Measuring deoxygenation effects on marine predators: A new animal-attached archival tag recording in situ dissolved oxygen, temperature, fine-scale movements and behaviour

Abstract

Global climate-driven ocean warming has decreased dissolved oxygen (DO) levels (ocean deoxygenation) leading to expansions of hypoxic zones, which will affect the movements, behaviour, physiology and distributions of marine animals. However, the precise responses of animals to low DO remains poorly understood because movements and activity levels are seldom recorded alongside instantaneous DO in situ. We describe a new animal-attached (dissolved oxygen measuring, DOME) archival tag with an optical oxygen sensor for recording DO, in addition to sensors for temperature and depth, a triaxial accelerometer for fine-scale movements and activity, and a GPS for tag recovery. All sensors were integrated on a single electronic board. Calibration tests demonstrated small mean difference between DOME tag and factory-calibrated DO sensors (mean relative error of 5%). No temporal drift occurred over a test period three times longer than the maximum deployment time. Deployments on four blue sharks (Prionace glauca) in the central North Atlantic Ocean showed regular vertical oscillations from the surface to a maximum of 404 m. Profiles from diving sharks recorded DO concentrations ranging from 217 to 272 μmol L−1, temperatures between 13°C and 23°C, and identified an oxygen maximum at ~45 m depth, all of which were consistent with ship-based measurements. Interestingly, the percentage of time sharks spent burst swimming was greater in the top 85 m compared to deeper depths, potentially because of higher prey availability in the surface layer. The DOME tag described blue shark fine-scale movements and activity levels in relation to accurately measured in situ DO and temperature, with the potential to offer new insights of animal performance in low oxygen environments. Development of a tag with physico-chemical and movement sensors on a single electronic board is a first step towards satellite relay of these data over broader spatiotemporal scales (months over thousands of kilometres) to determine direct and indirect responses of marine animals to heatwave and deoxygenation events.