Salmon lice

The salmon louse is a parasite that uses salmonids as a host. Although always present on wild salmonids in Norwegian waters, since the explosive growth of the aquaculture industry, the louse has gradually become a major environmental problem. As a consequence of fish farming the number of potential hosts for salmon lice has multiplied, and as a result the louse population has become so large that we see adverse effects on wild salmonid stocks in Norwegian coastal areas.


The salmon louse, Lepeophtheirus salmonis, is the ectoparasite that today constitutes the biggest problem in Norwegian aquaculture. Vast resources nationally and internationally are devoted to research on combating salmon lice. The salmon louse appears on most species in the genera Salmo (salmon and trout) and Oncorhynchus (Pacific salmon and rainbow trout) (Kabata 1979).


The main method to keep the population of louse in farms low is by using different pharmaceuticals. There are effective chemicals that can be given to the fish via the feed or supplied directly into the water. A concern is that the salmon lice can develop resistance against some of the pharmaceuticals. Some chemicals may thus lose their effect. Chemicals used can also have a negative impact on the environment surrounding the farm. Some farmers use biological delousing by using Wrasse. Wrasse fish is put together with the farmed fish and they eat lice. The experience has been variable, but several farmers have so much success with this method is that they have completely stopped using chemicals. Research is now being conducted to develop a vaccine against sea lice. If this becomes a reality it will contribute greatly to reducing the impact of sea lice to vulnerable populations of wild salmon and reduce the use pharmaceuticals.

Salmon lice continue to represent a significant problem for the stocks of wild salmonids in Norway’s coastal and fjord areas. The number of hosts is steadily rising, and absent a change in current strategies for combating salmon lice, this will lead to ever-increasing production of salmon lice in the years to come.


In 2000, the Ministry of Agriculture established a regulation on combating sea lice. The purpose of the regulation was to set out minimum measures to reduce the amount of sea lice in fish farms. For example if there are more than 0.5 adult female sea lice on average per fish between December to July treatment against sea lice shall be implemented.


At a threshold of 0.5 lice per fish, increased fish farming activities will lead to increased salmon lice production. To stop the growth of production of salmon lice and prevent increased infestation pressure on wild stocks, the threshold for the permitted number of lice per fish must continually be reduced. A lower limit for obligatory delousing, however, seems to many to be difficult to implement. The alternative would be to stipulate a carrying capacity for what a fjord area can tolerate of salmon lice. In that case fish farming activities must be adjusted on the basis of how much salmon lice the individual system tolerates.



We see that escaped salmon contribute heavily to the production of salmon lice along the Norwegian coast. Measures to reduce the escape of farmed fish will in this way help to reduce the infestation pressure on stocks of wild salmonids.