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Marine Ecology Progress Series

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MEPS 390:265-276 (2009)  -  DOI:

Seals, sandeels and salmon: diet of harbour seals in St. Andrews Bay and the Tay Estuary, southeast Scotland

R. J. Sharples1,2,*, B. Arrizabalaga1, P. S. Hammond1

1Sea Mammal Research Unit, Gatty Marine Laboratory, University of St. Andrews, St. Andrews, Fife, KY16 8LB, UK
2Commonwealth Industrial and Scientific Research Association (CSIRO), Marine and Atmospheric Research, GPO Box 1538, Hobart, Tasmania 7001, Australia

ABSTRACT: Harbour seal populations have declined by up to 40% around northern and eastern Britain since 2000 due to unknown causes; prey availability is one important factor that could be contributing to the decline. We estimate the diet and prey consumption of a population of harbour seals in southeast Scotland, using analysis of hard prey remains recovered from scats, to investigate change in the importance of sandeels over 6 yr spanning the local sandeel fishery closure. The study site includes Special Areas of Conservation for harbour seals as well as vulnerable salmon stocks. We estimate the extent of harbour seal predation on salmon in the area. In St. Andrews Bay, harbour seal diet was heavily dominated by sandeels, especially in winter and spring. Gadoids (whiting, cod) and flatfish (dab, plaice, flounder) were the other main prey. The proportion of sandeels in the diet was remarkably consistent over time (71 to 77%), but the average size of sandeels consumed increased following the closure of the fishery. In the Firth of Tay, sandeels were prevalent in winter, but the diet in the rest of the year was dominated by salmonids: salmon comprised 64% of the diet in summer and sea trout comprised 40% of the diet in autumn. Thus marked differences in diet were evident at a fine spatial scale. The effects of the sandeel fishery closure on harbour seals were equivocal, but harbour seals that haul out in SE Scotland are clearly dependent on sandeels; re-opening the fishery could thus have a negative impact and be inadvisable. We found evidence that local harbour seal predation could be impacting salmon stocks but the high uncertainty in estimates of seal diet and salmon stock size preclude the provision of management advice at this time.

KEY WORDS: Faecal sampling · Fisheries · North Sea · Prey availability · Phoca vitulina · Special Area of Conservation

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Cite this article as: Sharples RJ, Arrizabalaga B, Hammond PS (2009) Seals, sandeels and salmon: diet of harbour seals in St. Andrews Bay and the Tay Estuary, southeast Scotland. Mar Ecol Prog Ser 390:265-276.

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