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MEPS prepress abstract   -  DOI:

Environmental conditions are poor predictors of immature white shark Carcharodon carcharias occurrences on coastal beaches of eastern Australia

Julia L. Y. Spaet*, Andrea Manica, Craig P. Brand, Christopher Gallen, Paul A. Butcher

*Corresponding author:

ABSTRACT: Understanding and predicting the distribution of organisms in heterogeneous environments is a fundamental ecological question and a requirement for sound management. To implement effective conservation strategies for white shark (Carcharadon carcharias) populations, it is imperative to define drivers of their movement and occurrence patterns and to protect critical habitats. Here, we acoustically tagged 444 immature white sharks and monitored their presence in relation to environmental factors over a three-year period (2016–2019) using an array of 21 iridium satellite-linked (VR4G) receivers spread along the New South Wales coast of Australia. Results of generalized additive models showed that all tested predictors (‘month’, ‘time of day’, ‘water temperature’, ‘tidal height’, ‘swell height’, ‘lunar phase’) had a significant effect on shark occurrence (p<0.01). However, collectively, these predictors only explained 1.8% of deviance, suggesting that statistical significance may be rooted in the large sample size rather than biological importance. On the other hand, receiver location, which captures geographic fidelity and local conditions not captured by the aforementioned environmental variables, explained a sizeable 17.3% of deviance. Sharks tracked in this study hence appear to be tolerant to episodic changes in environmental conditions and movement patterns are likely related to currently undetermined, location-specific habitat characteristics or biological components, such as local currents, prey availability or competition. Importantly, we show that performance of VR4G receivers can be strongly affected by local environmental conditions, and provide an example of how a lack of range test controls can lead to misinterpretation and erroneous conclusions of acoustic detection data.