1. Faecal material collected in January, June, August and November 198
5 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 mea
sured to estimate diet composition by weight. The degree of otolith er
osion due to digestion was accounted for using empirically derived dig
estion coefficients. 3. Gadids dominated the diet, contributing approx
imately 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 plaic
e and megrim in the Outer Hebrides, and witch in the Minch/Inner Hebri
des. The dominant species again varied by area and season. 5. The cont
ribution of sandeels to the diet in the Hebrides was less than that fo
und in other areas of Britain (Orkney and the east coast). Pelagic sch
ooling 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 o
f the sandeel's preferred sea bed type (smooth areas of gravelly sand)
in the region. However, two species of flatfish, megrim and witch, we
re most prevalent in the diet in areas which did not correspond to the
ir supposed preferred sea bed depth or sediment type. 7. The greater n
umber 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 con
sumption, seen at this time of year in other areas of Britain.