Go to main content

Topic: Sea lice

The salmon louse is the most common parasite on farmed salmon, and the biggest disease problem in the aquaculture industry in Norway. Monitoring programs of salmon lice shows that the magnitude of the problem is increasing and that in many cases the lice have become resistant to frequently used treatments such as oral and bath treatments.

The salmon louse is a marine parasitic crustacean on salmonids (Atlantic salmon, sea trout and Arctic char), occurring naturally in the northern hemisphere.

Salmon lice does not tolerate low salinity and will eventually fall off when the fish returns to the river. Hence, the presence of salmon lice on the host was considered a sign that the fish had recently returned from the sea, but laboratory experiments have shown that salmon lice can survive on the fish for up to 14 days in fresh water.

The louse damages the host fish by eating mucus, skin and blood. This makes the fish more susceptible to other infections such as bacteria, viruses and fungi and also affects the salt balance of the fish. Large numbers of salmon lice can cause the fish to die, but also indirect damage, reduced growth and a reduction in suitable habitats (as the fish avoids areas with many lice) weaken the reproductive potential of wild salmonids.

Farming of Atlantic salmon (Salmo salar) in Norwegian fjords and along the coast has increased in recent decades, and it is a goal for both industry and the authorities that production should be further intensified. Since salmon farming takes place in open cages, parasites such as salmon lice can freely spread from farmed to wild fish. The large increase in the number of available hosts along the coast has led to an increased abundance of salmon lice on wild salmonids. Salmon lice are therefore a challenge for the sustainable growth in the aquaculture industry. Consequently, to reduce the amount of lice on wild fish, the government has introduced an limit on the number of allowed salmon lice on farmed fish