Occurrence ofLepeophtheirus salmonis(Copepoda: Caligidae) on sea trout (Salmo trutta) in the inner Oslo Fjord, south-eastern Norway

Abstract

The occurrence of the parasitic copepod Lepeophtheirus salmonis Kroyer on sea trout Salmo trutta L. was studied in the Askerelva river and in the adjacent Oslo Fjord, south-eastern Norway, where there are no salmon farms. Sampling was carried out in 1992 and 1993. Fishing was started in May, but no sea trout were caught in the river before August. Maximum prevalence (percent infested fish) in the monthly samples from the river was 50%, and the mean intensity (mean number of lice on the infested fish) in all river samples pooled was 19.9 parasites. Most were in the pre-adult or adult stage. The fish from the fjord were larger and had, in general, fewer lice than fish from the river (pooled mean intensity=7.6 lice), but the difference in mean intensity of infestation was not significantly different. There was no correlation between condition factor and the intensity of infestation on trout from the river nor from the fjord.