Abstract
DECC's Offshore Energy Strategic Environmental Assessment programme (SEA), requires robust evidence on which to base the relevant assessments. Currently little is known about the movements of grey seals (Halichoerus grypus) which haul out on the UK coast of the southern North Sea; only ten grey seals have previously been tracked. Since that deployment in 2005, there has been dramatic increases in the summer foraging and winter breeding populations of grey seals in the southern North Sea. Over the same time period, harbour seal (Phoca vitulina) populations on the UK south-east coast have recovered from earlier declines and are now at historic high levels. These populations are primarily based at The Wash and Blakeney which are part of The Wash and North Norfolk Coast Special Area of Conservation (SAC) for which harbour seals were a primary reason for designation). There have also been extensive windfarm developments with many more planned. These changes in population and environment may have affected the at-sea distribution of grey seals. In this DECC funded study, 21 tags were deployed at the two main haul out sites in the south-east UK: Donna Nook (part of the Humber Estuary SAC for which grey seals are a qualifying feature), and Blakeney. The data from these tags indicated that there are extensive movements between haul out sites on the UK south-east coast. Even once we exclude the breeding season, there are extensive movements within the foraging (non-breeding season) thus the grey seals in this area appear to encompass one, rather than multiple, populations. The disproportionately large foraging population, in comparison to the breeding population, on the UK south-east coast suggests that some individuals are foraging in the southern North Sea but breeding elsewhere. Supporting this suggestion, one female tagged at Donna Nook pupped in the northern North Sea. Four other tagged females appeared to pup on the UK south-east coast. There was extensive overlap between grey seal movements and, present and planned, windfarms; 17 of the 21 individuals entered at least one operational windfarm. There was no indication of overt avoidance or use of windfarms, or other anthropogenic structures. In comparison to 2005, it appears that offshore usage by seals emanating from Donna Nook in 2015 was not restricted to discrete patches. The dispersed foraging areas, and the greater maximum extent, may be a result of an increasing grey seal population and thus a depletion of prey resources or increase in competition at key foraging areas since 2005. This finding highlights the importance of updating at-sea usage maps with recent telemetry data, on the basis of which policy and marine spatial planning decisions may be made, especially when changes in population have occurred. With the rapidly increasing populations of both species it is likely that competition for resources will increase in the near future. Knowledge of the degree of overlap between grey and harbour seal foraging areas will provide a valuable baseline for comparison with the future, if and when, competition becomes important.