THE DIET OF GRAY SEALS AROUND ORKNEY AND OTHER ISLAND AND MAINLAND SITES IN NORTH-EASTERN SCOTLAND

1. Almost 1000 faeces were collected in February, June, August and Nov ember 1985 in order to quantify the diet of grey seals around Orkney; 82% of these contained fish otoliths. 2. Diet composition, by weight, was assessed by identifying and measuring otoliths from the faecal mat erial and correcting for reduction in otolith size as a result of dige stion using experimentally derived species-specific digestion coeffici ents. 3. Sandeels accounted for almost half the fish consumed, by weig ht. The rest of the diet was composed mostly of gadids (particularly c od), flatfish (particularly plaice), and sculpins. 4. Sandeels were mo re prevalent in February and the summer than in November. They were fo und least often in the eastern area. No significant regional or season al differences were found in the number of cod, haddock or saithe cons umed. Whiting featured strongly to the south in November. Ling occurre d most frequently in the west. Of the flatfish, plaice were consumed m ore in February than in November and more in the east than in the nort h and west. 5. The largest sandeels and plaice were found when these s pecies were also most prevalent in the diet, suggesting that seals may switch to these prey when and where they are larger. This was not fou nd for other species. 6. Some flatfish and sculpins were important in the diet locally, perhaps reflecting their restricted habitat requirem ents and feeding by seals on locally abundant prey. 7. Mature fish of a number of species were more prominent in the diet in areas and at ti mes of the year when spawning occurs, suggesting that grey seals take advantage of energy-rich prey when these are available.