An ocean habitat trap? Impacts of global oxygen-minimum zone expansions on threatened apex predator ecology

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Permanent oxygen minimum zones (OMZs) that extend to over 10 million km3 of ocean (ca. 8% of ocean volume) are expanding geographically and vertically due to climate-driven reductions in dissolved oxygen (DO). Potential impacts on marine animal distributions and abundance may be particularly significant for high-oxygen-demand apex predators, such as oceanic pelagic sharks, by reducing habitat volumes through OMZ shoaling and concentrating them further in surface waters where they become more vulnerable to fisheries. But predictions of how exploited oceanic fish actually respond to OMZ expansions are not based on mechanistic understandings, principally because direct measurements of oxygen tolerances during normal behaviour have not been determined for large predatory fish in the open ocean. The proposed research will bring about a step change in our understanding of OMZ impacts on oceanic ecology by applying our existing expertise in animal movement studies and by deploying new telemetry technologies for measuring oxygen environments actually encountered by free-living oceanic sharks moving above/within OMZs. This will enable major unknowns to be addressed concerning how oceanic sharks respond physiologically and behaviourally to OMZs, how oceanic shark habitats change with predicted OMZ expansion, and whether this will increase shark vulnerability to fishing gear. The project will achieve its objectives through linked field and modelling studies on two Red-Listed species, the warm-bodied (endothermic) shortfin mako, Isurus oxyrinchus, and the ectothermic blue shark, Prionace glauca, that are the two pelagic shark species most frequently caught in high seas fisheries. By focusing in depth on key processes underlying shark responses to DO in situ, our new modelling approaches will establish effects of future warming and OMZ shoaling on fish niches and determine how these shift distributions and alter capture risk by fisheries. The project represents a discipline-spanning approach linking physiology to ecology and oceanography, with wide-ranging outcomes for understanding global biotic responses to warming and ocean deoxygenation with direct relevance to sustainable fisheries and species conservation.