apex predators with a circumglobal tropical and warm-temperate distribution, with a general lack of population data for the central Indian Ocean. In Fuvahmulah, Maldives, tiger sharks display frequent use of the harbour area, attracted by discarded fish waste. Here, we document the population structure, residency, and reproductive characteristics of the world’s largest known tiger shark aggregation in a geographically-restricted area. Using non-invasive methods, photo identification and laser photogrammetry, we identified 239 individual tiger sharks over a 7-year study period. The aggregation was female-dominated (84.5%), with both large juveniles and adults present. Adult females were resighted over the entire study period displaying strong inter- and intra-annual site fidelity. Modelled residency using maximum likelihood methods suggests they spent 60.7 ± S.E. 7.5 days in Fuvahmulah, with a larger aggregation size, shorter residence periods and longer absence periods compared to juvenile females. Prolonged abdominal distensions of adult females indicate they likely stay near Fuvahmulah during gestation and reproduce biennially. Fuvahmulah seems to provide suitable conditions for gestation given the year-round provision of food and warm waters, exhibited by strong site fidelity and temporal residency. Our results show indications of a thriving population within the confines of protected waters.

Full text

Non-invasive methods characterise
the world’s largest tiger shark
aggregation in Fuvahmulah,
Maldives
Lennart Vossgaetter1,2, Tim Dudeck1,2, Jamie Crouch3, Maiah Cope3, Tatiana Ivanova3,
Ibrahim Siyan3, Abdullah Niyaz3, Mohamed Riyaz3 & Gonzalo Araujo4,5
Tiger sharks are apex predators with a circumglobal tropical and warm-temperate distribution, with
a general lack of population data for the central Indian Ocean. In Fuvahmulah, Maldives, tiger sharks
display frequent use of the harbour area, attracted by discarded sh waste. Here, we document
the population structure, residency, and reproductive characteristics of the world’s largest known
tiger shark aggregation in a geographically-restricted area. Using non-invasive methods, photo
identication and laser photogrammetry, we identied 239 individual tiger sharks over a 7-year study
period. The aggregation was female-dominated (84.5%), with both large juveniles and adults present.
Adult females were resighted over the entire study period displaying strong inter- and intra-annual
site delity. Modelled residency using maximum likelihood methods suggests they spent 60.7 ± S.E.
7.5 days in Fuvahmulah, with a larger aggregation size, shorter residence periods and longer absence
periods compared to juvenile females. Prolonged abdominal distensions of adult females indicate they
likely stay near Fuvahmulah during gestation and reproduce biennially. Fuvahmulah seems to provide
suitable conditions for gestation given the year-round provision of food and warm waters, exhibited by
strong site delity and temporal residency. Our results show indications of a thriving population within
the connes of protected waters.
Keywords Marine megafauna, Site delity, Gestation, Photo ID, LIR, Maximum-likelihood models
Across the globe, iconic predators such as sharks, wolves and lions are disappearing at the hands of human
development. Most of the remaining predators now exist in only a fraction of their historical range1,2. In the
ocean, this decline is especially evident in pelagic sharks and rays with many populations declining by > 70%
since 1970 3. More than one third of all elasmobranch species are now threatened with extinction4. Yet predators,
such as sharks, are essential for ecological balance due to their top-down regulatory impact on food webs5–7.
Sharks not only support ecosystem stability but can also provide economic value through tourism, beneting
dive operators, local tourism industries and governmental bodies8–10. While live sharks oer both economic and
ecological value, dead sharks play an economic role for sheries and local livelihoods worldwide11.
In the Maldives, shing for sharks was a common practice and resulted in a decrease of reef shark populations
at dive sites in the 1990s12,13. As a consequence, dive tourism declined leading to signicant economic losses for
local dive operators13. In response, the Maldivian government introduced legislation to protect sharks in their
entire exclusive economic zone in 2010 creating one of the largest shark sanctuaries in the world with an area of
916,011 km2 14,15. e Maldives is home to a large diversity of elasmobranchs8 but lacks scientic information
about critical habitats, behaviour and ecology with the exception of whale sharks Rhincodon typus and manta
rays, Mobula alfredi and Mobula birostris (e.g. 16,17). Recently, a large aggregation of tiger sharks Galeocerdo
cuvier was reported surrounding the oceanic island of Fuvahmulah in Southern Maldives (see Fig.1). Previously
neglected as a tourism destination, thousands of people now travel to Fuvahmulah annually to dive with this
iconic species (Fuvahmulah dive centres, pers. comm.). e Island is a designated UNESCO Biosphere Reserve
but scientic data on any of the local sh populations is lacking due to the recency of its commercial attention18.
