Drivers and constraints on offshore foraging in harbour seals

H M Vance,L Mikkelsen,M Johnson,J Teilmann,A Van Neer,U Siebert,S K Hooker

doi:10.1038/s41598-021-85376-2

H M Vance, L Mikkelsen + Show 5 more

Open Access

https://doi.org/10.1038/s41598-021-85376-2

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Abstract

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.

Highlights

Central place foragers are expected to offset travel costs between a central place and foraging areas by targeting productive feeding zones
Some individuals target resources closer to the haul-out[6,8,9], raising the question: why do some individuals accept the extra travel costs associated with foraging further offshore? it has been suggested that some populations, such as the Wadden Sea harbour seals, feed almost exclusively at offshore sites suggesting that these sites provide a resource which is not available closer to the haul-out[6,8,10,11]
Optimal foraging models predict that animals travelling further should forage longer to recoup travel costs, but trip length is likely constrained by other factors e.g., the need for sleep, digestion and/or social interaction

Summary

Introduction

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. It has been suggested that some populations, such as the Wadden Sea harbour seals, feed almost exclusively at offshore sites suggesting that these sites provide a resource which is not available closer to the haul-out[6,8,10,11]. Increased predation close to haul-out sites may reduce prey abundance around coastal sites Such zones of depletion, known as Ashmole’s halo[18], have been observed around seabird colonies and drive predators further offshore to forage. Despite the ubiquity of harbour seals around populated northern European coastlines, relatively little is known about what drives and constrains their offshore foraging

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