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

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.

There are two main potential science outcomes of the proposed research that may lead to significant impacts. Firstly, vertical space-use of oceanic blue and shortfin mako sharks may be shifted to shallower depths by the expanding eastern tropical Atlantic OMZ and sharks may remain that region for long periods of time due to greater foraging opportunities (as prey are similarly habitat compressed). As a result, the vulnerability of oceanic sharks to capture by fisheries may be much greater in the OMZ than in normoxic waters elsewhere in the Atlantic. This would identify the need to consider OMZs as areas requiring consideration of special management measures with respect to oceanic sharks and perhaps other pelagics (e.g. tunas, swordfish). OMZs may be particularly important areas that support large catches of sharks but that have gone undetected as major sources because of poor reporting of catches and the lack of regulation. Therefore, the proposed research may be of great interest to the International Commission for the Conservation of Atlantic Tunas (ICCAT) for informing plans to develop sustainable exploitation of oceanic sharks in the Atlantic. Similarly, the International Union for the Conservation of Nature (IUCN) World Commission on Protected Areas (WCPA) are interested in the project results because they address an emerging interest in assessing the status and global impacts of ocean deoxygenation. Furthermore IUCN WCPA are interested in exploring the potential of OMZs as a network of high seas Marine Protected Areas (MPA).
In the proposed research we plan to engage directly with ICCAT regarding shark catch limits in OMZs and with the IUCN WCPA on upper-trophic level impacts of ocean deoxygenation and to explore a potential role of OMZs as high seas MPAs. We plan to present the results of the proposed research formally to ICCAT as part of the annual intersessional meetings of the shark species group in addition to the species data preparatory meetings and stock assessment sessions (which the P.I. and Project Partner have attended as part of the EU delegation). A potential outcome will be bringing new data and understanding on shark movements, electronic tag recapture rates to estimate potential fishing mortality between different ocean areas, and the effects of environmental changes on risk of shark capture from fisheries, into the shark stock assessment process to inform recommendations for management. The proposed project may contribute results that identify OMZs as areas at particular risk from overfishing.
Engagement with the IUCN will be in collaboration with our Project Partner Prof Dan Laffoley who is the Marine Vice-Chair of the IUCN WCPA. We will work with Prof Laffoley in two linked areas. First, the WCPA are to produce an authoritative report on the impacts of ocean deoxygenation that will be launched at a future meeting of the World Conservation Congress. The proposed research will form the key source of new information detailing how IUCN Red-Listed oceanic sharks will respond to the combined effects of ocean deoxygenation, warming and fishing exploitation. Secondly, as part of the collaborative reporting of the effects of deoxygenation on sharks, we will explore with IUCN the potential for identifying OMZs as candidate high seas MPAs in the event that they are identified as key shark habitats that appear to be undergoing overfishing. We aim to feed project results into focused meetings with IUCN to determine whether there is scope and opportunity for proposing OMZs as MPAs as part of their Global Plan of Action.