The Diet of Grey Seals in the Inner and Outer Hebrides

Faecal material collected in January, June, August and November 1985 was used to investigate seasonal and regional variation in grey seal diet around the Western Isles of Scotland. A total of 511 individual faex was obtained from the Outer and 238 from the Minch/Inner Hebrides; 79% and 67%, respectively, contained fish otoliths and/or cephalopod beaks. 2. Fish otoliths and cephalopod beaks were identified, and measured to estimate diet composition by weight. The degree of otolith erosion due to digestion was accounted for using empirically derived digestion coefficients. 3. Gadids dominated the diet, contributing approximately 40% or greater in percentage by weight. The gadid species most commonly found were ling, cod and whiting, the predominance of which varied by area and season. 4. The main flatfish in the diet were plaice and megrim in the Outer Hebrides, and witch in the Minch/Inner Hebrides. The dominant species again varied by area and season. 5. The contribution of sandeels to the diet in the Hebrides was less than that found in other areas of Britain (Orkney and the east coast). Pelagic schooling fish (herring, mackerel and horse mackerel) contributed more to the diet than in other areas of Britain. 6. The significant variation in sandeel occurrence by area (many more sandeels taken off the Outer than the Minch/Inner Hebrides) can be explained by the distribution of the sandeel's preferred sea bed type (smooth areas of gravelly sand) in the region. However, two species of flatfish, megrim and witch, were most prevalent in the diet in areas which did not correspond to their supposed preferred sea bed depth or sediment type. 7. The greater number of cod and ling taken in summer compared to the rest of the year may compensate for the observed absence of an increase in sandeel consumption, seen at this time of year in other areas of Britain.