Development and application of fatty acid tracers to assess the impacts of white shark cage-diving on target and non-target species

Author: Lauren Meyer

Meyer, Lauren, 2019 Development and application of fatty acid tracers to assess the impacts of white shark cage-diving on target and non-target species, Flinders University, College of Science and Engineering

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Marine wildlife tourism is the fastest growing sector of the tourism industry, earning billions of dollars globally, and with it, a myriad of management and conservation challenges. White shark Carcharodon carcharias cage-diving is particularly popular and relies on provisioning, whereby bait (tuna heads and gills) and chum (minced tuna) is used to coax sharks to within view of tourists. Owing to a number of recorded impacts on various shark species, including changes in behaviour, movement, habitat use, and activity levels, wildlife provisioning remains a contentious issue amongst managers, tourism operators, and the public alike. The objective of my thesis is to determine the effects of cage-diving on the diet of target and non-target species using biochemical tracer results obtained via new sampling and analytical methods.

Although biochemical tracers, including fatty acids (FAs), are increasingly used to investigate feeding ecology of marine megafauna, their use in this setting requires further practical development in sample collection and storage scenarios, and quantitative development to understand how to appropriately interpret FA results. Specifically, I assessed the operational limitations of using FAs in the context of white shark tissue collection, determining that muscle and sub-dermal tissue biochemistry was not directly comparable, and that 50 mg of muscle was sufficient to obtain accurate FA profiles. Following this minimum tissue quantity, I modified a biopsy probe intended for underwater use, and showed its ability to collect sufficient tissue from white sharks, both underwater and from above the water’s surface. Together, these chapters provided the practical foundation to confidently apply FA analysis to samples collected from free-swimming white sharks around cage-diving vessels. Yet, analytical uncertainties remained, as our understanding of FAs generally come from controlled experiments on taxa comparatively easier to study. Chondrichthyans (sharks, rays, and chimaeras) have unique metabolic processes which may impact FA biochemical pathways, their deposition in tissues, and resulting data interpretation. I conducted a global analysis of 106 published FA profiles of chondrichthyans, and used a series of multivariate analyses and univariate model averaging to identify which FAs could trace specific aspects of chondrichthyan ecology (i.e. different habitats, water temperatures, trophic guilds, and phylogeny). Habitat type was distinguished by five individual FAs (16:0, 18:0, 22:6ω3, 20:5ω3 and 20:4ω6), allowing these FAs to be confidently used to trace specific foraging habitats (e.g. pelagic vs. reef). These operational (sample collection) and analytical (FA tracers) advances were then applied to white sharks and non-target fishes and rays at the Neptune Islands, Australia to determine if the cage-diving industry affected their diet and nutrition. I found no evidence of dietary shifts or reduced nutritional condition attributed to tourism-exposed residency at the Neptune Islands for the white sharks, despite other work detailing changes in daily activity and habitat use. Yet, all eight non-target species including pelagic fishes, reef fishes and rays, showed dietary shifts consistent with bait and chum consumption. These results showcase how the impacts of provisioning can extend beyond the charismatic species targeted by tourism operators, and also highlight how future research and impact management necessitates an ecosystem-approach, inclusive of non-target species.

Keywords: tourism, lipid, diet, trophic, biopsy, elasmobranch, provisioning

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2019
School: College of Science and Engineering
Supervisor: Charlie Huveneers