A student observing salmon lice. Photo: Trygve Poppe.

Counting salmon lice (Lepeophtheirus salmonis) on farmed fish is one of the routines of the compulsory field course in fish health, which veterinary students take each spring on Frøya, an island off the coast of central Norway. When recording salmon lice, ten fish from each of the two netpens (”fish cages”) are caught at random and anaesthetised in a container, Thereafter, the number of lice (including all development stages) on the fish are recorded.

The monitoring of the salmon lice population on farmed fish is also important for the wild salmon population, since the parasite can also cause considerable damage on wild salmonids. If salmon lice occur in large numbers, they can cause extensive skin damage, with secondary infections, disturbance of the salt and water balance, and even fish mortality as a result. Thus, if the salmon lice population in a fish farm exceeds a certain limit, the fish must be treated in order to reduce the infection pressure.

Two female (large) and one male salmon louse. The female salmon louse is characterized by the broad back part. The smallest light brown is a sea louse. Photo: Trygve Poppe.