Whale Shark Abundance Research Articles

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.

Whale shark abundance forecast: The interannual hotspot effect

Abstract Each year, whale sharks (Rhincodon typus) aggregate in the northern Gulf of California at Bahia de los Angeles, a small coastal village that recently developed whale shark swimming as economic activity during the summer and autumn. Since the number of animals using the bay fluctuates annually, we aimed to predict it 1 month before each whale shark season. We based our modelling approach on the hypothesis that interannual warming conditions would spatially restrict whale shark habitats, triggering larger aggregations in areas that remain productive. We photo‐identified whale sharks while surface feeding in Bahia de los Angeles during a 13‐year period, which allowed us to estimate a local abundance index per year through mark–recapture techniques. Then, we fitted these abundances as a function of several competing model structures, based on means of sea surface temperature and chlorophyll‐a, as well as their interannual anomalies from the linear trend, during spring, within the best‐known range of the species in the northern Gulf of California. The results of the ecological model showed that whale sharks visit Bahia de los Angeles in larger numbers during extreme warming conditions, with relatively high surface chlorophyll‐a concentrations, which was especially evident during the 2014–2016 northeast Pacific Marine Heatwave and El Niño. Presumably, the overall reduction in surface productivity associated with these events forced whale sharks to restrict their summer–autumn habitat to productivity hotspots, including Bahia de los Angeles. Our model allowed us to predict the number of animals visiting the site before each season starts, which has crucial proactive management implications, such as allowing authorities to regulate the ecotourism effort dynamically to reduce the probability of ship strikes and/or excessive stress to the animals. We propose an abundance baseline from the prediction for neutral interannual conditions. Synthesis and applications. We recommend local authorities apply the model described in this study during the late spring of each year to obtain predictions of the upcoming whale shark abundance and establish the maximum ecotourism effort allowed accordingly.

Zooplankton assemblage diversity in the whale shark Rhincodon typus aggregation area of Nosy Be (Madagascar)

Zooplankton assemblage was studied during seasonal aggregation of the whale shark Rhincodon typus in Nosy Be Island, a hotspot area in the Mozambique Channel. Two different sampling sites were identified in the outer neritic zone: a Feeding area, where whale shark was commonly sighted, and a Control area, never frequented by R. typus. The aim was to investigate zooplankton variability, in terms of composition, biomass and size spectra, in the two areas.Mesozooplankton was collected vertically form 10 m depth to the surface, in November–December 2018–2019 in Control and Feeding areas. A total of 36 zooplankton samples were collected during both campaigns; two different size classes, ≤2 mm and >2 mm and biomass (dry and wet weights) were analysed. Taxonomic composition was performed on 12 samples collected each year. In Feeding and Control areas mesozooplankton composition was similar and mainly represented by Copepoda. Wet and dry weights were higher in Control area compared to the Feeding, with a significant contribution of organisms >2 mm. A decrease of the total number of individuals was recorded in 2019, compared to 2018, characterized by a decrease in copepod abundance (−10%). Biomass decreased of 36.4% in 2019 with respect to 2018, mainly due to a decrease of organisms >2 mm. Similar mesozooplankton assemblage was recorded in the Control and Feeding areas of whale shark, whereas a significant increase in mesozooplankton size in Control area, evidenced by the highest biomass of organisms > 2 mm, could be due to the absence of predators. A decrease of biomass observed in 2019 needs to be further monitored, but the lack of significant differences between mesozooplankton assemblage in Control and Feeding areas suggests a possibility of multiple prey sources. Analysis of prey/predator interaction is recommended for understanding feeding habits and abundance of whale sharks in tropical regions.

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.

