Shark Aggregation Research Articles

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

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.

Observations on an Aggregation of Grey Reef Sharks (Carcharhinus amblyrhynchos) in the Mozambique Channel Off the Coast of Nosy Be (Madagascar) and Tools for Photo-Identification-A New Aggregation Nursery Site?

Following preliminary underwater observations of about 1000 h carried out monthly between 2012 and 2023 (except the years 2021 and 2022), 23 specimens of grey reef sharks were spotted and photo-identified off the coast of Nosy Be in Madagascar, on an emerging reef called "Mokarran" at a depth between 15 and 30 m. Over 10 years of observations, eight specimens were re-sighted, identified with a non-invasive photo-identification technique of part of the first dorsal and the caudal fin, and one specimen was re-identified after 1982 days from the first sighting, i.e., after more than 5 years. In addition, six specimens of probably pregnant females were also identified in the same area. The population was entirely made up of females. The aggregation area could represent a new nursery site which, if confirmed after further investigations, will require greater protection.

Effects of climate warming on energetics and habitat of the world's largest marine ectotherm

Responses of organisms to climate warming are variable and complex. Effects on species distributions are already evident and mean global surface ocean temperatures are likely to warm by up to 4.1 °C by 2100, substantially impacting the physiology and distributions of ectotherms. The largest marine ectotherm, the whale shark Rhincodon typus, broadly prefers sea surface temperatures (SST) ranging from 23 to 30 °C. Whole-species distribution models have projected a poleward range shift under future scenarios of climate change, but these models do not consider intraspecific variation or phenotypic plasticity in thermal limits when modelling species responses, and the impact of climate warming on the energetic requirements of whale sharks is unknown. Using a dataset of 111 whale shark movement tracks from aggregation sites in five countries across the Indian Ocean and the latest Earth-system modelling produced from Coupled Model Intercomparison Project Phase 6 for the Intergovernmental Panel on Climate Change, we examined how SST and total zooplankton biomass, their main food source, may change in the future, and what this means for the energetic balance and extent of suitable habitat for whale sharks. Earth System Models, under three Shared Socioeconomic Pathways (SSPs; SSP1–2.6, SSP3–7.0 and SSP5–8.5), project that by 2100 mean SST in four regions where whale shark aggregations are found will increase by up to 4.9 °C relative to the present, while zooplankton biomass will decrease. This reduction in zooplankton is projected to be accompanied by an increase in the energetic requirements of whale sharks because warmer water temperatures will increase their metabolic rate. We found marked differences in projected changes in the extent of suitable habitat when comparing a whole-species distribution model to one including regional variation. This suggests that the conventional approach of combining data from different regions within a species' distribution could underestimate the amount of local adaptation in populations, although parameterising local models could also suffer from having insufficient data and lead to model mis-specification or highly uncertain estimates. Our study highlights the need for further research into whale shark thermal tolerances and energetics, the complexities involved in projecting species responses to climate change, and the potential importance of considering intraspecific variation when building species distribution models.

Fatty acid composition of whale shark (Rhincodon typus), and zooplankton in two aggregation sites in the Gulf of California

The whale shark Rhincodon typus (Smith, 1828) is a filter-feeding species that feeds on zooplankton. There are whale shark aggregation sites in the Gulf of California, Bahia de La Paz in Baja California Sur, and Bahía de Los Angeles in Baja California. Whale shark ecotourism is an important and economically profitable activity for the coastal communities surrounding their habitat areas. Fatty acid (FA) profiles, including arachidonic acid (ARA), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA), are used as trophic tracers in marine food webs because they are essential for marine fish and provide information on the nutritional quality of zooplankton. In the present study, we analyzed the FA profile of zooplankton to trace them in the stomach contents, blood, liver, and muscle of stranded male whale sharks and the subdermal tissue of live whale sharks. Total lipids were extracted to calculate the energy density. Then lipids were methylated to obtain FA methyl esters (FAMEs) and analyzed in a gas chromatograph with a flame ionization detector. Permutational multivariate analysis of variance was applied to compare FA profiles. Zooplankton of both aggregation sites had 70 % similarity in their FA profiles. The FA profile found in the liver of whale sharks was similar to that of zooplankton in both aggregation sites (DHA > EPA > ARA). However, muscle and subdermal tissue had FA profiles significantly different from zooplankton, and there was preferential ARA assimilation in the subdermal tissue of stranded and live whale sharks. Therefore, FA profiles reflected just 50–60 % of the diet, suggesting that whale shark tissues retain FA according to their metabolic requirements or that surficial zooplankton do not provide ARA detected in subdermal tissue. Future studies should consider night sampling and FA analysis of demersal zooplankton in each aggregation site.

