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Global seascape ecology of the white shark

Jewell, Oliver Joseph David (2022) Global seascape ecology of the white shark. PhD thesis, Murdoch University.

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Abstract

The movements of predators are inherently connected to their prey and environment. Yet, quantifying the movements of predators in inaccessible areas, such as the marine environment, has traditionally been challenging. In recent years, technical advancements in the development and miniaturisation of biologgers and their attachment and retrieval devices have seen an unprecedented increase in our ability to answer questions addressing the movement ecology of highly mobile marine animals.

White sharks Carcharodon carcharias are long-lived, top marine predators that feed on a variety of prey in a range of environments. Despite being globally distributed, rare and threatened, they are found to seasonally aggregate in areas of high and predictable prey abundance. These aggregations offer a unique setting to answer questions on the movement ecology of a large marine predator, as many individuals, often varying in size and sex, are found in comparatively small areas that often serve as foraging grounds. In this thesis, I assess the seascape ecology of white sharks at local, regional and global scales using novel biologging tools and exploring their interactions with prey and the environment.

At a local scale, I used animal-borne video and environmental data collection devices (AVEDs) to determine how white sharks interact with their preferred prey in a complex marine seascape in South Africa. Previous studies identified white sharks forage diurnally in this seascape, and in response, Cape fur seals Arctocephalus pusillus pusillus refuge in kelp forest. The use of AVEDs revealed white sharks make repeated movements into kelp forest throughout the day while raising their activity and turning rates. Cape fur seals were seen in footage grouping in numbers, hunkering to the seafloor, and blowing bubbles as white sharks approached. Though no predation events were captured in the footage, the results combined with the previous studies revealed white sharks are capable of foraging in kelp forests.

At a regional scale, in the Northeast Pacific, I used biologging data from white sharks of three size-classes in four contrasting habitats in a multivariate statistical framework to elucidate both the internal and external determinants of movement and behaviour. I reveal distinct hierarchical similarity in movement characteristics, primarily driven by habitat, bathymetry in particular, and secondarily driven by size. Sharks in all habitats revealed distinct movement and behaviour between day and night, characteristic of a diurnal activity rhythm irrespective of circumstance. The availability of prey and access to deep water between these habitats are likely drivers of these differences. The two island habitats provide more elephant seal Mirounga angustirostris prey and are situated closer to the continental shelf edge. The other areas are either far from it and contain more harbour seals Phoca vitulina, or only host juvenile, piscivorous white sharks.

Finally, I tested if the diurnal activity rhythm detected in the Northeast Pacific white shark population was characteristic of the species globally. I used scale-free estimates of activity derived from accelerometers deployed on 104 white sharks of four populations and eight aggregation areas to test for local adaptations in activity rhythms. Overall, linear modelling revealed consistent diurnal activity rhythms, irrespective of size or population, suggesting strong conservation of activity rhythms in this species. Despite the overall conservation of diurnal activity, generalised additive mixed models revealed some degree of site-specific plasticity, with peak activity differing between sites. I suggest that strong conservation in the diurnality of white sharks is driven by sensory specialisation for foraging in well-lit environments. In contrast, behavioural plasticity in the peak of activity is driven by the availability of prey at a given location.

Together, my thesis reveals the major external and internal factors driving the movements and routines of white sharks. While white sharks display moderate plasticity in their movements and behaviours between different habitats, they also appear to be constrained to diurnal foraging, possibly due to the evolution of their sensory system making them diurnal specialists. As a threatened species that require protection yet can be potentially dangerous to water users, balanced management strategies involving multiple stakeholders are required in areas the species frequents. These strategies should consider the internal and external factors found influential on the movements and behaviours of white sharks. Given the results of this thesis, white sharks have a conserved diurnal circadian rhythm; potential challenges remain, as the time white sharks are more likely to be active reflects the time when people are more likely to be using coastal waters.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): College of Science, Health, Engineering and Education
Harry Butler Institute
Supervisor(s): Gleiss, Adrian, Beatty, Stephen, Jorgensen, S. and Chapple, T.
URI: http://researchrepository.murdoch.edu.au/id/eprint/63107
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