Residency and trophic ecology of juvenile whale sharks (Rhincodon typus) in the Western Indian Ocean

Abstract

The whale shark (Rhincodon typus) is the largest fish in the world, and while whale sharks are popular, charismatic megafauna supporting a major tourist industry, the global whale shark population has decreased by >50% in the past decade. Whale sharks are now listed as Endangered on the IUCN Red List of Threatened Species. Effective conservation of whale sharks depends on accurate and reliable information describing their trophic and spatial ecology. Whale sharks are found in predictable aggregations in certain coastal and island areas in the tropics and subtropics, presumably for feeding. ‘Whale shark season’ typically lasts for a short period at each site, and is often associated with an ephemeral food source, with individual sharks present for varying lengths of time. Conservation assessment of whale sharks through their entire range is difficult due to limited knowledge of their behaviour following dispersal from these coastal aggregation sites, as well as the poor understanding of their movements and geographical connectivity on a timeframe that is relevant to management. Whale sharks are relatively rare, oceanic, pelagic animals. The difficulty of observing the behaviour of mobile animals at sea, particularly in the open ocean, has encouraged the use of indirect biochemical methods to infer aspects of trophic and spatial ecology. Stable isotope analysis of consumer tissues provides a useful tool to investigate the retrospective movement and trophic ecology of mobile animals, and fatty acid analysis provides further information on diet. In this thesis I use stable isotope and fatty acid markers coupled with direct data on residency and movement of known individual whale sharks, derived through photo-identification to examine the trophic and spatial ecology of three male-dominated whale shark aggregations in the Western Indian Ocean and Arabian Gulf. I have assembled samples from 186 individuals, representing an estimated 7 to 55% of all individuals in their respective aggregations. Comparing the isotopic composition of whale shark tissues with known isotopic latitudinal gradients in diet, paired with long-term photo-identification data, I demonstrate limited latitudinal movement of individual sharks between important whale shark feeding aggregations in Mozambique, Tanzania, and Qatar. The relatively large isotopic dataset allows the first realistic assessment of isotopic niche breadth in whale shark feeding aggregations. Based on these results, I argue that eastern African and Arabian whale sharks should be treated as separate management units until their functional connectivity is shown to be significant. For the Tanzanian sharks, I have applied both stable isotope analysis and fatty acid analysis to this comparatively resident aggregation over a multi-year timescale. Stable isotope analysis indicates that the sharks forage primarily within the local food web. Fatty acid results show epipelagic feeding, and population-level responses to seasonal environmental changes. However, highly distinctive lipid class compositions within the local food web also suggest preferential routing of lipids in whale shark tissues. Preferential routing of essential fatty acids may be an unrecognised source of variance in elasmobranch dietary ecology, particularly filter feeding elasmobranchs, which urgently requires further study. Lastly I have used stable isotope analysis and dietary mixing models to investigate the coastal and offshore habitat use of an unseasonal whale shark aggregation in Mozambique. Here, isotope results suggest that whale sharks are feeding primarily on epipelagic zooplankton, and the sharks’ presence is tied to ocean-scale dynamics, with possible dietary contributions from epipelagic and deep-water sources. Mixing models imply the existence of an un-sampled dietary item, potentially from offshore, oligotrophic waters and / or dietary routing in whale sharks. Obtaining information on mobile marine animals is challenging. I also discuss the uses and limitations of stable isotope analysis techniques applied to whale sharks over varying spatial and temporal scales. I have shown that stable isotope analysis when used in conjunction with other methods can prove a valuable tool to shed light on their trophic and spatial ecology, but that interpretation of biochemical data is challenging particularly in the absence of experimental studies validating physiological and biochemical assumptions.

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ORCID for Clive Trueman: orcid.org/0000-0002-4995-736X

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