Population structure, residency, and abundance of whale sharks in the coastal waters off Nosy Be, north‐western Madagascar

Abstract Between September and December, whale sharks (Rhincodon typus) aggregate in the coastal waters off Nosy Be, an island in north‐western Madagascar. Swimming with these sharks has become an important tourism activity, but no formal protection is in place in Madagascar to protect this endangered species from the potential negative effects of tourism or other human impacts. Boat‐based surveys (n = 405) were conducted from tourism vessels from September to December, 2015–2019. For most sightings (98%), whale sharks were sighted while foraging for bait fish at the surface, in association with mackerel tuna (Euthynnus affinis) and seabirds (Sternidae). A total of 408 individual whale sharks were individually photo‐identified over this period. All individuals were immature, and 82% of sexed sharks were male. Sharks ranged from 3.0 to 8.0 m in total length (TL), with a mean TL of 5.65 ± 0.94 m (n = 66) for females and 5.46 ± 1.09 m for males (n = 295). Most sharks (72% of the identified individuals) were only identified once within the study period. Movement modelling showed an open population with a short mean residence time of 7.2 days. Resightings were recorded from up to 12 years apart (2007–2019). Ten sharks were seen in all five seasons during 2015–2019. A basic POPAN mark–recapture model estimated a total population size of 681 (608–763) sharks over the 2015–2019 period. Nosy Be waters are an important foraging ground for juvenile whale sharks. Sighting data demonstrate that a high proportion of the sharks’ preferred habitat lies outside existing protected areas, but within an identified Key Biodiversity Area. National species‐level protection and increased spatial management is warranted to secure the continued presence of whale sharks in this region.

Whale shark-tuna associations, insights from a small pole-and-line fishery from the mid-north Atlantic

Fisherman have known and explored the association between tuna and whale sharks for centuries. Just as in the past, present day industrial fishers search for the presence of whale shark as cue to locate tuna schools. These whale shark-tuna associations have been usually assumed to result from the same ecological drivers that govern the attraction of tunas and other fishes towards floating objects or Fish Aggregation Devices (FAD). Yet, this assumption has never been explicitly investigated. As a result, the ecological (evolutionary) significance of the whale shark-tuna associations and the potential impacts of fishing these associations remain unclear. Here, we set out to test a set of predictions expected under the framework of the ‘indicator-log’ and the ‘meeting-point’ hypotheses, originally proposed in the context of the tuna-FAD associative behaviour, by contrasting inter-annual fishing patterns, over 16-years, across highly variable whale shark abundance periods and comparing whale shark associated tuna schools and free swimming schools in the Azores. In addition we also analysed the tuna fishing fleet behaviour over periods of high and low whale shark abundance across the archipelago.We found an overall south and eastward migration of fishing effort towards lower latitudes, where most whale sharks were reported, compared to the low whale shark occurrence period. Overall we found that the whale shark associated school differed from free swimming schools and fishing yields were also influenced by the presence of the whale shark. Specifically, we found that skipjack, bigeye, albacore and yellowfin tuna are caught in higher quantities when associated with whale sharks. Whale shark associated schools were more diverse and yielded smaller fish on average than free-swimming schools, as predicted under the theoretical framework of tuna-FAD associations. Our results highlight the possibility that whale shark behaviour may play an active role in the association dynamics, and that these associations may be mutually beneficial. We argue that whale sharks may represent a “meeting point” for the aggregation of small tunas and that this behaviour is more likely to be size dependent than species oriented. Finally, we discuss the potential implications of our findings for the management and conservation of both tunas and whale sharks.