1Leibniz Centre for Tropical Marine Research, 28334 Bremen, Germany. 2University of Bremen, 28334 Bremen,
Germany. 3Fuvahmulah Dive School, Fuvahmulah 18011, Maldives. 4Marine Research and Conservation
Foundation, Lydeard St Lawrence, Somerset, UK. 5Environmental Science Program, Department of
Biological and Environmental Sciences, College of Arts and Sciences, Qatar University, Doha, Qatar. email:
lenny.vossi@gmail.com
OPEN
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Tiger sharks are large apex predators with a circumglobal distribution in warm-temperate and tropical
waters19. In the Indian Ocean, females reach maturity at a total length (TL) of 3.0–3.5m, whereas males mature at
2.8–3.2m20–22, with a maximum TL of 5.5m23. e tiger shark is the only member of the family Carcharhinidae
with aplacental viviparous (ovoviviparous) reproduction24. eir adaptability as generalist predators allows
them to function as apex and meso predators in dierent ecosystems25. e species is globally assessed as Near
reatened in the IUCN Red List of reatened Species with decreasing population trends19. Due to the tiger
shark’s low reproductive output and genetic diversity, the species is vulnerable to overexploitation by targeted
shark sheries and shark control programs, and as bycatch in commercial and artisanal sheries globally19,26.
e reproductive cycle is currently one of the most enigmatic aspects of their biology25, even though highly
relevant to species management27. Initial work from the North Atlantic suggests tiger sharks follow a biennial
cycle28,29. Castro (2009) proposed that they have a gestation period of 12 months with a synchronous reproductive
cycle, where mating occurs before pregnant females pup in late spring and summer24. In Hawaiian tiger sharks
however, a triennial cycle was observed with a gestation period of 15–16 months30. Studies using nuclear
markers (microsatellites) revealed two major populations, one in the Atlantic and one in the Indo-Pacic31,32.
Due to the genetic connectivity within the Indo-Pacic, several studies assumed that Indo-Pacic tiger sharks
follow a triennial reproductive cycle e.g.22,33. However, Manuzzi et al. (2022) used genomic analysis to highlight
the occurrence of localised cryptic populations of tiger sharks in Eastern Australia at ner geographical scales
than previously understood34. As dierent populations may have diering life history traits, reproductive cycle
generalisations should be regarded with caution35. With a lack of data throughout the rest of the Indo-Pacic
and the possibility of localised populations, the reproductive cycle length remains unclear, particularly for tiger
sharks in the Maldives.
In the Indo-Pacic, the species’ space use has been extensively studied in Hawai’i36,37, Eastern Australia33,38,
Western Australia39,40, Southeast Africa41,42 and the Eastern Pacic43,44. Despite tiger sharks having been studied
in the Indian Ocean, movements to–from the Maldives have not yet been identied26,41,45. eir space use is
highly variable depending on the location, habitat and life stage with large intraspecic variation. Tiger sharks
can migrate vast distances (i.e. 1000s of km), but have also been shown to display site delity in adults and
residency in juveniles33,42,43,46. In Hawai’i, the species has been shown to display partial migrations in which
some individuals display residency, whereas others, usually adult females, migrate oshore37. In the Galapagos
Fig. 1. Location of Fuvahmulah within the Maldives Archipelago in the Indian Ocean. e dive sites, where
tiger sharks are frequently spotted and where most footage originates from, are marked in the map (Map was
created using QGIS 3.28.1-Firenze, URL: https://qgis.org/download/).
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Marine Reserve (GMR), Ecuador, juvenile and subadult tiger sharks remain resident year-round with tracked
individuals spending a remarkable 93% of their time within the GMR43. In the Bahamas, female subadult and
adult tiger sharks display site delity to a large, shallow sand bank for reproductive purposes during the boreal
winter. e warm and protected waters are used for gestation47. In Eastern South Africa and Mozambique, adult
tiger sharks exhibited relatively restricted space use along the continental shelf likely linked to abundant food
sources in the region41.
In Fuvahmulah, the local traditional tuna shery dates back for centuries and discards have historically been
tossed into the ocean48. According to local anecdotal information, large sharks have always gathered around
shing boats waiting for sh discards and depredation opportunities (M. Ibrahim, pers. comm.). Aer the
harbour was built in 2004, sh waste started accumulating in the surrounding harbour area, which since 2017
serves as a dive site for tiger shark tourism. e rst registered local dive school in Fuvahmulah, Fuvahmulah Dive
School (FDS), began collecting videos and photos of tiger sharks in 2016. e footage allows for the recognition
of individuals through external markings and pigmentation patterns over time49. Photographic identication
(photo-ID) is a wide-spread, non-invasive method to study populations of megafauna50. e method has
been applied to a variety of species with distinct markings such as white sharks (e.g.51), striped hyenas (e.g.52),
whale sharks (e.g.53), manta rays (e.g.54), cetaceans (e.g.55), marine turtles (e.g.56), anurans (e.g.57) and others,
to describe population structure, abundance, residency, demographics, and animal movement between study
sites. e fundamental assumption of photo-ID is that the natural markings are unique enough to reliably
distinguish individuals within a population and that these do not change over time50,58. Most research exploring
the demographic structure, reproductive patterns and residency behaviour of tiger sharks has relied on either
sheries-dependent data (e.g.59) or acoustic and satellite telemetry (e.g.60). rough the recent commercial
development of tiger shark diving in Fuvahmulah, a substantial quantity of footage has been collected by dive
centres and guests. With increased camera usage by recreational divers, citizen science can aid signicantly in
the data collection for photo-ID studies61–63. While previous research has employed photo-ID to study tiger
sharks64,65 applying up to 14 visual traits for identication49, this study represents the rst extensive investigation
of the species with this method.
