MONTHLY VARIATION IN THE DIET OF HARBOR SEALS IN INSHORE WATERS ALONGTHE SOUTHEAST SHETLAND (UK) COASTLINE

We studied the diets of harbour seals Phoca vitulina along the southea st Shetland (UK) coastline by analysing prey remains found in faeces ( N = 733) at haul-out sites. A total of 44 325 fish otoliths were recov ered. Sandeel (Ammodytidae) otoliths were the most numerous (38704), f ollowed by Gadidae (4707). Otoliths were measured and experimentally d erived digestion coefficients were applied (correcting for digestion i n the seal's gut) to estimate the size of ingested prey fishes. Gadids accounted for an estimated 53.4% of the annual diet by weight, sandee ls 28.5% and pelagic fishes 13.8%. The dominant gadid fishes were whit ing Merlangius merlangus (25.3%) and saithe Pollachius virens (11.1%). The range of species observed in the diet was similar to that recorde d in other areas of the UK. One exception to this was garfish Belone b elone accounting for 34.1% of the diet in September (1996), which is a species not previously reported for harbour seal diets in UK waters. There were strong seasonal patterns in the contribution of sandeels an d gadids, with sandeels being important in spring and early summer, an d gadids in winter. Pelagic species (mainly herring Clupea harengus, g arfish and mackerel Scomber scombrus) were important in late summer an d autumn. Observed seasonal patterns are similar to those previously r ecorded for harbour seal diets in the Moray Firth area of Scotland and appear to coincide with changes in prey availability. A comparison of the utility of using only otoliths to estimate seal diet with all ide ntifiable structures showed that using otoliths alone underestimated t he contribution of pelagic fish and overestimated the importance of ga dids and sandeels.