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Feeding ecology of whale sharks at Ningaloo Reef, Western Australia

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Marcus Zamora, L 2017 , 'Feeding ecology of whale sharks at Ningaloo Reef, Western Australia', PhD thesis, University of Tasmania.

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Abstract

The largest fish in the world, the iconic whale shark, is a highly migratory species of high conservation risk, that seasonally aggregates at predictable coastal locations in tropical and temperate regions around the world. A number of logistical challenges, however, have hindered our ability to understand key aspects of their ecology, which is imperative in assisting management efforts. In the last decades, biochemical approaches offer tools to identify patterns and drivers of the feeding habits of marine animals at large temporal (from weeks to seasons) and spatial (regional to ocean basin) scales. In this thesis, the feeding ecology of whale sharks were investigated using signature fatty acid and stable isotope analyses of whale shark sub-dermal tissue and a wide range of putative prey species including zooplankton and micronekton collected at Ningaloo Reef, Western Australia.
Fatty acid analysis of whale shark sub-dermal tissue indicated large intraspecific variability in diet, with the differences observed likely due to individual specialization and also changes in the primary and secondary production between years of sampling. Significant differences were found between fatty acid profiles of whale sharks and their potential prey collected at Ningaloo Reef, the latter showing signatures typical of pelagic systems. High relative levels of the omega-6 long-chain (≥ C\(_{20}\)) polyunsaturated fatty acid (LC-PUFA) - arachidonic acid (20:4ω6) - in whale shark profiles suggested that these animals have a wide foraging range with important contribution to their diet from benthic and deep-water habitats including demersal zooplankton.
An experiment assessed the appropriate treatment of sub-dermal tissue and the prey species samples prior to stable isotope analysis. Failing to remove known sources of isotopic variability (lipids, urea or inorganic carbonate) from samples can potentially confound the reconstruction of food webs and movement patterns. Lipid extraction by chloroform:methanol:water and removal of urea by rinsing with deionized water was recommended as a means to standardize δ\(^{13}\)C and δ\(^{15}\)N values in whale shark sub-dermal tissue, and elasmobranchs in general. For the analysis of δ\(^{13}\)C values of zooplankton and micronekton, an aliquot of the sample should be lipid extracted and acidified, with the acidification step employed only if inorganic carbonate is present. In contrast, the erratic effect of lipid extraction and removal of inorganic carbonate in δ\(^{15}\)N values in zooplankton and micronekton, indicated the need for an untreated subsample to be used for the analysis of nitrogen. Previous published mathematical normalizations used to adjust isotopic values were not suitable for all the taxa and tissue types in this study.
Based on stable isotope analysis, whale sharks have an estimate trophic position of 2.6. Enriched \(^{13}\)C signatures in whale sharks compared to collected prey, suggested that these large elasmobranchs are highly reliant on inshore benthic food webs, similar to findings observed in the signature fatty acids. Size- and sex-specific habits of feeding among whale sharks are revealed. Values of δ\(^{15}\)N increased with size of whale sharks, indicating they consume higher trophic level prey as they age. An ontogenetic transition in habitat from offshore to more coastal habitats as whale shark grows (from 3 to 8 m) was also observed.
By improving the applicability and ecological interpretation of non-invasive biochemical approaches, this study has advanced our understanding of the feeding ecology of whale sharks, and has in turn set the path for future studies to use these cost-effective techniques in other aggregations. Given that global populations of whale sharks are in decline, information provided in this study will assist in the development of conservation and management strategies to ensure connectivity for this highly mobile elasmobranchs.

Item Type: Thesis - PhD
Authors/Creators:Marcus Zamora, L
Keywords: elasmobranchs, sharks, fatty acid and stable isotope analysis, diet, trophic ecology
Copyright Information:

Copyright 2017 the author

Additional Information:

Chapter 2 appears to be the equivalent of a post-print version of an article published as: Marcus, L., Virtue, P., Pethybridge, H. R., Meekan, M. G., Thums, M., Nichols, P. D., 2016. Intraspecific variability in diet and implied foraging ranges of whale sharks at Ningaloo Reef, Western Australia, from signature fatty acid analysis, Marine ecology progress series, 554, 115-128. Copyright © 2016 Inter-Research

Chapter 3 appears to be the equivalent of a post-peer-review, pre-copyedit version of an article published in Marine biology. The final authenticated version is available online at: https://doi.org/10.1007/s00227-017-3153-6

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