Author: Joshua Donovan Dennis
Dennis, Joshua Donovan, 2024 Beyond Jaws: exploring the unseen influence of white shark cage-diving on silver trevally (Pseudocaranx georgianus), Flinders University, College of Science and Engineering
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Wildlife tourism often uses food-based attractants to aggregate focal species, unintentionally attracting and feeding non-focal species, the impact of which is poorly understood. In South Australia's Neptune Islands Group Marine Park, bait and berley (southern bluefin tuna Thunnus maccoyii) is used to entice white sharks (Carcharodon carcharias) to cage-diving vessels, inadvertently attracting silver trevally (Pseudocaranx georgianus). These silver trevally form large aggregations around cage-diving vessels as they consume the bait and berley, often impeding tourists' views of the focal white sharks. The impacts of the white shark tourism industry on the spatiotemporal distribution and behaviour of silver trevally, and potential flow-on effects on their physiological processes (i.e., growth, reproduction, healing) is unknown. This study aimed to comprehensively assess how the aggregatory behaviour and feeding of bait and berley may affect silver trevally movements, abundance, growth, and physiological condition.
Due to impacts on the spatiotemporal distribution and activity of similar non-focal species, I first monitored the movements, distribution, and activity of 25 silver trevally in response to cage-diving operators using a fine-scale acoustic telemetry array (Chapter 2). The number of days silver trevally were present per week and the number of hours per day at the Neptune Islands increased by 32% and 20%, respectively, when operators were present. However, a seasonal exodus by 76% of individuals triggered by low water temperature suggested that silver trevally are not permanent residents of the Neptune Islands, and still undergo natural movements away from this near-continuous source of food. Cage-diving tourism also reduced the core space use of silver trevally, aggregating them at the surface (< 5 m depth), close to food-based operators. Despite changes in space use and residency, overall activity did not substantially increase when operators were present, despite frequently observed bursts of acceleration. The near-continuous feeding on bait and berley, large amount of time spent at the Neptune Islands, and the lack of increased activity might result in an energy surplus in silver trevally and affect growth, reproduction, and physiology.
With an understanding of the spatiotemporal distribution and activity of the silver trevally (Chapter 2), the size of the affected population remained unknown. Therefore, I developed and tested novel mark-resight methods (Chapter 3), using the most precise method to quantify and assess trends in the population size of silver trevally occupying the Neptune Islands (Chapter 4). I tagged 700 silver trevally with conventional identification tags, undertook monthly surveys over two years, and used the acoustically tagged silver trevally from Chapter 2 to estimate resighting probability to improve the precision of modelled population size. I estimated up to ~4000 silver trevally at the Neptune Islands, with the population size decreasing with temperature, aligning with the seasonal exodus observed in Chapter 2. However, the number of silver trevally was not affected by the intensity of cage-diving operations.
I then tested the physiological effects of silver trevally exposure to bait and berley using age-at-length, bioelectrical impedance, and fatty acid analysis (Chapter 5). The silver trevally at North Neptune Islands that are frequently exposed to bait and berley were larger than silver trevally of the same age from locations with similar habitats, but without supplemental feeding from cage-diving operations. This was supported by the higher levels of Eicosapentaenoic and Oleic acid in silver trevally from North Neptune Islands. Eicosapentaenoic and Oleic acid, which are known to be high in the bait and berley used, are also known to be important for multiple physiological functions and to increase the overall growth performance, welfare, and condition of teleosts. However, mortality rate and body condition were similar across locations, indicating that while silver trevally may grow faster, consumption of bait and berley may not lead to negative effects on the health or fitness of silver trevally.
This is the first study to assess the ecology of silver trevally in temperate southern Australia, and how it is influenced by wildlife tourism, providing a baseline for impacts of supplemental feeding on a non-focal species. I found effects on the movement, behaviours, and growth of silver trevally, but broader impacts on their health and physiology were undetected. Importantly, despite the effects described here, exodus from the Neptune Islands during cold periods, and the effect of temperature on population size, indicates that silver trevally are still undergoing natural movements and behaviours triggered by thermal cues. Overall, my study comprehensively assesses the ecology of silver trevally through a multidisciplinary approach, showing that small and non-focal species can be affected by provisioning, despite under representation in management frameworks and investigative studies.
Keywords: Silver Trevally, Wildlife Tourism, non-focal, Spatiotemporal distribution, Acoustic Telemetry, Mark-resight, Mark-recapture, Abundance, Growth, Body Condition
Subject: Biology thesis
Thesis type: Doctor of Philosophy
Completed: 2024
School: College of Science and Engineering
Supervisor: Charlie Huveneers