Are salmon stressed by anti-salmon louse medication?

Studies carried out by the National Institute of Nutrition and Seafood Research (NIFES) show that the active component in one of the most common anti-salmon louse drugs causes temporary moderate stress in the treated salmon.

One of the most common ways of treating farmed salmon against salmon louse (a crustacean) is to give the fish medicated feed containing active components. Emamectin benzoate is the most used active component in medicated feed of this kind, being commonly used in Norway, Scotland and Chile. Since emamectin benzoate affects salmon louse, possible toxic effects on wild living crustaceans has been in focus nationally and internationally. It is also of interest to examine whether the drug used for treatment may have adverse effects on the fish during medication. Researchers at NIFES have studied the stress response in salmon that have been given a standard treatment with emamectin benzoate.

The study showed a temporary moderate oxidative stress response, and a delayed inflammatory reaction in salmon liver, says researcher Pål A. Olsvik at NIFES.

Examination of liver genes
Several of the basic metabolic processes in salmon take place in the liver, and the effect of treatment with an anti-louse drug can be measured by studying the expression of genes in liver cells. Changes in the expression of 16 000 liver genes were mapped from the first day of treatment until day 35.

Only a small number of liver genes were up-regulated or down-regulated as a result of the treatment. The analysis of the expression of genes in the liver cells showed a temporary oxidative stress response after seven days and a moderate inflammatory reaction after 14 days, says Olsvik.