Due to limited scientic information on tiger sharks in the Indian Ocean, there exists an imminent need
to determine reproductive parameters, population structure and critical habitats. In the present study, we use
photographic data from 2016 to 2023 and laser-photogrammetry to investigate: (a) the demographic structure
of tiger sharks visiting Fuvahmulah, (b) reproductive indications and (c) the level of site delity/residency to
this oceanic island in the Maldives (Fig.1). Hereby, this study presents the rst scientic description of this tiger
shark aggregation, with implications for the management and conservation of tiger sharks in the Maldives.
Results
Survey eort
Between Dec 7th 2016 and Sep 30th 2023, we collected footage from a total of 788 separate dive surveys: 772
at the Tiger Harbor (TH) dive site, six at Farikede (FK) and ten from Oshore Plateau (OP) (Fig.1). We saved
32,495 photographs and frame grabs from video material of sucient quality to identify individual sharks.
Photo identication
A total of 239 individual tiger sharks were identied, exhibiting a signicant female bias (F = 202, M = 37,
Chi-squared test, χ2 = 113.9, p < 0.0001). We logged 6,035 individual encounters throughout the study period
(encounter meaning one identication of one individual). e majority (n = 5,986, 99.3%) of these encounters
took place in TH. Of all encounters, 93.7% (n = 5,653) were females. Most individuals (n = 186) were sighted on
more than one dive survey resulting in a resighting rate of 77.8%. On average, individuals were resighted on 25.3
(SD = 28.8) dive surveys with a maximum of 128 encounters for individual F-011, being present on 16.6% of all
dive surveys at TH. A total of 89 sharks (37.2%) were seen in only one year, whereas 150 individuals (62.8%) were
sighted over multiple years (≥ 2) and 53 individuals (22.2%) were seen ≥ 5 years (Supplementary Fig. S1). On
average, we encountered 10.4 (SD = 5.6) individuals per dive survey, with a maximum of 40 sharks encountered
during a single dive survey of 61min length on Apr 21st 2022. During a one-year period of highest sampling
eort from July 2021 until June 2022, we identied 186 individuals. When inspecting cumulative identications
per month, males tend to be almost absent from TH during the months of July (n = 1) until September (n = 1)
and had highest sighting rates from November until April. Although females were present year-round, female
sighting rates were signicantly higher during the months of the Northeast monsoon including transitional
months compared to the months of Southwest monsoon (Fig.2b, Student’s t-test, t=-4.778, p = 0.0007). e
number of dive surveys did not play a signicant role in determining the number of identications (Fig.2b).
Size estimates
Total length (TL) was visually estimated for a total of 213 sharks: 175 (82.16%) females and 38 males. From those
estimated, we used laser photogrammetry to measure 52 individuals on 65 occasions from November 2021
until April 2022 at TH. ere was no signicant dierence between estimates and measured values (t=−0.7745,
p = 0.4422) with a mean dierence of -2.1cm ranging from − 49.0cm to 34.1cm (see Supplementary Table S1).
erefore, visual size estimates were assumed to be sucient for approximate TL estimation of the tiger sharks.
Tiger sharks ranged from 2.0 to 4.5m in TL with a mean size of 3.24m (n = 213, SD = 0.52m). Males ranged
from 2.0m to 3.5m in size, whereas females ranged from 2.0to 4.5m. ere was no signicant dierence between
the mean size of males and females (t = 1.5541, p = 0.1253). However, all sharks that were estimated > 3.5m TL
were females. Most sharks were larger than the size at maturity resulting in 57.0% of the females considered
adults, which were responsible for 68.5% (n = 4136) of all encounters (Fig.3). Male tiger sharks encountered
in TH all ranged from 3.0 to 3.5m and were sexually mature. Males ranging from 2.0 to 2.5m were only
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Fig. 2. (a) Countershading delineation in six dierent tiger sharks. e six dierent individuals (F-001,
F-002, F-018, F-009, F-011, F-025) display intraspecic variation of the countershading delineation anterior
to the pectoral ns. is feature was most useful in dierentiating the individuals. (b) Cumulative number of
identications of tiger sharks by sex per month visiting tiger harbour. e orange line indicates the cumulative
number of dive surveys per month.
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encountered at OP (n = 5). Female tiger sharks of all size classes were encountered at TH suggesting the presence
of juvenile and adult individuals.
Reproductive indications
A total of 54 pregnancies of 39 individuals (33.9% of all adult females) were documented from tiger sharks
present in Fuvahmulah. For 35 pregnancies of 32 sharks, picture quality was sucient to assess their standardised
width over time. ere was a signicant dierence in standardised widths between sharks considered pregnant
[median(IQR) = 0.45 (0.44–0.46)] and sharks that consequently returned [median(IQR) = 0.36(0.35–0.37)]
aer an absence period with presumed parturition (U = 0, p < 0.0001). Figure 4a depicts the increase of the
standardised width over time until they were not sighted at the dive site anymore. When they return, a signicant
change in their body width was obvious, highlighting the morphological dierence aer diering periods
of absence (Fig.4b). e timing, size and body development suggests these female tiger sharks return from
parturition at a dierent location. During continuous sampling eort, 40 pregnancies of 36 individuals were
recorded, allowing quantication of their absence periods. e median sharks’ absence for presumed parturition
was 97 days ranging from 35 to 345 days (IQR = 69–126 d).
