Effects of salmon lice Lepeophtheirus salmonis on sea trout Salmo trutta at different times after seawater transfer

The physiological and behavioural effects and skin damage caused by salmon lice Lepeophtheirus salmonis (Krøyer) infections on sea trout Salmo trutta L. smolts were greater in fish infected with lice 2 wk after seawater transfer than in fish infected 6 wk after seawater transfer. The initial prevalence of infection was 100% for both groups and the intensity of infection decreased significantly with time over 5 wk. Significantly fewer of the fish infected 2 wk after seawater transfer had resumed feeding by the end of the experiment, leading to a loss of body condition. Furthermore, these fish suffered more severe damage to the skin and detrimental changes in physiological integrity than fish infected 6 wk after seawater transfer as a direct consequence of feeding preadult lice, leading to osmoregulatory failure and death. Although this study was carried out in laboratory conditions, results indicate that lice infections may potentially have a detrimental impact on the survival of wild smolts after seawater transfer.