SALMON LICE, LEPEOPHTHEIRUS-SALMONIS, INFESTATION AS A CAUSAL AGENT OF PREMATURE RETURN TO RIVERS AND ESTUARIES BY SEA-TROUT, SALMO-TRUTTA,JUVENILES

A field experiment conducted in the River Lonningdalselven in spring 1 992 supports the hypothesis that salmon lice, Lepeophtheirus salmonis, infestations may cause premature return of sea trout, Salmo trutta, j uveniles, either to estuaries or to rivers. When lice infested (expose d) and uninfested (control) sea trout juveniles (post smelts) were rel eased simultaneously into the sea, exposed fish returned to the estuar ine area earlier compared with controls. Within the following two days , exposed sea trout migrated further into freshwater. At that time the y were infested with a median of 62.5 lice, dominated by chalimus larv ae and late juveniles. Exposed sea trout suffered from an osmoregulato ry failure in sea water and this is considered one reason for infested fish returning to brackish water. While only a few control fish retur ned to the estuary on the day of release, some more returned to freshw ater the following four days. During this time they had become heavily infested with copepodids, and carried a median of 150.0 lice. It is s uggested that physiological stress and high infection pressure in the sea results in sea trout juveniles returning to estuaries and freshwat er.