Registreringer av lakselus på laks, sjøørret og sjørøye i 2004

Abstract

Results from 2004 showed that the lice infection pressure varied between the different monitoring stations in time and space and that the overall pattern was moderate to high infection in all areas. This is in spite of the fact that the fish farmers have improved their implementation of delousing strategies in fish farms. The density of fish farms is high all along the coast. The total biomass may therefore be so high that the "legal' lice numbers per fish not will be sufficiently low to reduce the infection pressure to sustainable levels. This may be the reason for the relatively high infection levels seen on sea trout in Eresfjorden, Hitra and Vik in Vesterålen. Slightly increased infections were also found in Altafjorden (Finnmark) during the summer period. In Troms and Finnmark, most of the fish farmers delouse during late autumn. This may result in the low infection pressure during spring and early summer, but also lead to pronounced infection pressure during late summer and autumn. Therefore, fish farmers must increase their focus on delousing this part of the year. Another source of infection may be from ascending wild salmonids. The infection pressure during the postsmolt phase of Atlantic salmon in 2004 was relatively low in Altafjorden and Trondheimsfjorden compared to earlier findings from the fjords in Western Norway. The trawling for postsmolts from Trondheimsfjorden in 2004 was concentrated to week 20 and a low infection rate was found. In the Altafjord we observed for the first time since the postsmolts surveys started in this fjord that the postsmolts were infected by salmon lice but both intensity and prevalence was low. Parallel captures of wild sea trout and Arctic charr in the littoral zone in Altafjorden and at Hitra outside the Trondheimsfjord showed that the infection pressure on sea trout and Arctic charr was high during the postsmolt stage of Atlantic salmon. The risk and consequences of the infection on Atlantic salmon postsmolts will therefore depend on the intensity of the infection pulse and, whether this pulse coincides with the smolt migration period. Low sea temperatures in the spring and early summer in Finnmark may cause the infection pressure and the smolt migration period not to coincide, and that the postsmolts may migrate to the open sea without being infected by salmon lice in these areas. In addition, the salmon farmers have focused on keeping low lice numbers during the smolt migration period. This may be the reason to the reduced salmon lice infection pressure for postsmolts of Atlantic salmon all along the Norwegian coast. The status for sea trout is not so positive. Monitoring of the lice pressure on sea trout in 2004 still show high infection levels of salmon lice. On most of the locations there are no positive effects of the National Action Plan Against Salmon Lice on Salmonids. It appears that the aim of less than 10 lice per sea trout will be difficult to achieve. In areas with intensive fish farming activity the delousing limits should be reduced below the prevailing limits. Increased focus must be on delousing in fish farms during summer. Field monitoring, as performed in 2004, must be continued in order to assess whether the impact of delousing strategies in fish farms are sufficient, and to generate necessary scientific knowledge to be able to manage the salmon lice problems on wild and farmed salmonids. We must concentrate sampling to the monitoring stations where we already have long time data series on sea trout, Arctic charr and Atlantic salmon.