1 Background, Grey seal population biology

In a study of animals captured at the Farne Islands and Abertay Sands, McConnell et al. (1999) found that movements were on two geographical scales: (a) long and distant travel (up to 2,100 km away); and (b) local, repeated trips to discrete offshore areas. Long-distance travel included visits to Orkney, Shetland, the Faroes, and far offshore into the Eastern Atlantic and the North Sea. 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 possibly even the Faroes. The grey seals that forage in the North Sea can thus be considered as coming from a single ecological population.

In 88% of trips to sea, individual seals returned to the same haul-out site from which they departed. The durations of these return trips were short (typically 2-3 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 component of grey seal diet. This, and the fact that dives in these areas were primarily to the seabed, implies that these were foraging areas. The limited distance from a haul-out site of return trips (about 40 km) indicates that the ecological impact of seal predation may be greater within this coastal zone, rather than further offshore.

This is confirmed by recent work at the SMRU in which data from larger numbers of seasl tagged at Farne Islands, Isle of May and Abertay Sands have confirmed the general movement patterns. In addition, a mathematical and statistical modelling framework has been developed that uses satellite-linked telemetry and other data to generate predicted distributions of where grey seals spend their time foraging around the British Isles (Matthiopoulos et al. in preparation). Figure 11 shows the distributionof grey seals tagged at the Farnes, Isle of May and Firth of Tay. Activity is concentrated in the north off the Farne Islands, a particularly important area for grey seals (McConnell et al. 1999) and at a wide range of offshore banks. Recent aerial survey counts of grey seals during summer harbour seal surveys suggests that the level of foraging effort in the southern North Sea is increasing.

Figure 10. Swimming tracks of grey seals tagged at haulout sites in the Farnes/Lindisfarne area and at AberTay/Isle of May area. a. & b. are tracks of weaned grey seal pups, tagged before their first trip to sea. c. & d. are tracks of adult grey seals.