Seasonal occurrence of Lepeophtheirus salmonis and Caligus elongatus (Copepoda: Caligidae) on sea trout (Salmo trutta), off southern Norway

The occurrence of the parasitic copepod Lepeophtheirus salmonis Kroyer and Caligus elongatus Nordmann on sea trout Salmo trutta L. was investigated in the Arendal archipelago, southern Norway, an area without salmon farms. The occurrence of both parasite species followed a seasonal pattern. Prevalence (percent infested fish) and median intensity (median number of lice on the infested fish) of both species increased in the spring, and declined in the winter. A prevalence of 100% was reached in 1993, 1994 and 1995 for L. salmonis, while the prevalence of C. elongatus generally peaked at about 90%. In March, only adult L. salmonis were found, and small numbers of pre-adults appeared in April. The proportion of chalimi of L. salmonis never reached more than 15% each month of the study. The infected fish were without wounds and apparently in good condition. Neither the condition factor of the host, nor host age were correlated with the intensity of L. salmonis or C. elongatus. (C) 1998 International Council for the Exploration of the Sea.