Laboratory and field investigations of salmon lice [Lepeophtheirus salmonis (Kroyer)] infestation on Atlantic salmon (Salmo salar L.) post-smolts

Hatchery-reared 1-year-old Atlantic salmon post-smolts (Salmo salar L.), ar tificially infected with salmon lice [Lepeophtheirus salmonis (Kroyer)] cop epodids, were found to suffer from primary alterations (increased cortisol levels) at early lice stages. Secondary alterations, such as osmotic stress (increased chloride levels), first occurred after the preadult stages of t he lice appeared. Fish with the highest salmon lice infections died through out the experiment. Seven years of field investigation of Trondheimsfjorden showed that Atlantic salmon post-smolts descending coastal waters can beco me heavily infected with salmon lice. The migrating post-smolts were only i nfected with the chalimus stages, showing that the fish had only recently l eft the rivers. The infection level, however, varied considerably between t he years, and, in 1998, the infection was higher than previous years. The e xperimental results have been combined with the field data to appraise the consequences of the infection.