Fighting salmon lice

Publish date: January 9, 2009

Existing treatments for salmon lice can be roughly divided between biological methods, i.e. the use of wrasse, and chemical treatment of salmon infested with salmon lice (Roth et al., 1993; Costello, 1993). Both methods are discussed in detail in their own chapters in this report. Among other important measures, coordinated delousings may be mentioned.


In the salmon lice regulation laid down by the Ministry of Agriculture on 1 February 2000 pursuant to Sections 16 and 29 of Act no. 54 of 13 June 1997 relating to measures to counteract diseases in fish and other aquatic animals (the Fish Diseases Act), threshold limits were introduced for obligatory delousing of fish farms. If in the period from 1 December to 1 July a count shows an average per fish of 0.5 or more adult female lice, or a total of 5 or more adult female lice and mobile stages, in individual cages, treatment for salmon lice is to be performed for the entire locality. In the period from 1 July to 1 December, the entire locality is to be deloused if there is an average of 2 or more adult females, or a total of 10 adult females and mobile stages per fish in an individual cage (Lovdata).


In 1996, work began on the "National Plan of Action to Combat Salmonid Lice". The working group had representatives from the Norwegian Animal Health Authority, the Norwegian Directorate of Fisheries, the Directorate for Nature Management, the Norwegian Fish Farmers’ Association (NFF) and the Aquaculture Veterinarians’ Association. Together they formulated several goals in combating salmon lice. The plan of action’s long-term objective is to reduce the harmful effects of lice on farmed and wild fish to a minimum. Five short-term objectives were also defined:


1. measures are to be planned and coordinated in regional collaborations.

2. the prevalence of lice in at least 95% of the localities is to be documented.

3. the prevalence of lice on wild fish is to be documented.

4. measures used to combat salmon lice are to be documented in at least 95% of the fish farms.

5. organised delousing is to be planned and carried out during the cold season.


Pharmaceuticals used for combating salmon lice

Treatments of lice-infested farmed fish can be divided into three main categories, which are usually used in combination: wrasse, delousing baths, and medicated pellets. The first, wrasse, has few environmental drawbacks, but certain limitations on practical use. The two others subject the fish and the marine environment to toxic substances, and must therefore be thoroughly evaluated with a view to environmental impacts.

For more information read the full articles on salmon lice pharmaceuticals.



The use of wrasses to control lice is an effective means of control that does not add pharmaceuticals to the marine environment. Wrasse is an umbrella designation of fish in the wrasse family (Labridae) that can feed on ectoparasites, i.e. parasites that are attached to the outsides of farmed fish.


For more information read the full article on wrasse.


Other measures against salmon lice


Lights in the cages may mean fewer lice

There is an established connection between how deep the salmon go in the fish farm and how many lice the salmon get. The deeper the salmon go, the smaller the infestation. Experiments have shown that submerged lights in the cages lure the salmon down into deeper water, with lower louse infestations as an outcome. This method is most relevant during the late autumn and winter. In the spring and summer the natural light will override the artificial light Hevrøy, 1998 and Boxaspen, 2001), making the method relatively less interesting under Norwegian conditions.


Stimulating the salmon’s resistance

Tests performed at a fish farm in Scotland have shown that the salmon’s resistance against lice was strengthened when the feed product Respons Proaktiv, an amino acid-based feed that contains glucanes and extra vitamins, was added. Glucanes are polysaccharides that consist of glucose from yeast cell walls. Glucanes’ positive impact on the salmon’s immunity and general health has been documented. The group of salmon fed glucanes had on average 24.4 per cent fewer lice than the control group, a statistically significant difference. It is not completely clear how glucanes help the fish against infestation by salmon lice (Ritchie, 1999).


Vaccines against salmon lice

The "Eukaryotic Parasites in Fish" project is working to develop preventive measures against salmon lice. One approach is building on the fact that the lice suck and digest blood from their host. If vital components of this blood digestion can be characterised, they may conceivably be used as antigens in a vaccine against salmon lice. Researchers believe, however, that there is far to go before such a vaccine is available. (Institute for Marine Research website,


Therapy recommendations of the Norwegian Medicines Agency

In 2000 the former Norwegian Medicines Control Authority, now the Norwegian Medicines Agency, published therapy recommendations for salmon lice. The purpose of the recommendations is to ensure the effective treatment of farmed fish, minimise the spread of salmon lice to wild salmon, limit eco-toxic effects and prevent the emergence of resistance against pharmaceuticals in salmon lice. These recommendations are divided according to the size of the fish and, in part, by the season in which treatment takes place. The recommendations would probably look somewhat different today, because it is now known that emamectin is also appropriate on large fish. The most important thing is alternating between different treatments, so as to avoid the development of resistance.


Fish smaller then 500g:


Treatment – summer:

1. Wrasse

2. Oral treatment (medicine pellets), preferably emamectin

3. Synthetical pyrethroids


Tratment – winter/early spring

1. Syntetical pyrethroids


Fish between 500 – 1000g:


1. Wrasse (spring/summer)

2. Syntetical pyrethroids

3. Oral treatment


Fish over 1000g:


1. Syntetical pyrethroids


In addition to following the Norwegian Medicines Agency’s therapy recommendations, proper routines for prevention and treatment will help to reduce louse infestations on the fish as well as the use of pharmaceuticals. For example, letting sites lie fallow will keep different generations of fish apart, and general good care of the fish will reduce the need for treatment. Players in the fish farming industry have specified such an integrated approach to the louse problem with the concept "integrated pest management" (Richie, 2002).We can also note that wrasse were not included as part of the recommendations for large fish. The Norwegian Medicines Agency should update its recommendations if wrasse can be successfully obtained for large fish.


Treatment of salmon lice

Common to all pharmaceuticals intended to combat salmon lice is that they are toxic to a number of organisms, especially crustaceans, which are the subphylum salmon lice belong to. However, the toxic effects of the substances are relatively local, in the sense that individuals located a distance from the fish farm are not exposed to toxic doses of the agents. How large an area around the fish farm that is affected will vary with the type of substance and local environmental conditions, such as currents and aquatic chemistry.


The pharmaceuticals are also relatively persistent, with half-life periods of several months in sediment, though they are biodegradable, unlike heavy metals and other environmental toxins.


From an environmental perspective, salmon delousing may be viewed as a necessary evil. Failure to act against large infestations of salmon lice in fish farms will not only create health problems for the farmed fish, but also create an untenable situation for emigrating wild salmon. Wrasse have the potential to render the use of antiparasitic drugs superfluous, but until this potential is realised, using these agents cannot be avoided.


The use of wrasse against salmon lice can and should be a more central form of treatment, but several challenges remain before the use of chemicals can be replaced by a non-polluting louse treatment. A greater effort especially in disseminating knowledge about routines for this nonpolluting form of treatment is therefore imperative. Bellona will evaluate the policy instruments that in an appropriate and cost-effective manner can force the increased use of biological methods to combat salmon lice.