Whale shark abundance, spatial distribution and effective tourism areas in the northern Mexican Caribbean

ABSTRACT Every year, from June to September, the whale shark (Rhincodon typus) is present in the Mexican Caribbean and has become a highly attractive tourist resource. In this study, we assess the spatial and temporal distribution of whale sharks and estimate the size of the effective tourism area from data collected between 2016 and 2021. The main whale shark aggregation site in the Mexican Caribbean is called ‘El Azul’, it has an average size of 400 km2. Using aerial survey data, we found 37 whale shark aggregations where we counted 1–170 individuals and 16–150 tourist boats. The average size of the area covered by these groups was 2.9 km2. The highest density of whale sharks was recorded in 2020 (48 whale sharks per km2) and the lowest in 2021 (5 whale sharks per km2). Estimating the effective tourism area (where whale shark tourism takes place) within the entire aggregation area is important to refine tourism carrying capacity calculations and to avoid miscalculations that could lead to over-exploitation of whale shark tourism activities with negative consequences for the species. The information from this study is directly applicable to adaptive ecotourism management, improving the sustainability of whale shark tourism, and biological monitoring.

Emergent community architecture despite distinct diversity in the global whale shark (Rhincodon typus) epidermal microbiome

Microbiomes confer beneficial physiological traits to their host, but microbial diversity is inherently variable, challenging the relationship between microbes and their contribution to host health. Here, we compare the diversity and architectural complexity of the epidermal microbiome from 74 individual whale sharks (Rhincodon typus) across five aggregations globally to determine if network properties may be more indicative of the microbiome-host relationship. On the premise that microbes are expected to exhibit biogeographic patterns globally and that distantly related microbial groups can perform similar functions, we hypothesized that microbiome co-occurrence patterns would occur independently of diversity trends and that keystone microbes would vary across locations. We found that whale shark aggregation was the most important factor in discriminating taxonomic diversity patterns. Further, microbiome network architecture was similar across all aggregations, with degree distributions matching Erdos–Renyi-type networks. The microbiome-derived networks, however, display modularity indicating a definitive microbiome structure on the epidermis of whale sharks. In addition, whale sharks hosted 35 high-quality metagenome assembled genomes (MAGs) of which 25 were present from all sample locations, termed the abundant ‘core’. Two main MAG groups formed, defined here as Ecogroup 1 and 2, based on the number of genes present in metabolic pathways, suggesting there are at least two important metabolic niches within the whale shark microbiome. Therefore, while variability in microbiome diversity is high, network structure and core taxa are inherent characteristics of the epidermal microbiome in whale sharks. We suggest the host-microbiome and microbe-microbe interactions that drive the self-assembly of the microbiome help support a functionally redundant abundant core and that network characteristics should be considered when linking microbiomes with host health.

Occurrence of whale sharks (Rhincodon typus) in the Kwatisore Waters within the Cenderawasih Bay National Park as a basis for carrying capacity estimations and assessments for ecotourism suitability

Kwatisore waters in the Cenderawasih Bay National Park area is one of the whale sharks’ habitats in Indonesia. As shark sightings seem to occur daily there is the opportunity for ecotourism development in the area. This study aims to quantify the occurrence of whale sharks at Kwatisore, and estimate the carrying capacity of the area, together with its suitability for ecotourism based on whale shark aggregations. Primary data collection on the appearance of whale sharks was carried out every day through a census technique on 7 designated areas (Apr to Aug, 2022). Every whale shark that appears on the surface of the water is photographed and identified by an ID number based on the I3Sv2.0 and its sex determined. During the census period, 709 whale sharks’ sightings were recorded mostly in the morning. There were 15 individuals in total, all males with a total length ranging from 3-7 m. The carrying capacity of the area for whale shark ecotourism in Kwatisore waters is a maximum of 252 people per day who are allowed to interact with whale sharks, while the tourism suitability index is included in the very appropriate category for whale shark tourism.

Occurrence of whale sharks (Rhincodon typus) in the Kwatisore Waters within the Cenderawasih Bay National Park as a basis for carrying capacity estimations and assessments for ecotourism suitability

Kwatisore waters in the Cenderawasih Bay National Park area is one of the whale sharks’ habitats in Indonesia. As shark sightings seem to occur daily there is the opportunity for ecotourism development in the area. This study aims to quantify the occurrence of whale sharks at Kwatisore, and estimate the carrying capacity of the area, together with its suitability for ecotourism based on whale shark aggregations. Primary data collection on the appearance of whale sharks was carried out every day through a census technique on 7 designated areas (Apr to Aug, 2022). Every whale shark that appears on the surface of the water is photographed and identified by an ID number based on the I3Sv2.0 and its sex determined. During the census period, 709 whale sharks’ sightings were recorded mostly in the morning. There were 15 individuals in total, all males with a total length ranging from 3-7 m. The carrying capacity of the area for whale shark ecotourism in Kwatisore waters is a maximum of 252 people per day who are allowed to interact with whale sharks, while the tourism suitability index is included in the very appropriate category for whale shark tourism.