Evaluating abundance trends of iconic species using local ecological knowledge

Abundance is commonly used to assess the status of wildlife populations and their responses to changes in management frameworks. Monitoring abundance trends often requires long-term data collection programs, which are not always carried out. One alternative to scientific surveys is to utilize the local ecological knowledge (LEK), from people in continuous interactions with the environment. We developed a semi-quantitative approach to assess shark population trends by using the LEK of non-extractive resource users. We carried out structured interviews with dive guides regarding the abundance trends of six shark species in the Galapagos Marine Reserve (GMR) across decades since the 1980s. Based on dive guides' LEK, we developed a virtual abundance change (VAC) model to assess the changes in abundance across decades. Our VAC analysis showed a 50% decline in hammerhead sharks and 30% decline in whitetip reef sharks. Silky sharks and Galapagos sharks were perceived to suffer an initial decline by 25% and 30% then stabilized. Whale shark abundance did not appear to have changed. Finally, blacktip sharks showed an apparent recovery after a decline by 25%. Furthermore, our VAC results were comparatively similar to empirical datasets from the GMR and neighboring protected areas of the Eastern Tropical Pacific. Our study highlights the value of LEK in assessing the state of marine resources in data-limited management regions. Our VAC method offers an alternative approach by which LEK can provide valuable insights into the historical trends of species abundance.

Acoustic backscatter at a Red Sea whale shark aggregation site

An aggregation of sexually immature whale sharks occurs at a coastal submerged reef near the Saudi Arabian Red Sea coast each spring. We tested the hypothesis that these megaplanktivores become attracted to a prey biomass peak coinciding with their aggregation. Acoustic backscatter of the water column at 120 kHz and 333 kHz –a proxy for potential prey biomass – was continuously measured spanning the period prior to, during, and subsequent to the seasonal whale shark aggregations. No peak in acoustic backscatter was observed at the time of the aggregation. However, we observed a decrease in acoustic backscatter in the last days of deployment, which coincided the trailing end of whale shark season. Organisms forming the main scattering layer performed inverse diel vertical migration, with backscatter peaking at mid-depths during the day and in the deeper half of the water column at night. Target strength analyses suggested the backscatter was likely composed of fish larvae. Subsurface foraging behavior of the whale sharks within this aggregation has not been described, yet this study does not support the hypothesis that seasonal peaks in local whale shark abundance correspond to similar peaks in prey availability.

Observations on the Whale Shark (Rhincodon typus) in the Dutch Caribbean

Records of whale sharks in the Caribbean are relatively sparse. Here we document 24 records of whale sharks (Rhincodon typus Smith 1882) for the Dutch Caribbean, four for the windward islands of Saba, St. Eustatius and St. Maarten, and twenty for the southern Caribbean leeward islands of Aruba, Curaçao and Bonaire. The results suggest a higher abundance of whale sharks in the southern, leeward part of the Dutch Caribbean, likely associated with seasonal upwelling-driven productivity known for the southeastern Caribbean area. A bimodal seasonal pattern as documented for Venezuela was not as pronounced in our findings for the Leeward Dutch Caribbean and whale sharks were recorded in 9 months of the year. In the Windward Dutch Caribbean all (4) records so far were for the winter months of December-February. Most records involved large and solitary animals in contrast to areas elsewhere suspected of being nursery habitat. According to local sources, whale sharks were most often associated with feeding tunas and sea surface swarms of crab megalopae.

Can citizen science monitor whale-shark aggregations? Investigating bias in mark–recapture modelling using identification photographs sourced from the public

Context The conservation status of the whale shark, Rhincodon typus, remains uncertain. Throughout their range, whale sharks are a focus for wildlife tourism and many identification photographs taken by tourists have become available online, presenting a potentially valuable source of data for monitoring populations using mark–recapture techniques. However, the suitability of these photographs for mark–recapture models has yet to be investigated. Aims We explore the suitability of identification-photographs available from online databases to produce assessments of life-history parameters and conservation status of whale sharks in the Maldives. Methods To test the validity of using publically sourced images, we used photo-identification images collected from both experienced researchers and tourists between 2003 and 2008 to construct two databases. Images taken by tourists were compiled from online databases. Researcher and public databases were analysed separately and the results of mark–recapture models then compared. Key results The dataset constructed from online public databases did not violate the assumptions of mark–recapture modelling. Estimates of parameters and abundance obtained from models using these data were similar to those produced using data provided by experienced researchers. Conclusions Publically sourced data allowed for the accurate estimation of abundance of whale sharks. These estimates were not confounded by the suitability of photographs, probably because of the high encounter rate in the aggregation, the high residency rate of sharks and the retrospective nature of photo-identification, which limited heterogeneity in capture probability between marked and unmarked sharks. Implications Our findings support the use of publically sourced data for use in mark–recapture studies of whale sharks, at least in situations where sharks are resident to the location. This approach will be useful in regions where data collected by tourists are available online, and research funding is limited.