Two consecutive pregnancies were recorded for nine individuals and three consecutive pregnancies were
recorded for three individuals. Consecutive periods of gestation and parturition were not signicantly dierent
from an expected two-year period (Chi-squared test, χ2 = 2.7783, p = 0.9994). Footage of pregnant sharks was
separated on average by 788.7 (SD = 69.0) days or 2.16 years.
Residency and lagged identication rate
e residency models were run for (i) the entire population, (ii) adult females, and (iii) juvenile females. Model
H, which included parameters for emigration, reimmigration and mortality of tiger sharks, had the lowest
QAIC indicating the best t for the data of all three data sets (Table1). According to the model, overall tiger
sharks spent a mean of 65.5 ± S.E. 76.1 [95%CI (55.5–79.4)] days in Fuvahmulah and a mean of 107.9 ± S.E.
Fig. 3. Total length (TL) of visual size estimates from 213 sharks incremented by 0.5m. All females 3.5m and
larger were considered adults. All males 3.0m and larger were adults.
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14.7 [95%CI (82.2-139.7)] days away. Aggregation size was estimated to be 43.1 ± S.E. 3.3 [95%CI (37.2–50.4)]
individuals present in the study area on any given day.
Similarly, adult female tiger sharks spent a mean of 60.7 ± S.E. 7.5 [95% CI (50.2–72.9)] days in Fuvahmulah
and a mean of 110.4 ± S.E. 15.8 [95% CI (81.2–135.5)] days away. Aggregation size was estimated to be 25.9 ± S.E.
2.2 [95% CI (22.0–30.5)] individuals present in the study area on any given day. In contrast, juvenile female tiger
sharks spent a mean of 93.0 ± S.E. 42.7 [95% CI (32.7–182.1)] days in Fuvahmulah and a mean of 76.9 ± S.E.
55.7 [95% CI (19.3–219.7)] days away, whereas the aggregation size was estimated to be 16.0 ± S.E. 2.1 [95%
CI (12.0–20.2)] individuals present in the study area on any given day. According to the model results, juvenile
females spent shorter time periods away from the study site than adults, while remaining for longer periods
when present. eir overall aggregation size was also considerably lower, which is consistent with the amount of
juveniles vs. adult females identied. e results for all tiger sharks was similar to the results for the adult females
except for aggregation size.
e LIR plot of all sharks showed a rapid decline from day 1 to 190 days (Fig.5). Aerwards the LIR reached
an asymptote at an LIR of 0.007 to 0.009 until 714 days, followed by a further slow decrease to 0.005 until day
2153 never reaching zero. is suggests long-term delity to the dive site by at least some individuals. Between
Fig. 4. (a) Standardised width of presumably pregnant sharks over time. Day 0 indicates the last measurement
before the sharks’ absence period, where we presume parturition may take place. Sharks scored 1 or 0 were
visually assessed as ‘pregnant’ or ‘not pregnant’, while ‘not scored’ refers to a shark’s appearance, where we did
not infer pregnancies based on their visual appearance (see Methods for more details). A linear regression
model, including standard error, is tted to the data until Day 0. (b) An example of one individual’s presumed
pregnancy (F-049). Number in brackets provides a corresponding day value to a). e standardised width
of this shark increased until day 0 followed by a period of absence for 93d. Upon its return, the standardised
width of this shark had signicantly declined.
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190 and 369 days, the LIR increased, indicating periodicity in the visitation of the dive site by some tiger sharks
at half and one-year time periods.
e LIR plot for juvenile females showed high probability of resighting from day 1 to 6 days. LIR remained
high until 687 days, or 1.9 years. Aer 2,158 days the LIR reached zero, implying permanent emigration or
mortality (Fig.5). However, most juveniles have not been identied for periods longer than three years. e LIR
plot for adult females displayed a rapid decline from day 1 to 189 days. Aerwards the model curve stabilised
at a value of 0.014, never reaching zero. Between 190 and 368 days, the LIR increased signicantly and roughly
maintained this level until 1457 days, indicating strong periodicity in the visitation of the dive site by some adult
female tiger sharks as well as inter-annual site delity.
Discussion
is study provides the rst assessment of the population structure, reproductive patterns and residency
behaviour of tiger sharks at a hotspot in the central Indian Ocean using non-invasive methods. While adult
females showed inter- and intra-annual site delity with temporal residency periods, large juveniles showed high
residency with shorter periods of absence suggesting that they possibly remain resident in close geographical
proximity to the island. Fuvahmulah hosts an unprecedentedly large aggregation of tiger sharks year-round,
which appears to play a critical role for the population’s reproductive cycle i.e. as a gestation area for adult
females.
Population structure
e island supports, to our knowledge, the largest documented number of individual tiger sharks encountered in
one geographically restricted area with 239 tiger sharks identied in the six-year study period. Few studies about
tiger shark aggregations using photo-ID have been published to date. Nakachi (2019) identied 69 individuals
from opportunistic photo ID data in Hawai’i over a 16-year study period49. Clua et al. (2013) documented the
presence of 46 individuals over an eight-day period at a blue whale carcass in New Caledonia64. In Tahiti, Bègue
et al. identied 55 individuals at a provisioning site over an eight-year study period65. e large number of tiger
sharks in Fuvahmulah is likely driven by the daily, year-round provisioning activities. While the provisioning
activities for tourism started in 2017, sh waste from the local tuna shery has likely been discarded for
generations48,66. is low-eort food source probably attracted tiger sharks to the island long before the tourism
activities started and altered their distribution in the area. Consequently, we assume that this large aggregation
is a consequence of human activity and that sharks within this study likely present a skewed picture of natural
tiger shark population dynamics.