Patterns of overlapping habitat use of juvenile white shark and human recreational water users along southern California beaches.

Juvenile white sharks (JWS) of the Northeastern Pacific population are present in nearshore southern California waters and form mixed size class (~1.5-3 m) aggregations for weeks to months, often within 500 m of shore. These nearshore beach habitats are heavily used for human recreation (e.g., surfing, swimming, body boarding, wading, and standup paddleboarding) and the amount of spatio-temporal overlap between JWS and humans is currently unknown. Increases in human population and the Northeastern Pacific population of white sharks have raised concern over human beach safety. To determine spatio-temporal JWS-human overlap at various spatial scales (e.g., across the entire southern California coastline, across different distances from shore, and within specific beach locations), 26 beach locations across southern California were surveyed monthly resulting in 1644 aerial drone surveys between January 2019 to March 2021. Thirteen environmental variables were assessed to predict when spatio-temporal overlap between JWS and water users was highest. Coast-wide distribution of JWS was clumped, limiting human-shark co-occurrence to specific locations, with 1096 of 1204 JWS observations occurring at Carpinteria and Del Mar Beach locations. Nearshore distribution indicated JWS are often close enough to the wave break to interact with some water users (median = 101 m, range = 2-702 m), although JWS had the most spatial overlap with stand-up paddlers. Daily human-shark co-occurrence was 97% at beaches where JWS aggregations had formed, and human activity showed high spatial overlap at shark aggregation sites. Although there is higher seasonal human-shark spatio-temporal overlap where aggregations form in southern California, the number of unprovoked shark bites across southern California is extremely low. This study provides evidence that high human-shark spatio-temporal overlap does not lead to an increased bite frequency in southern California, and there are a number of possible explanations as to why JWS are not biting water users despite daily encounters.

Comparing spatial management tools to protect highly migratory shark species in the Eastern Mediterranean Sea hot spots

Bycatch of non-target species is a pressing problem for ocean management. It is one of the most concerning issues related to human-wildlife interactions and it affects numerous species including sharks, seabirds, sea turtles, and many critically endangered marine mammals. This paper compares different policy tools for ocean closure management around a unique shark aggregation site in Israel's nearshore coastal waters. We provide a set of recommendations based on an optimal management approach that allows humans to enjoy marine recreational activities such as fishing, while maintaining safe conditions for these apex predators which are vital to the local marine ecosystem. To learn more about recreational fishers' derived benefits, we use a benefit transfer method. Our main conclusion is that dynamic time-area closures offer sustainable and effective management strategies. Since these closures are based on near real-time data, they might successfully preserve specific species in limited areas (i.e., small areas).

Preliminary study of shark microbiota at a unique mix-species shark aggregation site, in the Eastern Mediterranean Sea.

Sharks, as apex predators, play an essential ecological role in shaping the marine food web and maintaining healthy and balanced marine ecosystems. Sharks are sensitive to environmental changes and anthropogenic pressure and demonstrate a clear and rapid response. This designates them a "keystone" or "sentinel" group that may describe the structure and function of the ecosystem. As a meta-organism, sharks offer selective niches (organs) for microorganisms that can provide benefits for their hosts. However, changes in the microbiota (due to physiological or environmental changes) can turn the symbiosis into a dysbiosis and may affect the physiology, immunity and ecology of the host. Although the importance of sharks within the ecosystem is well known, relatively few studies have focused on the microbiome aspect, especially with long-term sampling. Our study was conducted at a site of coastal development in Israel where a mixed-species shark aggregation (November-May) is observed. The aggregation includes two shark species, the dusky (Carcharhinus obscurus) and sandbar (Carcharhinus plumbeus) which segregate by sex (females and males, respectively). In order to characterize the bacterial profile and examine the physiological and ecological aspects, microbiome samples were collected from different organs (gills, skin, and cloaca) from both shark species over 3 years (sampling seasons: 2019, 2020, and 2021). The bacterial composition was significantly different between the shark individuals and the surrounding seawater and between the shark species. Additionally, differences were apparent between all the organs and the seawater, and between the skin and gills. The most dominant groups for both shark species were Flavobacteriaceae, Moraxellaceae, and Rhodobacteraceae. However, specific microbial biomarkers were also identified for each shark. An unexpected difference in the microbiome profile and diversity between the 2019-2020 and 2021 sampling seasons, revealed an increase in the potential pathogen Streptococcus. The fluctuations in the relative abundance of Streptococcus between the months of the third sampling season were also reflected in the seawater. Our study provides initial information on shark microbiome in the Eastern Mediterranean Sea. In addition, we demonstrated that these methods were also able to describe environmental episodes and the microbiome is a robust measure for long-term ecological research.

