Effects of salmon lice (Lepeophtheirus salmonis) on wild caught sea trout (Salmo trutta) under experimental conditions
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- Institutt for biologi 
An increasing net cage farming of salmonids in Norway are leading to increased production of infective salmon louse (Lepeophtheirus salmonis) larvae in coastal areas. One of the most vulnerable salmonids for salmon lice infestation is sea trout (Salmo trutta), whose population in some parts of Norway has declined in areas with intensive farming. The present study investigated mortality and the physiological effects induced by salmon lice. This was done by using natural and artificially infested wild sea trout post-smolts which were divided into two different laboratory experiments. Sea trout used in the experiment were captured during the summer of 2015 in areas around the Romsdalsfjord, in western Norway. Increasing salmon lice density was shown to negatively affect mortality in naturally infested fish. The physiological effects indicated that artificially infested fish did not suffer from chronic stress or osmotic imbalance due to salmon lice. This was probably due to a too low infestation intensity and because a loss of salmon lice in the mobile stages. The results showed, however, that it was possible to capture, transport and conduct laboratory experiments on wild sea trout post-smolts. The experiment on natural infested sea trout should be repeated with a higher number of individuals with a homogeneous distribution of salmon lice. For the artificially infested sea trout, a higher salmon lice intensity should be used to provoke increased physiological responses. Knowledge of physiological effects and morality induced by salmon lice is important in order to direct management efforts towards a critical threshold of salmon lice in wild sea trout and other salmonid species.