Spatial ecology of the shortfin mako shark and the impacts of ocean deoxygenation on behaviour and vulnerability to fisheries

Abstract

Determining animal movement patterns, habitat selection and ecophysiological tolerances are vital components of understanding a species spatial ecology. These factors govern behaviour and distributions across spatial scales, directly influencing the spatial overlap between marine predators and anthropogenic stressors such as industrial fisheries. This is particularly important for species that have suffered large-scale population declines as a result of human activities, such as large pelagic sharks. Of particular concern for these species is how increases in ocean temperatures coinciding with decreases in dissolved oxygen concentrations - as a consequence of anthropogenic climate change - will alter 3-dimensional space use and vulnerability to fisheries. Such information is vital to implementing targeted management measures that are specific to regional environmental conditions and robust to the impacts of climate change. The shortfin mako shark (Isurus oxyrinchus) is a globally distributed, endangered predatory shark species with a range of physiological specialisations such as regional endothermy. The high oxygen demands associated with this physiology are likely to render mako sharks susceptible to the impacts of decreasing oxygen concentrations, making this species an ideal case study to examine the ecological impacts of ocean deoxygenation and other climate stressors on pelagic sharks. Using a range of remote tracking technologies, this thesis explored various aspects of mako shark spatial ecology within the North Atlantic across a range of spatio-temporal scales. At fine scales, motion sensitive biologgers revealed high swimming speeds and aerobic oxygen demands, similar to other regionally endothermic fishes, suggesting that mako sharks are likely to be particularly susceptible to declining oxygen concentrations. At the mesoscale, mako sharks displayed a range of complex behaviours including area restricted searching, seasonal partial migrations and site fidelity with likely foraging behaviours linked to increased concentrations of prey proxies and proximity to seamounts within the Azores archipelago. Mako sharks exhibited pronounced spatial segregation between ecoregions at the basin scale with distinct habitat selection patterns. Connections between aggregations were maintained by a limited number of individuals making long distance movements which spanned across the entire North Atlantic basin. Additionally, individual mako sharks interacted extensively with the eastern tropical Atlantic oxygen minimum zone which is marked by the presence of permanent low oxygen waters at mesopelagic depths. The vertical movements of sharks tracked within these waters were strongly influenced by sea surface temperatures, primary productivity and subsurface oxygen concentrations. Sharks were subject to reductions in habitat volumes because of increasing temperatures and declining oxygen concentrations in 2023. Fisheries catches were highest within core habitats where the effects of declining oxygen concentrations were greatest leading to concerns over how future climate change will alter fisheries vulnerability for mako sharks and other threatened species globally. Collectively these results highlight the need to study spatial ecology at appropriate spatial scales and to incorporate region specific behaviour and habitat selection into future management plans to promote population recovery.

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ORCID for Matthew James Waller: orcid.org/0000-0001-9839-3452

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