A first look at whale sharks in Hawaiian waters: Using citizen science to study the world's largest fish, Rhincodon typus

Abstract The world's largest fish, the whale shark (Rhincodon typus), is a circum‐tropically distributed and globally endangered species, that is widely studied at predictable aggregation sites. The main Hawaiian Islands are not known to have large aggregations of whale sharks; however, they have been anecdotally reported here with some regularity. To date, little is known about this charismatic species in Hawaiian waters. This study is the first effort to examine whale shark demographics and movements in a previously unstudied region of the world. Here, citizen science was used to investigate the abundance, seasonality (if any) and occurrence of whale sharks in the waters around the main Hawaiian Islands. A total of 309 individual whale sharks were identified from sightings between 1991–2020 by their unique spot patterns, most of which (74%) were reported between 2018 and 2020. Estimated whale shark total length ranged from 2–12 m (mean 6.1 m), with both juvenile and mature males and females represented in the dataset. The best‐fit Lagged Identification Rate (LIR) model for these data suggests that individuals sighted in Hawaiian waters are transient in nature, supporting the empirical data (88% of sharks sighted only once). Although using citizen science data can have inherent biases, these results present a conservative first assessment of the demographics of whale sharks in Hawaiian waters. For an Endangered species assessed as Largely Depleted by the IUCN Green Status of Species, highlighting Hawai’i as an important habitat for the whale shark – mainly as a migratory corridor or navigational waypoint – is a crucial step in understanding their ecology in the Pacific in order to develop effective management plans.

An open spatial capture-recapture framework for estimating the abundance and seasonal dynamics of white sharks at aggregation sites

The present study provides the first estimate of abundance for the white shark at a new aggregation site in the western North Atlantic, which required the development of a novel modeling framework to accommodate the species’ migratory behavior. Estimates of abundance are needed to evaluate the performance of existing conservation measures for white shark populations worldwide but have historically been infeasible to obtain in the region. Following the recent emergence of Cape Cod, Massachusetts, USA, as a seasonal aggregation site, we conducted a photographic capture-recapture survey and identified 393 individual white sharks from 2015-2018. As conventional capture-recapture models do not adequately represent the species’ migratory behavior, we extended an existing open spatial capture-recapture framework to allow for movements into and out of the surveyed area and accommodate variation in residency and habitat use among individuals. Using simulations, we demonstrated that failing to account for these processes resulted in biased estimates of abundance that would be misleading if used as the basis for management advice. We applied the model developed to describe the seasonal dynamics of the Cape Cod aggregation site and estimated a superpopulation size of 800 (393-1286) individuals, which provides an important baseline for this species of conservation concern. Because it directly links changes in abundance over time to the demographic processes underpinning them, the model described provides a more mechanistic understanding of the dynamics of white shark aggregations and improves the applied relevance of the results for the conservation and management of the species.

Improving sightings-derived residency estimation for whale shark aggregations: A novel metric applied to a global data set

The world’s largest extant fish, the whale shark Rhincodon typus, is one of the most-studied species of sharks globally. The discovery of predictable aggregation sites where these animals gather seasonally or are sighted year-round – most of which are coastal and juvenile-dominated – has allowed for a rapid expansion of research on this species. The most common method for studying whale sharks at these sites is photographic identification (photo-ID). This technique allows for long-term individual-based data to be collected which can, in turn, be used to evaluate population structure, build population models, identify long-distance movements, and assess philopatry and other population dynamics. Lagged identification rate (LIR) models have fewer underlying assumptions than more traditional capture mark recapture approaches, making them more broadly applicable to marine taxa, especially far-ranging megafauna species like whale sharks. However, the increased flexibility comes at a cost. Parameter estimations based on LIR can be difficult to interpret and may not be comparable between areas with different sampling regimes. Using a unique data-set from the Philippines with ~8 years of nearly continuous survey effort, we were able to derive a metric for converting LIR residency estimates into more intuitive days-per-year units. We applied this metric to 25 different sites allowing for the first quantitatively-meaningful comparison of sightings-derived residence among the world’s whale shark aggregations. We validated these results against the only three published acoustic residence metrics (falling within the ranges established by these earlier works in all cases). The results were then used to understand residency behaviours exhibited by the sharks at each site. The adjusted residency metric is an improvement to LIR-based population modelling, already one of the most widely used tools for describing whale shark aggregations. The standardised methods presented here can serve as a valuable tool for assessing residency patterns of whale sharks, which is crucial for tailored conservation action, and can cautiously be tested in other taxa.