e aggregation was dominated by large juvenile and adult females, despite having a sex ratio close to 1:1
in uterus24,30. is supports size-sex segregation as commonly observed in sharks20,67,68 and possibly indicates
reproductive needs. e sex-biased habitat use patterns in Hawai’i were found to be driven by the females’
propensity for inshore habitats, whereas males tended to occupy areas farther oshore37. Similarly, at Tiger
Beach, Bahamas, juvenile and adult females also dominate, with males being almost completely absent except
for a few large individuals47. Tiger Beach has various similarities to TH in Fuvahmulah: both sites are shallow
with warm-water areas adjacent to o-shore, pelagic ecosystems, tiger sharks are provisioned, and pregnant
females are observed47,69. It has been postulated that the warm waters of Tiger Beach function as a female refuge
from male harassment and provide warm temperature gestation grounds47. is aligns with the reproductive
indications we monitored at TH: adult female sharks stay around Fuvahmulah during their presumed gestation
period for extended time periods until they leave for parturition and aer an absence period of ca. 2–5 months
they return to the dive site. However, male harassment has been witnessed in one photo series during a safety
stop in blue waters o TH, where a large male shark swimming signicantly faster than the usual cruising speed
Model Model description Parameters (i)∆
QAIC (ii)∆
QAIC (iii)∆
QAIC
A Closed 1/a1 = N6129.0 4284.9 2791.3
B Closed a1 = N6129.0 4284.9 2791.3
C Emigration/mortality a1 = Emigrat ion rate;
1/a2 = N3695.2 3180.8 530.0
DClosed: Emigration + reimmig ration a1 = Emigrat ion rate;
a2/(a2 + a3) = proportion of population in study area at any time 170.6 6.6 537.5
E Emigration/mortality a1 = N;
a2 = Mean residence 3695.2 3180.8 530.0
FEmigration + reimmigration + mort ality NA 3218.1 2997.7 490.9
GEmigration + reimmigration a1 = N;
a2 = Residency time in; a3 = residency time out 170.6 6.6 537.5
HEmigration + m ortality + reimmigration
a1 = N;
a2 = Residency time in;
a3 = Residency time out;
a4 = Mor tality
0.0 0.0 0.0
Tab le 1. Residency model parameters as preset in SOCPROG 2.9 101 and goodness of t assessed through the
Quasi-akaike Information Criterion (QAIC). (i) shows model results for the entire population, (ii) for adult
females, and (iii) for juvenile females. N is the population size.
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of tiger sharks was chasing a large, possibly pregnant female (LV, pers. observation). Shortly before contact,
both turned away and swam separate ways (supplementary Fig. S2). Fuvahmulah waters do not seem to protect
females from male harassment as has been postulated for Tiger Beach47. However, intersexual aggression
remains a rarity despite the long-term presence of a few adult males at TH (Fig.2b; authors, pers. observation).
Fig. 5. Lagged Identication Rates (LIR, mean ± S.E.) for all tiger sharks (top panel) and for juvenile (red) and
adult (blue) female tiger sharks visiting Fuvahmulah. Model H (pale lines) is represented in all cases including
emigration, reimmigration and mortality as model presets.
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Despite the large number of tiger sharks and the presence of juveniles and adults, small juvenile tiger sharks
(< 2.0m) were not observed surrounding Fuvahmulah. Tiger sharks are known cannibals70 and a large diversity
of other apex predators including Sphyrna lewini, Carcharhinus albimarginatus and C. amblyrhynchos surrounds
the island71. erefore, smaller juveniles probably avoid the area before they reach a size of ca. 2m TL to evade
the elevated predation risk. Such size segregation is common among tiger sharks (e.g.20,60).
While visual observations were able to determine sexual maturity in males due to large, calcied claspers,
female maturity could not be visually determined. Considering the notable variation in the size at maturity
among tiger sharks in the Indian Ocean [females: 3.00m21 to 3.59m20), we recognize that there exists a level of
ambiguity regarding the classications made in this study. Our adoption of > 3.0m TL as a length-at-maturity is
a conservative estimate, more likely to categorise adult individuals ≤ 3.0m as juveniles rather than the converse.
Reproductive indications
It was proposed that Indo-Pacic and Atlantic tiger shark populations may have dierent lengths in reproductive
cycles, being biennial in the North Atlantic24 and triennial in Hawaiian waters30. However, here we show evidence
for a biennial reproductive cycle in tiger sharks in the Maldives. Fuvahmulah is located almost on the equator
and has a year-round sea surface temperature of 26–30 degrees Celsius (dive log data from dive computers).
e continuous presence of warm waters accelerates embryo development and reduces the gestation period72.
