Drivers and constraints on offshore foraging in harbour seals
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Central place foragers are expected to offset travel costs between a central place and foraging areas by targeting productive feeding zones. Harbour seals (Phoca vitulina) make multi-day foraging trips away from coastal haul-out sites presumably to target rich food resources, but periodic track points from telemetry tags may be insufficient to infer reliably where, and how often, foraging takes place. To study foraging behaviour during offshore trips, and assess what factors limit trip duration, we equipped harbour seals in the German Wadden Sea with high-resolution multi-sensor bio-logging tags, recording 12 offshore trips from 8 seals. Using acceleration transients as a proxy for prey capture attempts, we found that foraging rates during travel to and from offshore sites were comparable to offshore rates. Offshore foraging trips may, therefore, reflect avoidance of intra-specific competition rather than presence of offshore foraging hotspots. Time spent resting increased by approx. 37 min/day during trips suggesting that a resting deficit rather than patch depletion may influence trip length. Foraging rates were only weakly correlated with surface movement patterns highlighting the value of integrating multi-sensor data from on-animal bio-logging tags (GPS, depth, accelerometers and magnetometers) to infer behaviour and habitat use.
Vance , H M , Hooker , S K , Mikkelsen , L , van Neer , A , Teilmann , J , Siebert , U & Johnson , M 2021 , ' Drivers and constraints on offshore foraging in harbour seals ' , Scientific Reports , vol. 11 , 6514 . https://doi.org/10.1038/s41598-021-85376-2
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DescriptionThis study was funded by the German Federal Agency of Nature Conservation under the projects “Effects of underwater noise on marine vertebrates” (Cluster 7, Z1.2-53302/2010/14) and “Under Water Noise Effects – UWE” (Project numbers FKZ 3515822000). Catches were funded and supported by the Schleswig-Holstein’s Government-Owned Company for Coastal Protection, National Parks and Ocean Protection. We thank all helpers during the seal catches. Development of the tags was aided by a Marie Curie-Sklodowska Career Integration Grant (EU-FP7) to MJ and by MASTS, the Marine Alliance for Science and Technology Scotland. MJ is supported by the Aarhus University Research Foundation and the EU H2020 research and innovation programme under Marie Skłodowska-Curie grant 754513.
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