Diet of harbour seals at Mousa, Shetland, during the third quarter of 1994

Seats (N=200) of harbour seals (Phoca vitulina) were collected from haul-out sites on the Island of Mousa, south-east Shetland, during July, August and September 1994. A total of 5,531 fish otoliths were identified in seats and measured to reconstruct fish lengths and weight. Gadid fishes dominated the diet of harbour seals on Mousa during the study period, both in terms of weight and the frequency with which they occurred. There was evidence for the existence of temporal trends. Over the period July-September the importance of gadids in the diet increased while that of sandeels declined. These temporal trends are probably associated with changes in prey availability. Comparisons of the size of sandeels, herring and whiting eaten by seals with trawl catches suggest that seals were probably not size-selective, but a corollary of this is that, while otolith size is reduced during passage through seal guts, application of correction factors derived from captive feeding experiments led to over-estimates of fish size.