An Assessment of the True Damages caused by Grey Seals, Halicoerus grypus, in the Swedish Baltic Net Fishery after Atlantic Cod, Gadus morhua

The conflict between grey seals, Halichoerus grypus, and the Swedish cod fisheries in the Baltic Sea has steadily increased during the past 10 years. Seals locate the fishermen’s nets, damage the catch and tear holes in the nets. In this study we investigated the damages caused by seals on the net fishery for cod, Gadus morhua, in the central Baltic Sea. Damage includes both the visible damage and the hidden damage i.e. the fish that are taken whole without leaving any visible trace or damages to the nets. To gather the required data an observer joined two professional fishermen for a period of tree months in 2005. During that period all catch were recorded together with any visible damages. On some experimental links of nets the caught fish were marked without being removed from the net. The net was reset and when retrieved again the number of damaged or lost marked fish was noted. Corrections to the data were made to account for fish that spontaneously fall off. On 101 out of 184 links set out damaged fish were found. Damaged fish was most often found as small remains or with only the head of the fish left. At 20 out of 31 occasions when fish were left in the net, marked fish were missing or found damaged indicating a seal visit. 64.3 percent of the marked cod were removed from the damaged links without leaving any visible trace, and an additional 10.1 percent were damaged adding up to a total damage of 74.4 percent of the catch. There was also a significant difference in catch per effort (cod caught per 100m*24h) between damaged and non-damaged experimental links. Our findings show that the damage to the cod fishery caused by seals is large and that damage can not only be estimated by observing and counting retrieved damaged fish. The hidden damage is significant an