The harbour seal Phoca vitulina as a predator in the Skagerrak

Abstract

Harbour seals, Phoca vitulina, in the Skagerrak preyed upon 31 species offish during the last half of 1989 but only six species (herring Clupea harengus, cod Gadus morhua, blue whiting Micromesistius pou-tassou, whiting Merlangius merlangus, Norway pout Trisopterus esmarkii and lemon sole Microstomia kitt) contributed substantially to the diet. This was essentially the same composition as documented in a similar study in 1977-79, with the exception that herring more than doubled its importance in 1989. Based on measurements of seasonal changes in mass and on estimates of the energy required for maintenance and growth, average daily energy requirement was calculated as 4680 kcal per seal in both 1979 and 1989, despite considerable changes in the population structure. Using estimated energy densities for selected prey this corresponds to 3.67-4.15 kg fish per seal per day depending on the diet composition. Total consumption offish was proportional to population size and was in the magnitude of 2400 tons in 1989. The consumption of commercially important species did not exceed 1% of the catches of these species by fisheries in the Skagerrak. Young harbour seals utilized an abundance of prey in 1989 for accelerating their body growth and they may be particularly vulnerable to depletion of fish stocks by the fisheries. The harbour seal feeds mainly on soft, vegetation poor sea beds at moderate depths, from where it feeds on some of the available fish species. In this habitat harbour seals can, and do, utilize both the benthic, demersal and pelagic fish communities. Size and productivity of available feeding habitat, together with access to haul-outs, partly explain the local and regional distribution of harbour seals.

Key words:

• Harbour seal
• Phoca vitulina
• feeding habits
• population energetics
• multispecies assessment