Habitat use and feeding behaviour of harbour seals (Phoca vitulina) in Vesterålen

Abstract

A pilot project was carried out by the Institute of Marine Research (IMR) to test the functionality of GPS phone tags (by SMRU) on harbour seals (Phoca vitulina) in the Vesterålen archipelago, Norway. Acoustic surveys were performed to investigate the abundance of fish in these waters and scat sampling was used to assess the diet composition of the seal population inhabiting the area. Five juvenile females were tracked between August 2007 and March 2008 and their diving behaviour and habitat use was studied. The dive profiles recorded from the tags were classified in the attempt to identify the dives connected with feeding behaviour. The tagged seals foraged close to the coast and at relatively shallow depths (50% of the dives between 12-32 m) showing marked individual differences in the choice of feeding grounds. Analysis of scat sampling suggested that gadoids dominated the diet (62%) followed by herring (Clupea harengus) (35%). Comparison between the abundance of fish species in the study area and in the diet composition indicated that harbour seals have no prey preference. Harbour seals appeared to prey on small size classes of gadoids. Therefore fish size, rather than species, could be a potential selection criterion in foraging. The local abundance of fish in the area is likely to allow harbour seals to feed on what is available close to the haul out sites. Targeting of small size classes might therefore be a consequence of habitat selection based on accessibility rather than size selection.