Satellite telemetry of grey seals caught with trap-nets in the Bothnian Sea during years 2008–2009

The key results of the project about spatial behaviour of grey seals (Halichoerus grypus) in the Baltic Sea during years 2008–2009 are presented in this report. This knowledge can be useful in mitigating the seal-fishery conflict and assuring the sustainable management of grey seal stock in the Baltic. Altogether 12 male grey seals were equipped with GPS/GSM-devices, which collect information about movements, dives, haul outs and water temperature. Grey seals were caught in the Bothnian Sea near the towns of Merikarvia and Pori by using commercial trap-nets equipped with “non-return” gates. Ten GPS/GSM-devices collected enough data for the analyses. Home ranges (95 % MCP and a-LoCoH methods) and home range core areas were estimated for autumn and winter. Core area locations were compared to the trap-net fishing effort in each statistical area (about 50 x 50 km) in autumn (Sept-Nov). Movements of the grey seals were compared to the ice cover on the basis of satellite pictures.

Only males (N=18) were trapped in the experimental traps. The autumn home ranges (N=10) were on the coastal areas (less than 30 m depth). Nine home ranges were in the westcoast of Finland and quite close to the experimental traps. Autumn core areas of eight GPS/GSM-grey seals were near the mouth of rivers Merikarvianjoki and Eurajoki. These areas were also important for the trap-net fishery during September-November. Some individuals (N=3) were also recaptured in the experimental traps. The grey seals preferred open water areas to ice covered. Winter 2008–2009 was mild and the grey seals (N=2) stayed near the river deltas also during the winter as only areas very near to the coasts were covered with ice. Winter 2009–2010 was colder and ice covered larger areas of the Baltic Sea. The studied grey seals (N=4) moved from Bothnian Sea to the main basin of the Baltic Sea.

In accordance to previous studies, the results of this study show that grey seals can move long distances but often concentrate their movements to certain restricted area for periods of time. The male seals in this study seem to prefer coastal areas and river mouths during autumn. Also some trap-net fishing is concentrated near river mouths. River mouths can attract fish, for example spawning whitefish. In addition, cooling waters of nuclear power plant near the river mouth of Eurajoki can attract fish. The results of this study suggest that some grey seals can specialise in feeding in fish traps and in their proximity. However, both grey seals and fishermen are interested in good fishing waters. The results of this study do not show whether grey seals are attracted to the area by abundance of fish or trap-net catches.