Oceanographic and atmospheric phenomena influence the abundance of whale sharks at Ningaloo Reef, Western Australia

Seasonal observations of whale shark abundance recorded by ecotourist operators at Ningaloo Reef, Western Australia from 1999 to 2004 were compared with weekly regional and global oceanographic and atmospheric variables, including average sea surface temperatures, along-shelf wind shear, sea level and the Southern Oscillation Index (SOI). Estimates of these physical variables were derived from either ground-based data or from remote sensing instruments. A generalised linear mixed-effects modelling (GLMM) approach with random sampling and model simulation was used to determine the relationships between the number of whale sharks and all model variants of the environmental parameters, using information-theoretic weights of evidence to rank models. SOI and wind shear had the most support for explaining the deviance in weekly whale shark abundance at Ningaloo Reef during a season. The SOI and wind shear variables positively influenced whale shark abundance such that more sharks were sighted when the Southern Oscillation was stronger and along-shelf winds were increasingly prevalent. This may reflect changes in the strength of oceanographic processes such as the Leeuwin Current (in response to the Southern Oscillation) and wind/current driven upwelling which may affect the abundance of whale sharks transported to the region and/or the availability of their prey by driving productivity changes.

Aerial survey as a tool to estimate whale shark abundance trends

Aerial surveys have been used to estimate population abundance of both terrestrial and marine species; in the marine environment this has largely been used for air-breathing species that spend time regularly at the surface. Whale sharks spend a large proportion of their time close to the surface and so are amenable to aerial survey techniques. This study presents the results of six years of synoptic aerial belt-surveys done nearly daily during the peak whale shark season around the island of Mahe, Seychelles. A total of 580 survey flights were flown providing 699.7 hours of survey record. A seasonal peak of shark sightings per hour was recorded in September or October in most years with the maximum on a single survey of 28.4 h - 1 in October 2006. The aerial survey data were used to generate an estimate of relative population abundance indicating that highest mean annual relative population estimate was also in 2006, with an estimate of 38, while the lowest mean estimate was 11 in 2004. These estimates were then compared to weekly capture-mark-recapture estimates of abundance based on unique individual identification data. The results indicate that the use of aerial survey data alone may give an acceptable indication of instantaneous relative population abundance but further refinement is necessary to estimate absolute regional abundance.

The Seasonal Aggregation of Whale Sharks at Ningaloo Reef, Western Australia: Currents, Migrations and the El Niño/ Southern Oscillation

Whale sharks, Rhincodon typus, seasonally aggregate in coastal waters off Ningaloo Reef, Western Australia. We review the oceanographic setting of the region and present evidence that such aggregations form as a result of migratory behavior associated with climatic and oceanographic processes. We utilise records of whale shark abundance collected at Ningaloo Reef from dedicated searches by boat and aircraft and from log sheets recorded by the tourism industry. Measures of whale shark abundance derived from log sheet data sets were moderately correlated with the Southern Oscillation Index and weakly correlated with coastal sea level, an index of the strength of the Leeuwin Current, and sea surface temperature over the period 1993 to 1998. Abundances of whale sharks derived from boat searches from 1983 to 1992 were also correlated with fluctuations in the Southern Oscillation Index, except during a three year period from 1988 to 1990. We conclude that, at least in some years, there appears to be a link between the abundance of aggregating whale sharks and the physical and biological oceanography of the region, with greater whale shark numbers in La Nina years. The lack of correlation in other years may be due to a combination of uneven quality of data and/or aggregations occurring in response to a complex interaction between the physical and biological oceanography of the region.