Consequences of premature return by sea trout (Salmo trutta) infested with the salmon louse (Lepeophtheirus salmonis Kroyer): Migration, growth, and mortality

Premature return of sea trout (Salmo trutta) was studied by collecting ascending trout in a fish trap in the mouth of the Lonningdalselven River, western Norway. Postsmolts ascended the river in June and older migrants in July. Ascending fish were heavily infested with salmon lice (Lepeophtheirus salmonis) and many were in poor physical condition. Louse infestations differed between the two groups of anadromous trout. Postsmolts had a median infestation intensity of 206 lice, mainly juveniles. Older migrants carried a median intensity of 43.5 lice, with a high proportion of preadult and adult lice. Forty-one percent of the postsmolts that ascended the river migrated back to the sea, after a median river stay of 37.5 days. These fish had recovered from the severe louse attack but experienced a median decrease of 23.5% in body mass and no increase in length. All older migrants stayed in the river for the rest of the season. Low mortality (3.6%) was observed in prematurely ascending postsmolts but 19.7% of the older migrants died within the ist week of their return.