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Marine Ecology Progress Series

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MEPS 170:269-281 (1998)  -  doi:10.3354/meps170269

Seasonal variation in the diet of harbour seals in the south-western North Sea

Ailsa J. Hall1,*, John Watkins2, Philip S. Hammond1

1NERC Sea Mammal Research Unit, Gatty Marine Laboratory, School of Environmental and Evolutionary Biology, University of St Andrews, Fife KY16 8LB, United Kingdom
2Conservation Research Ltd, 110 Hinton Way, Cambridge CB2 5AL, United Kingdom

ABSTRACT: Seasonal variation in harbour seal diet in the south-western North Sea was investigated from faeces collected monthly, over a 2 yr period, from a high-water haulout site in the Wash on the east coast of England. A total of 12444 fish otoliths from 31 species was recovered from 708 faeces; otolith measurements were corrected for partial digestion and used to estimate the proportion, by weight, of each species in the diet. Overall, the diet was dominated by whiting (24%), sole (15%), dragonet (13%) and sand goby (11%). Other flatfish (dab, flounder, plaice: 12%), other gadoids (bib, cod: 11%), bullrout (7%) and sandeels (3%) were also consumed. A strong seasonality in diet was apparent which can be summarised as: whiting, bib and bullrout dominated from late autumn through early spring; sand goby peaked during winter and early 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 particular, appeared mainly to be linked to availability (in terms of prey distribution and abundance, feeding or spawning activity and, perhaps, prey size) but this was not always the case. In a few species (whiting, dab 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 feeding in different areas, but there was also evidence of a maximum preferred prey size for harbour seals.

KEY WORDS: Faecal sampling · Fish otoliths · The Wash · Foraging areas · Prey size · Prey availability · Predator preference

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