ICES CM 2005/X:12 Working paper may not be cited without prior reference to the authors Impact of grey seals in the herring gillnet fishery along the Swedish Baltic coast Sara Königson, Arne Fjälling and Sven-Gunnar Lunneryd Institute of Coastal Research, Swedish Board of Fisheries, Contact author: Sara Königson, Institute of Coastal Research, Swedish Board of Fisheries, Box 423, 401 26 Göteborg, Sweden tel: +46 31 7430422 email:
[email protected] The gillnet fishery for herring Clupea harengus in the Baltic Sea is subjected to intense damages by grey seals, Halichoerus grypus. Fishing takes place mainly during spring and autumn. Damage caused by seal peak in late autumn and for which reason fishery during this part of the year has more or less disappeared. In this study we investigated the hidden damage, i.e. the amount of fish completely removed from nets without a trace as well as the catch losses due to the presence of seals. Observers joined a commercial fisherman on his regular fishing trips. Catch and damaged fish were noted and systematic observations for seals were carried out during every fishing occasion. On some fishing trips herring was caught in a 30 meter long net of commercial type. All fish was then marked, without being removed from the net. The net was thereafter reset and linked to the local fisherman’s gear. The percentage of marked fish that was damaged or missing the next day, was used as an indication that seals had visited the nets. If a large part of the marked fish were missing it was concluded that seals had visited the nets.