Variations in harbour seal Phoca vitulina diet and dive-depths in relation to foraging habitat

Abstract

We use information on the at-sea distribution of radio-tagged seals to identify the summer foraging areas used by 31 harbour seals from two different haul-out sites in the Moray Firth, N.E. Scotland. We then determine whether seals tend to occur over particular sediment types or water depths as hypothesized by Härkönen (1987b), and assess whether local geographical variations in diet composition can be related to local differences in available foraging habitat.

The majority of seals foraged within 30 km of their haul-out site, consequently there was broad overlap between the foraging areas used by animals from the same site, but little overlap in the areas used by seals from different sites. Most seals foraged in water depths of 10-50 m with mainly sandy sea-bed sediments. Data on the distribution of dive depths of five seals which were fitted with time-depth-recorders indicated that seals were generally diving on, or close to, the sea-bed. Occasional pelagic dives between the more common benthic dives were also observed. Between-site differences in the seals' use of different water depths and sea-bed sediments suggest that local geographical variations in diet were related to local differences in foraging habitats, but that habitat use also differed between individual seals.