Additionally, the substantial amounts of sh discards provide gestating females a low-eort food source. It is
likely that several individuals can temporarily fuel their energetic requirements by scavenging on these discards,
a behaviour observed in other large-bodied shark species at similar provisioning sites73,74. However, similar
studies would be needed to prove this hypothesis. Numerous other potential prey items live in the ecosystems
surrounding Fuvahmulah. We lmed how a tiger shark tries to bite a hawksbill turtle Eretmochelys imbricata
while breathing (Supplementary Fig. S3). Moreover, the island supports a local tuna shery for yellown unnus
albacares and skipjack tuna Katsuwonus pelamis. Fishers frequently complain about depredation events66. Shark
depredation rates can reach up to 26% of all hooked shes in commercial and recreational sheries and is
energetically highly ecient for sharks75. Tunas with massive chunks missing are occasionally observed in the
local shing market, likely due to depredation events by tiger sharks (Supplementary Fig. S4). However, these
events have not been quantied for the tuna shery in Fuvahmulah.
To conclude, we suggest that pregnant females may be using Fuvahmulah waters in part to benet from
the year-round warm waters, and in part to access low-eort food. Similar behaviours have been documented
in a variety of other species of pregnant elasmobranchs (e.g.76–78). ese ideal conditions for reproducing
and gestating females may allow this population to reproduce faster than in more challenging environmental
conditions with stronger seasonal inuences such as Hawaiian waters79. However, North Atlantic tiger sharks
have similar seasonal inuences as Hawaiian tiger sharks and yet exhibit biennial cycles. Unfortunately, data
gaps remain throughout the rest of the Indo-Pacic regarding their reproductive cycle. Given the possibility of
localised tiger shark populations on smaller geographical scales34 and some evidence for biennial reproduction
presented here, we recommend avoiding generalisations of reproductive cycle lengths within the Indo-Pacic.
We acknowledge that photographic observations provide limited insight into reproductive biology. Pregnancy
was determined based on external appearance, with a consistent abdominal distension over time, which allowed
us to quantify the morphological change of sharks before and aer presumed parturition. Short-time distended
abdomens (e.g. <7 d) likely coincided with the consumption of large amounts of food64, and was observed
regularly. However, the degree of distension observed in supposedly full-term pregnant females has never been
observed in short-time distensions. Combined with the subsequent return to the dive site aer a period of
absence with considerably altered body conditions, we argue that parturition during this time is extremely likely
(Fig.4).
Residency
It is common in tiger sharks that large, adult individuals undertake frequent oceanic migrations while displaying
site delity, whereas juveniles tend to have smaller home ranges (e.g.37,38,46). Adult females o Fuvahmulah
displayed a high degree of long-term site delity. is is evident from the stabilising LIR over time, long absence
periods with subsequent returns and the high number of sharks sighted throughout multiple years along the
entire study period (Supplementary Fig. S1). In combination with the high resighting rate, this evidence indicates
that they are temporal residents with strong inter-annual site delity. e long absence periods are likely due
to far-ranging migrations either for foraging or for reproductive purposes37. Displaying longer residence times
and shorter absence periods, juveniles are more likely to stay close to the surrounding waters of Fuvahmulah.
We observed several juveniles staying on the periphery of the dive site when adult females were feeding during
footage inspection. ey tend to approach the site when larger sharks stop feeding (authors, pers. obs.). Since
tiger sharks follow strict hierarchical patterns during feeding aggregations64,80,81, intraspecic competition makes
photographic detection of adult tiger sharks more likely at a feeding site. Hence, a large proportion of juveniles is
probably underrepresented in the data set. erefore, large juveniles (TL > 2.0m) are potentially resident to the
greater area of the waters surrounding Fuvahmulah before reaching maturity and the necessary strength to take
on larger migrations38. Similarly, large juvenile sharks displayed year-round residency in tropical oshore atoll
ecosystems of the Coral Sea33. Likewise, small and large juvenile sharks investigated at the Galapagos Marine
Reserve spent a remarkable 93% of their time within the reserve43.
e inherent limitations of photo-ID at a provisioning site highlights the potential bias towards a fraction of
the population i.e. presumably bolder, shallow-dwelling, more aggressive individuals coming in for an easy meal.
For instance, several shark species form social networks with non-random associations between individuals
(e.g.82–84). Despite tiger sharks having been thought to be solitary in nature, evidence from the Bahamas suggest
that their associations are in some cases non-random, especially at provisioning sites85. us, the occurrence
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of tiger sharks at TH is biased by intraspecic competition, non-random associations, and intraspecic
behavioural variations. We hypothesise that TH is dominated by a subset of the population that shows above-
average dominance and aggression. Less dominant sharks are likely chased away or avoid close interactions with
larger and dominant sharks. At TH, juvenile females generally avoided the feeding area when adult females were
present. However, these behaviours are variable and require further investigation. With most photo-ID data
from TH (99%), our analyses show the residency to the dive site and not to the waters surrounding Fuvahmulah.
It is likely that a large proportion of the population was present in the area but avoided the dive site and thus,
remained undetected.
