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Long-term variation in harbour seal diet in Orkney (NE Scotland) and relationships with fish abundance

Pierce G.J1,2, Ieno E.N1,3, Santos M.B1,2, Zuur A.F1,3, Edridge A1 & Thompson P.M4

1Oceanlab, University of Aberdeen, Main Street, Newburgh, Aberdeenshire, AB41 6AA, UK;

2Instituto Español de Oceanografía, Centro Oceanográfico de Vigo, P.O. Box 1552, 36200, Vigo, Spain;

3Highland Statistics Ltd, 6 Laverock Road, Newburgh, Ellon, Aberdeenshire, AB41 6FN, UK

4Lighthouse Field Station, University of Aberdeen, George Street, Cromarty, Ross-shire, IV11 8YJ, UK

Long time series of dietary data can potentially provide information on how predator diets respond to changing prey abundance, e.g. due to overfishing and/or climate change. The present study examines long-term variation in the summer diet of harbour seals (Phoca vitulina) in Orkney (NE Scotland), based on faecal sampling at haulout sites on the island of Eynhallow during 1986-2006. In summer, haulouts are used mainly by harbour seals while grey seals (Halichoerus grypus) use the island mainly in autumn and winter. Samples were collected in the summers of 1986-88, 1993-96, 1998, 2000-03 and 2005-06. The main component of the diet in summer is sandeels (Ammodytidae, mainly Ammodytes spp.). Average sandeel size in the diet increased by around 6% over the study period while numbers of sandeels in scats declined. Both harbour seal and sandeel abundances have fallen markedly since the mid-1990s but, although dietary trends were statistically significant, harbour seal diet has remained remarkably constant over the study period, suggesting a degree of dietary specialization. Thus harbour seals in Orkney may currently depend on sandeels as their main food source.

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