Impact of grey seals in the herring gillnet fishery along the Swedish Baltic coast

No abstracts are to be cited without prior reference to the author.

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. Adjustments were made for the marked fish that fell off during the handling of nets. 14 occasions of 19 rounds were considered as occasions with seal damages. In those cases more than 86 percent of the marked fish was missing and 5 percent was found left but damaged. The overall catch, including the commercial fisherman’s during occasions with marked fish, did decrease significantly when seals had been present. The catch losses (difference in catch) were much larger than the few observed seals possibly could have consumed in that time period. Observations from all set links showed that on those occasions when seals had been observed while setting the nets the catch was much lower than when seals had not been observed. The low catch when seals are close by and the very few remains of herring found, indicates that the mere presence of seals affected the catch level. This supports the commonly held view among fishermen that seals 'patrol' along nets and thus scare fish away.