Conclusions
Fuvahmulah is a bright spot for tiger shark conservation in the Indian Ocean given their protected status within
Maldivian waters and being the world’s largest documented aggregation. Likely, female tiger sharks use the area
for gestation with strong site delity, and thus, the waters o Fuvahmulah serve as a critical habitat for the
population. However, the methods applied in this study are limited and provide only initial insights into this
aggregation at a provisioning site. It remains unknown where they migrate or where the parturition sites are
located. In future studies, we recommend using ultrasonography to conrm the reproductive status of the tiger
sharks and to validate our abdominal distention results and assumptions. Satellite telemetry studies on gestating
females could indicate their migratory routes when absent86, and more interestingly, a novel intrauterine tag
(see Sulikowski and Hammerschlag 2023) could indicate exact parturition locations87. Furthermore, telemetry
methods could reveal their geographic connectivity to other populations and whether Fuvahmulah tiger
sharks truly spend most of their time within protected waters of the Maldives shark sanctuary. Nevertheless,
the existence of conservation measures in the Maldives is tightly coupled to the economic incentives of shark
tourism10. us, sustainable practices at the provisioning site are of critical importance to provide a net
conservation benet66,88. To our knowledge, in the Maldives, there are currently no laws or guidelines regulating
provisioned shark dives, and “codes of conduct” are voluntary and dive-centre specic. At other provisioning
sites, successful management strategies have been implemented, such as a locally managed MPA in Fiji89,
guidelines and legislation that regulate SCUBA dives90, and policies that manage provisioning activities91. To
minimize future conict, we recommend incorporating all stakeholder’s interests into local management plans
that support sustainable ecotourism in one of the world’s largest shark sanctuaries.
Methods
Study site
e Maldives is a collection of coral atolls that form a chain extending from 7°N to approximately 0.5°S.
Fuvahmulah is the biggest single reef island in the Maldives, not being part of a larger atoll and therefore, a
true oceanic island along the Chagos-Laccadive Ridge. e Island is located ~ 30km south of the equator, and is
surrounded by a narrow fringing reef with steep slopes reaching to the ocean oor (Fig.1).
In the harbour area, tiger sharks are now provisioned daily, year-round, with the shers’ tuna (unnus
albacares and Katsuwonus pelamis) discards (i.e. sh heads, guts, bones) to support local dive tourism. is dive
site is referred to as ‘Tiger Harbour’ (TH) in this document (Fig.1). At the dive site ‘Farikede’ (FK), tiger sharks
are frequently observed unprovisioned, while exploratory baited dive surveys were conducted in oshore waters
(OP) 1.0 to 2.5 nautical miles further south of FK (Fig.1).
Photo identication
Footage of tiger sharks was collected from dive guides, recreational divers (citizen scientists) and researchers.
Sampling eort was highest from May 2021 until September 2023 due to the presence of a person dedicated to
saving collected footage on the island (supplementary Fig. S5). All pictures of sucient resolution, lighting, and
framing of a shark were extracted from the raw footage (supplementary Fig. S6). Individual sharks were identied
through a variety of natural markings such as the external pigmentation patterns anterior to the pectoral ns
along the counter-shading delineation (Fig.2a), stripe patterns, and n shapes49,64. A new individual was only
added to the catalogue when the footage quality was sucient to identify at least two dierent identiable traits
on the le side of the individual (see supplementary Fig. S6 for examples). Additional identiable traits from
both sides were collected further on to enable identication from both sides. e sharks were cross-referenced
with a catalogue and each identication was double checked by the principal investigator as well as conrmed by
another scientist to minimise human error. Sex was determined from the presence (male) or absence (female)
of claspers assessed via sucient pictures. Maturity in males was assessed through clasper size and calcied
appearance20. Additional footage of tiger sharks was retrieved during exploratory usage of a remote underwater
video station deployed by the Manta Trust and FDS (n = 9 encounters).
Size estimates
Tiger shark sizes were estimated by researchers based on objects of known lengths as a visual reference on the
video les and during the dives (i.e. a large chain block, regular dive guides)92–94. Estimates for TL were made
in 0.5m increments (e.g. 2.0, 2.5, 3,0m, etc.). Given that tiger sharks can grow up to 5.5m TL23, we deemed
the increments used in our data analysis to be adequate for this study. All estimates were made by the same
researcher, who has viewed all footage and conducted > 300 dive surveys in Fuvahmulah. For a subset of sharks,
we used laser photogrammetry to control for the accuracy of the estimates. Two adjustable, screw-mounted,
green lasers (520nm, 5 mW) were equipped on a PVC rig 60cm apart. Videos and pictures were taken on a
Panasonic LUMIX GH5 Mk II. Accuracy and distortion of our set-up was assessed following Deakos (2010)95.
Before each dive, the lasers were calibrated at 3 and 8m distance using a whiteboard with two dots exactly 60cm
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apart. Underwater measurements were made at 3–4m distance to the sharks for consistency. Aer each dive,
accuracy was reassessed to ensure the laser position had not changed. For each video, at least three frames were
extracted, and the average was built for one measurement. Appropriate frames were chosen to minimise parallax
error and caudal n exing94–96. Laser photogrammetry measurements were compared to previous estimations
of the same individuals with a paired t-test. TL was obtained from laser photogrammetry PCL measurements
using tiger shark morphometrics from La Réunion island22. e equation used was:
TL = 1.169PCL + 35.697 (r2 = 0.98, n = 136), from Pirog et al. (2020).
Size dierences between sex were compared with the Welch two-sample t-test. Maturity status of female
sharks cannot be determined through external appearance. Hence, we used TL to separate mature from
immature female individuals and categorised them as adults and juveniles. Based on published size-at-maturity
estimates in the Indian Ocean20–22, we calculated the mean to designate sharks in this study accordingly. Given a
mean of 3.32m, female sharks that were estimated > 3m were considered as adults and females estimated ≤ 3m
were considered juveniles. All tiger sharks were assigned their life stage aer size estimates from 2022. If the
individuals were not present in 2022, their latest size estimate was used.
