Interannual variation in the summer diets of harbour seals Phoca vitulina at Mousa, Shetland (UK)

The main prey species in the summer (July-September) diets of harbour seals (Phoca vitulina) on the Island of Mousa (Shetland, UK), 1994-1997, were wa iting (Merlangius merlangus), herring (Clupea harengus), sandeel (Ammodytid ae) and garfish (Belone belone). Norway pout (Triopterus esmarkii) was nume rically important but comprised no more than 11% of prey weight in any year . There were marked bet,, een-year fluctuations in the relative importance of these prey, with whiting comprising 16-34% (by weight) of the diet, herr ing 12-28%, sandeels 7-18% and garfish 7-22%, Additional data on spring (Ap ril-June) diet were available for 1995-1997: sandeels were the most importa nt prey (% weight) in all three years (51-60% of the diet), while herring ( 8-48%) and gadids (2-22%) varied in importance. The average size of fish ea ten was larger than that reported in comparable studies from other areas. H arbour seals appear to have selected larger sandeels, whiting and Norway po ut than (lie average size available in the area, as indicated by survey tra wls, although between-year changes in the size of Norway pout in the diet d id to some extent reflect availability. Interannual variation in the import ance of Norway pout in the diet appeared to track trend,) in abundance, alt hough the short time series precluded detection of a statistically signific ant correlation. Thus, sonic of our results are consistent with harbour sea ls feeding opportunistically while others point to selectivity, particularl y for prey size. Estimated consumption of fish by the 500 harbour seals on Mousa during summer ranged from 152-195 tonnes.