Movements and foraging areas of grey seals in the North Sea

Summary 1. Grey seals Halichoerus grypus Fab. are large, numerous marine top predators. Fears concerning competition with fisheries have prompted calls for control measures. However, little is known about the areas where grey seals forage or the distances they may travel. 2. The movements of 14 grey seals caught at the Farnes in north-east England (12) and Abertay in eastern Scotland (2) between August 1991 and July 1993 were investigated using Argos Satellite Relay Data Loggers (SRDLs). A total of 1461 seal days of location and behavioural data (mean 104·3 days per seal) covered all months of the year except February and March. 3. The seal movements were on two geographical scales: long and distant travel (up to 2100 km away); and local, repeated trips from the Farnes, Abertay and other haul-out sites to discrete offshore areas. 4. Long distance travel included visits to Orkney, Shetland, the Faroes, and far offshore into the Eastern Atlantic and the North Sea. During travel the seals moved at speeds of between 75 and 100 km day–1 (0·87 and 1·16 m s–1). Most of the time, long distance travel was directed to known haul-out sites. The large distances travelled indicate that grey seals that haul out at the Farnes are not ecologically isolated from those at Orkney, Shetland and the Faroes. 5. In 88% of trips to sea, individual seals returned to the same haul-out site from which they departed. The durations of these trips were short (mean 2·33 days) and their destinations at sea were often localized areas characterized by a gravel/sand seabed sediment. This is the preferred burrowing habitat of sandeels, an important part of grey seal diet. This, and the fact that dives in these areas were primarily to the seabed, leads us to conclude that these were foraging areas. The limited extents of return-trips from a haul-out site (mean 39·8 km) suggest that the direct impact of seal predation may be greater on fisheries within this coastal zone, especially those near seal haul-out sites, rather than on fisheries further offshore. 6. An average of 43% of all the seals’ time was spent within 10 km of a haul-out site, although localized foraging areas were identified considerably further offshore. Proximity to a haul-out may provide safety from predation. Alternatively, these periods may be used for rest or social interaction, or we may be underestimating foraging activity near haul-out sites. 7. We suggest that the movement patterns observed in this study may persist through time and across the grey seals which haul-out at the Farnes. We also suggest that a study such as this could be combined with diet studies and haul-out censuses to map foraging intensity. Such information is an essential component of seal–fishery interaction models, upon which management decisions should be based.

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