SEASONAL-VARIATION IN THE DIET OF HARBOR SEALS IN THE SOUTH-WESTERN NORTH-SEA

Seasonal variation in harbour Seal diet in the south-western North Sea was investigated from faeces collected monthly, over a 2 yr period, f rom a high-water haulout site in the Wash on the east coast of England . A total of 12 444 fish otoliths from 31 species was recovered from 7 08 faeces; otolith measurements were corrected for partial digestion a nd used to estimate the proportion, by weight, of each species in the diet. Overall, the diet was dominated by whiting (24 %), sole (15%), d ragonet (13 %) and sand goby (11%). Other flatfish (dab, flounder, pla ice: 12%), other gadoids (bib, cod: 11%), bullrout (7 %) and sandeels (3 %) were also consumed. A strong seasonality in diet was apparent wh ich can be summarised as: whiting, bib and bullrout dominated from lat e autumn through early spring; sand goby peaked during winter and earl y spring; dragonet, sandeels and flatfish (except sole) dominated from late spring to early autumn; and sole peaked in spring. Harbour seal diet composition in general, and seasonal changes in diet in particula r, appeared mainly to be linked to availability (in terms of prey dist ribution and abundance, feeding or spawning activity and, perhaps, pre y size) but this was not always the case. In a few species (whiting, d ab and plaice), seasonal changes in consumption appeared to be related to the availability of other species. Differences in harbour seal and grey seal diets in the same area were consistent with the 2 species f eeding in different areas, but there was also evidence of a maximum pr eferred prey size for harbour seals.