8.9 The significance of ocean deoxygenation for Elasmobranchs David W. Sims 8.9 The significance of ocean deoxygenation for Elasmobranchs David W. Sims Marine Biological Association of the UK, The Laboratory, Citadel Hill, Plymouth PL1 2PB, UK; Ocean and Earth Science, University of Southampton, National Oceanography Centre Southampton, European Way, SO14 3ZH, UK. Email:
[email protected] Summary • All of the >1000 species of sharks, skates and rays are obligate water-breathers with comparatively high absolute oxygen demands being relatively large-bodied, active predators. With broad distributions across aquatic habitats exhibiting large variations in physico-chemical variables including oxygen concentration indicates elasmobranch physiology, behaviour and ecology to be strongly influenced by oxygen depletion. • Many elasmobranchs show rapid behavioural responses to hypoxic water by increased activity associated with avoidance. Nonetheless, elasmobranchs also appear capable of withstanding mild hypoxia with circulatory and/or ventilatory responses, perhaps even for extended periods. However, such strategies may be insufficient to endure moderate, progressive or prolonged hypoxia or anoxia. • As water temperatures rise with climate warming most elasmobranchs (as ectotherms) will exhibit elevated metabolic rates and will be increasingly less able to tolerate the effects of even mild hypoxia associated with ocean deoxygenation. Thus, sustained hypoxia in warmer coastal waters is likely to lead to shifts in elasmobranch distributions. • Expansion of oxygen minimum zones (OMZs) of the open ocean in particular are predicted to have significant population-level implications for pelagic elasmobranchs as they become habitat compressed into surface layers by shoaling hypoxic water. • Surface layers overlying OMZs appear to be space use hotspots of pelagic sharks that may be increasingly likely to undergo significant ‘habitat compression’ (reduced habitat volumes) with expanding OMZs, potentially increasing their susceptibility to surface fisheries, e.g.