Foraging behaviour and prey consumption by grey seals (Halichoerus grypus)-spatial and trophic overlaps with fisheries in a marine protected area

The roles of the marine national park of the Iroise Sea (France) are to maintain marine biodiversity, including the southernmost grey seal colony in the Eastern Atlantic, whilst managing sustainable human activities. This study compares the fish biomass taken by local seals and the landings by man in the Iroise Sea. Sixteen seals were satellite tracked from 1999 and 2003, providing location and behavioural (diving) data from which foraging locations were estimated. One individual spent a third of its foraging time in direct, long distance trips (200-350 km) across the English Channel, but most seals spent the majority (68.5%) of their foraging time in the Iroise Sea, making return trips within 30 km of their departure haulout sites. The energetic consumption of the seal colony, taking sex and age classes into account, was assessed and combined with seal abundance estimates and dietary data to assess the total prey consumption by seals, for each prey species. We estimated that during the study period, the grey seal colony in the Iroise Sea consumed around 115 tons of fish per year. The main source of uncertainty of this calculation came from the confidence intervals in total seal abundance estimates. This consumption comprised 13.6 tons of sea bass, 4.3 tons of pollack and 2.7 tons of sole, representing 16.4, 1.8, and 5.2% of landings in the same areas for these three fish species, respectively. Within the four ICES rectangles where grey seals foraged, overlap between seals and fisheries was greatest in rectangle 25E5 (72.7% of all foraging dives), where grey seals haul out, and less in rectangles 25E4 (11.7%), 26E5 (10.8%), and 26E4 (4.8%).