Monthly variation in the diet of harbour seals in inshore waters along the southeast Shetland (UK) coastline

We studied the diets of harbour seals Phoca vitulina along the southeast Shetland (UK) coastline by analysing prey remains found in faeces (N = 733) at haul-out sites. A total of 44325 fish otoliths were recovered. Sandeel (Ammodytidae) otoliths were the most numerous (38704), followed by Gadidae (4707). Otoliths were measured and experimentally derived digestion coefficients were applied (correcting for digestion in the seal's gut) to estimate the size of ingested prey fishes. Gadids accounted for an estimated 53.4% of the annual dlet by weight, sandeels 28.5% and pelagic fishes 13.8%. The dominant gadld fishes were whiting Merlanglus rnerlangus (25.3%) and saithe Pollachlus virens ( l 1 1 % ) . The range of species observed in the diet was similar to that recorded in other areas of the UK. One exception to this was garfish Belone helone 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 s t l u ~~y bedsonai patterns in the contribution of sandeels and gadids, with sandeels being important in spring and early summer, and gadids in winter. Pelagic species (mainly herring Clupea harengus, garfish and mackerel Scomber scombrus) were important in late summer and autumn. Observed seasonal patterns are similar to those previously recorded 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 identifiable structures showed that using otoliths alone underestimated the contribution of pelagic fish and overestimated the importance of gadids and sandeels.