CONSEQUENCES OF PREMATURE RETURN BY SEA-TROUT (SALMO-TRUTTA) INFESTEDWITH 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 R iver, western Norway. Postsmolts ascended the river in June and older migrants in July. Ascending fish were heavily infested with salmon lic e (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 ju veniles. 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 seve re louse attack but experienced a median decrease of 23.5% in body mas s and no increase in length. All older migrants stayed in the river fo r the rest of the season. Low mortality (3.6%) was observed in prematu rely ascending postsmolts but 19.7% of the older migrants died within the ist week of their return.