Interactions between harbour seals, Phoca vitulina, and fisheries in complex coastal waters explored by combined Geographic Information System (GIS) and energetics modelling

A 1680 km 2 coastal archipelago area in Norway around Sandoy (62 N6 E), with a resident population of approximately 750 harbour seals was modelled in a geographic information system (GIS). The proportions of different habitat types available to harbour seals for foraging were estimated. Empirical data on levels of activity and foraging of VHF radio-tracked harbour seals were used to parameterise an energetics simulation model based on activity, body size, and composition. The harbour-seal energy requirement was simulated for a period from 20 June to 31 August. The diet of seals in the area was established from otoliths in faecal samples. The daily food requirements of the 750 harbour seals averaged 3 t based on a diet of mainly Gadidae. A procedure to extrapolate from individual movements and foraging activities of radio-tracked seals to population-level habitat use of foraging seals was adopted. The habitat use of the population was then used to integrate the results of the energetics simulations into the GIS model. The distribution of fishing operations was included and the co-occurrence of fishing operations and seals was analysed. The largest potential for interaction between fisheries and harbour seals was a bottom-set gillnet fishery at 100–200-m depth just off the slope separating the archipelago and adjacent shelf waters. Seals were foraging on fish species targeted by this fishery and the entanglement of seals in this type of fishery was assumed to have an effect both on seal population growth rate and on gillnet fishing efficiency. An estimated total of 32.1 t of fish was removed by seal predation from the areas actually fished by bottom-set nets (117 km 2 ) during the 73 days simulation period. The corresponding figures for the areas fished by Danish seine (140 km 2 ) and shrimp trawl (153 km 2 ) were 20.4 and 40.6 t of fish, respectively. While seal predation of fish probably caused negative effects on gillnet and Danish seine catches the removal of benthic-feeding fishes may cause a positive effect on shrimp catches. 2002 International Council for the Exploration of the Sea

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