Densities of planktonic Lepeophtheirus salmonis before and after an Atlantic salmon farm relocation

Relocating Atlantic salmon Salmo salar L. farms away from important salmonid rivers is possibly a means to reduce Lepeophtheirus salmonis (Kroyer, 1837) interactions between farmed Atlantic salmon and wild salmonids. However, to date, no data have been presented on the consequences of salmon farm relocation in terms of densities of planktonic L. salmonis. Near-surface plankton were sampled at 5 stations during 2 full farming production cycles; 1 before and 1 after a salmon farm relocation within a sea loch/fjord. The relocation comprised the removal of a salmon farm and the reallocation of its fish biomass to a different farm within the same management area. Over 85% of the larval sea lice we identified in sub-samples were L. salmonis; we therefore assumed that the majority of the larvae found were of that species. Densities of L. salmonis larvae increased significantly at all stations post relocation, except at the station near the vacated farm site where mean densities of nauplii were significantly lower and mean densities of copepodids were lower without statistical significance. The removal/relocation of the salmon farm significantly reduced the production of L. salmonis larvae, but did not significantly reduce the infection pressure, as represented by densities of the infectious copepodid stage, at the vacated farm site. This finding indicates that planktonic L. salmonis were transported to the vacated farm site from sources at a minimum of 5 to 8 km distant.