Drivers and constraints on offshore foraging in harbour 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

H. M. Vance; S. K. Hooker; L. Mikkelsen; A. van Neer; J. Teilmann; U. Siebert; M. Johnson

2021

Scholarcy 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, 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
  • Our understanding of harbour seal foraging behaviour has largely been drawn from inferences based on surface movement patterns and dive shape acquired at low sampling rates from biotelemetry t­ ags
  • Fine-scale movement sensors have previously been used on harbour seals in the German Wadden S­ ea, sampling rates were too low to detect the fast movements associated with prey capture ­attempts
  • Using distinctive acceleration transients as a proxy for prey capture attempts, we found that harbour seals forage almost continuously throughout the trip including on the outbound and return journeys, suggesting that offshore patches are not so rich as to make opportunistic foraging during travel inefficient
  • Increased resting comes at the expense of foraging time and we found a daily decrease of about 25 PCAs
  • To compare foraging inferences from our high-bandwidth acceleration data to those taken from movement patterns and dive shapes, we computed hourly median and max dive depth values as well as hourly median and max dive shape indices across the three phases of each offshore trip

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