Reproductive indications
Adult females were presumed pregnant when an abdominal distension was consistently observed over long time
spans of up to ve months69,76,97. e minimum requirement for a presumed pregnancy was evidence from two
separate dives at least two weeks apart consistently showing a similar or increasing degree of abdominal distension.
ese sharks were scored as ‘pregnant’ in our analysis. Based on an abrupt transition in physical appearance from
an abnormally large abdominal distension to a normal appearance or a concave curvature towards the interior
of the body, following a period of absence, we inferred that, presumably, parturition had occurred during that
time. ese sharks were scored as ‘not pregnant’. e presumed pregnancies are referred to as ‘pregnancies’
throughout this manuscript. Based on previous doubts in whale sharks about assigning reproductive status
based on visual inspections alone98, change in body width throughout presumed pregnancies was documented
from photographs99,100. Photogrammetric width measures have been shown to successfully detect pregnancies in
cetaceans where life history information was known99,100. However, this approach has not been applied to sharks
yet. Since our data set included photographs of the same sharks throughout their presumed gestation period
and subsequent return aer parturition, we used this method to quantify the observed morphological changes
over time. To develop a dimensionless and scale-invariant metric for comparisons between photographs, we
standardised the body widths by a length measure. In this photogrammetry approach, we used the shark’s width
from the posterior end of the rst dorsal n vertically down (90° to swimming direction) and the length from
the anterior base of the pectoral n to the anterior base of the pelvic n to minimise error due to the sharks’
propulsive tail exing (Supplementary Fig. S7). For this analysis, we used a subset of pregnancies, where the
picture quality allowed for the quantication of shark widths by having at least ten sucient pictures spread over
at least six months across presumed pregnancies (supplementary Fig. S7). We calculated the ratios starting one
year prior to a shark leaving for apparent parturition and took three measurements aer its subsequent return.
Sharks that did not fall into the category ‘pregnant’ or ‘not pregnant’, were le as ‘not scored’. To assess if there
was a consistent trend in abdominal distension throughout presumed gestation, we applied a linear regression
model to the width data until the sharks le.
Residency and lagged identication rate
Maximum likelihood techniques were used to estimate the parameters of residency models using the soware
SOCPROG 2.9101. ese techniques use datasets of individual identications, where the identications themselves
are used as a measure of eort. us, this approach is appropriate for opportunistic data, where sampling periods
are distributed neither randomly nor systematically. As such, this method has been successfully applied to various
photo identication data sets that are characterised by opportunistic data collection and uneven sampling
(e.g.53,55,102,103). e models developed by Whitehead (2001) were applied to estimate residency times55. ese
models include various combinations of emigration, immigration, and mortality with preset parameters testing
for closed and open population models. e results evaluate the time spent within the study area, the time of
absence from the study site aer emigration or mortality, and the population size on any given sampling occasion
(day). e lagged identication rate (LIR) is the probability that an individual animal is re-sighted at the study
site aer a certain time lag55. LIR plots provide insights into the animal movement and residence behaviour
over time and have been applied in this context to whale sharks53, manta rays54 and cetaceans55 amongst others.
Goodness of t of the models was evaluated using the quasi-Akaike information criterion (QAIC) to account for
overdispersion of the data104. e best-t model underwent 1000 bootstrap iterations to obtain standard errors
of residency parameters and 95% condence intervals for the calculated LIRs. As juvenile and adult females
represent most identications, models were run for (i) the entire population, (ii) adult females, and (iii) juvenile
females. If not stated otherwise, all analyses and visualisations were performed in R (version 4.2.2)105.
Ethics declaration
e study was conducted following the guidelines and under the research permits issued by the Environmental
Protection Agency (annually renewable permit: EPA/2021/PA-F01 and EPA/2022/PA-F02) and the Ministry
of Fisheries, Marine Resources and Agriculture, Maldives (annually renewable permit: 30-D/PRIV/2021/190).
e methods employed were non-invasive in nature, ensuring no harm was caused to the animals involved. All
methods used were in accordance with ARRIVE guidelines.
Data availability
All data is available upon reasonable request to the corresponding author.
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Received: 8 April 2024; Accepted: 13 September 2024
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Acknowledgements
is work was possible through the support of Fuvahmulah Dive School and dedicated sta members (including
A. Farish, S. Waseem, H. Rasheedh) collecting video footage of tiger sharks across the seasons. Data collection
was also supported by a wide range of citizen scientists and K. Zerr (Manta Trust) sharing their footage with us.
Our gratitude is further extended to L. Müller for additional help with study design.
Author contributions
LV, GA, TD and TI designed and conceived the project. LV prepared the manuscript and data analysis. JC and
LV collected data during the period of highest sampling eort from 2021 until 2023. MC designed the original
photo identication catalogue and collected data in 2019 and 2020. AN, MR, IS and TI collected data throughout
the entire study period. AN lmed the predation event on a hawksbill turtle. All authors contributed to manu-
script revision.
Funding
Open Access funding enabled and organized by Projekt DEAL.
Additional information
Competing interests
e authors declare no competing interests.
Additional information
Supplementary Information e online version contains supplementary material available at https://doi.
org/10.1038/s41598-024-73079-3.
Correspondence and requests for materials should be addressed to L.V.
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