First description of a sex segregated aggregation of silky sharks (Carcharhinus falciformis) and the frequency and distribution of mating wounds off the tip of the Baja California Peninsula

First description of a sex segregated aggregation of silky sharks (Carcharhinus falciformis) and the frequency and distribution of mating wounds off the tip of the Baja California Peninsula

Correction to: First description of a sex segregated aggregation of silky sharks (Carcharhinus falciformis) and the frequency and distribution of mating wounds off the tip of the Baja California Peninsula

Correction to: First description of a sex segregated aggregation of silky sharks (Carcharhinus falciformis) and the frequency and distribution of mating wounds off the tip of the Baja California Peninsula

Correction to: Preliminary insights of a mixed‑species shark aggregation: a case study of two carcharhinids from the Mediterranean Sea

Correction to: Preliminary insights of a mixed‑species shark aggregation: a case study of two carcharhinids from the Mediterranean Sea

Preliminary insights of a mixed-species shark aggregation: a case study of two carcharhinids from the Mediterranean Sea

Preliminary insights of a mixed-species shark aggregation: a case study of two carcharhinids from the Mediterranean Sea

Fear at the top: killer whale predation drives white shark absence at South Africa’s largest aggregation site

Risk-induced fear effects exerted by top predators are pervasive in terrestrial and marine systems, with lasting impacts on ecosystem structure and function. The loss of top predators can disrupt ecosystems and trigger trophic cascades, but the introduction of novel apex predators into ecosystems is not well understood. We documented the emigration of white sharks Carcharodon carcharias in response to the presence of a pair of killer whales, Orcinus orca, at a large white shark aggregation site in South Africa. Between February and June in 2017, five white shark carcasses washed up on beaches in Gansbaai, Western Cape Province, four of which had their livers removed. Sightings per unit effort (sea days) and telemetry data demonstrated that white sharks emigrated from Gansbaai following these predation events, and in response to further sightings of this pair and other killer whale pods in the vicinity. Tagging data demonstrated the immediate departure of white sharks from Gansbaai, and some sharks were subsequently moving east. Contrary to expected and well-documented patterns of white shark occurrence at this site, their sightings dropped throughout the following 2.5 years; change-point analysis on both datasets confirmed these departures coincided with killer whale presence and shark carcasses washing out. These findings suggest that white sharks respond rapidly to risk from a novel predator, and that their absence triggered the emergence of another predator, the bronze whaler Carcharhinus brachyurus. Predator–prey interactions between white sharks, other coastal sharks, and killer whales are increasing in South Africa and are expected to have pronounced impacts on the ecosystem.

Pieces in a global puzzle: Population genetics at two whale shark aggregations in the western Indian Ocean.

The whale shark Rhincodon typus is found throughout the world's tropical and warm‐temperate ocean basins. Despite their broad physical distribution, research on the species has been concentrated at a few aggregation sites. Comparing DNA sequences from sharks at different sites can provide a demographically neutral understanding of the whale shark's global ecology. Here, we created genetic profiles for 84 whale sharks from the Saudi Arabian Red Sea and 72 individuals from the coast of Tanzania using a combination of microsatellite and mitochondrial sequences. These two sites, separated by approximately 4500 km (shortest over‐water distance), exhibit markedly different population demographics and behavioral ecologies. Eleven microsatellite DNA markers revealed that the two aggregation sites have similar levels of allelic richness and appear to be derived from the same source population. We sequenced the mitochondrial control region to produce multiple global haplotype networks (based on different alignment methodologies) that were broadly similar to each other in terms of population structure but suggested different demographic histories. Data from both microsatellite and mitochondrial markers demonstrated the stability of genetic diversity within the Saudi Arabian aggregation site throughout the sampling period. These results contrast previously measured declines in diversity at Ningaloo Reef, Western Australia. Mapping the geographic distribution of whale shark lineages provides insight into the species’ connectivity and can be used to direct management efforts at both local and global scales. Similarly, understanding historical fluctuations in whale shark abundance provides a baseline by which to assess current trends. Continued development of new sequencing methods and the incorporation of genomic data could lead to considerable advances in the scientific understanding of whale shark population ecology and corresponding improvements to conservation policy.