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 Ha Èrko È nen (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 ®ve seals which were ®tted 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.

Distribution and diving of harbour seals ( Phoca vitulina ) in Svalbard

“…Deep-diving behaviour depends on a number of external factors, such as local bathymetry and predator and prey ecology (Schreer and Kovacs 1997;Tollit et al 1998). In the present study, most diving was shallower than 250 m, and half of the dives shallower than 40 m. Deeper and longer-lasting diving occurred further oshore closer to the shelf break.…”

“…In the literature, harbour seals are noted as usually feeding at the sea-bed (HaÈ rkoÈ nen and Heide-Jùrgensen 1991; Bjùrge et al 1995;Tollit et al 1998). Because of the limitations in the accuracy of the SLDRs, it is not known if harbour seals in Svalbard were diving to the sea-bed, but based on the dive-depth frequencies and the bathymetry of the area, this seems likely.…”

“…These deep dives were observed when deep waters occurred relatively close to haul-out sites (Tollit et al 1998). Harbour seal feeding areas are typically located 10±70 km away from their preferred haul-out sites : 10,20,40,60,90,120,150,200,250,300,350,400,450 and >450 m. Duration histogram bin lower limits are: 1,4,7,10,13,16,19,22,25 and >25 min (Thompson et al 1991;Bjùrge et al 1995;Tollit et al 1998;Lesage et al 1999). In the Prins Karls Forland area, the continental shelf break is only 35 km oshore.…”

“…Harbour seals are generally considered to feed close to the sea-bed and at moderate depths (4±200 m) (HaÈ rkoÈ nen and Heide-Jùrgensen 1991; Bjùrge et al 1995;Tollit et al 1998;Lesage et al 1999). In Svalbard, most of the haul-out sites are located in areas which include shallow-water shelves less than 200 m deep, but also deep waters down to depths of more than 1,000 m.…”

Techniques for real-time, active tracking of sea lions

“…Study methods were chosen that did not necessitate recapturing individuals to retrieve archival tags. Additionally the lack of road access in Southeast Alaska precluded active VHF tracking from land (Tollit et al 1998). We tested the efficacy of a variety of techniques (including boat-based VHF and acoustic telemetry) and show for the first time that it is possible to track sea lions at sea, both in daylight and darkness, and under a range of sea conditions.…”