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 Carcharodon 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 3 yr period (2016-2019) using an array of 21 iridium satellite-linked (VR4G) receivers spread along the coast of New South Wales, 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. 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.
Highlights
Variation in the spatial distribution of wild animals is largely determined by shifts in habitat use of individuals
Given the described importance of environmental drivers in the distribution and movements of white sharks and the susceptibility of the eastern Australasian population to habitat modification (Depart-Spaet et al.: Drivers of white shark occurrence ment of Sustainability, Environment, Water, Population and Communities 2013), we explored a range of environmental and temporal variables that could influence the occurrence of immature white sharks along the coast of New South Wales (NSW), eastern Australia
Receiver performance was likely influenced by both environmental conditions and biological noise, providing an example of how a lack of controls can lead to misinterpretation of shark occurrence patterns
Summary
Variation in the spatial distribution of wild animals is largely determined by shifts in habitat use of individuals. Coastal habitats along the Australian east coast are regularly frequented by juvenile and sub-adult (hereafter referred to as immature) white sharks Carcharodon carcharias (Linnaeus 1758) which, except for occasional across-ocean excursions (Bruce et al 2019, Spaet et al 2020), primarily move among a relatively small number of interconnected habitats and the 120 m depth contour (Bruce et al 2006, Werry et al 2012) These animals belong to a single, relatively small population White sharks are listed as Vulnerable based on International Union for Conservation of Nature (IUCN) Red List criteria (Rigby et al 2019), and have been afforded protection under various national jurisdictions and international treaties, such as listing in Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), and the Convention on the Conservation of Migratory Species of Wild Animals (CMS) This has fostered wide-ranging research and conservation efforts over much of their global distribution (Huveneers et al 2018). Elucidating the mechanisms behind white shark distribution and movements is a prerequisite to the implementation of ecologically sound conservation strategies (Southall et al 2006, Certain et al 2007) and has recently been identified as one of the top 10 research priorities for this species globally (Huveneers et al 2018)
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