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. The most relevant wrasse species are the goldsinny or salmon wrasse (Ctenolabrus rupestris) for salmon under 2 kilograms, and the ballan wrasse (Labrus bergylta) for larger salmon. Other species that can be used for louse control are the corkwing wrasse (Crenilabrus melops), the rock cook (Centrolabrus exoletus) and cuckoo wrasse (Labrus bimaculatus) (Andreassen and Kvenseth, 2000; Nogva, 2000).

An experiment conducted at Villa Miljølaks AS's fish farm in Vestnes in Møre og Romsdal county shows that wrasse can provide an effective treatment against lice (Kvenseth et al., 2002). During the period of the experiment, the farm was subjected to repeated infestations of salmon lice. Every time, the lice were eaten up by the wrasse before they reached sexual maturity. The advantage of this form of treatment for salmon lice is that the wrasse performs continuous control of the louse situation. For their part, medications or delousing baths are incapable of keeping louse infestations down between treatments. The wrasse's appetite increases as the louse grows. Used correctly, a female louse with eggs is a rare sight in a well-run salmon farm that actively uses wrasse.

Practical limitations

Once the wrasse have cleaned the nets and the salmon, they can do damage by biting the fin rays or the eyes of the salmon. Therefore it is essential to monitor the development of the total food available to the wrasse. If such a situation arises, the number of wrasse in the cage needs to be adjusted, for example with the aid of a modified fish pot baited with mussels (Kvenseth et al., 2003).

Another limitation is the supply of wrasse. The need is estimated to be six million goldsinny, if all fish farmers used these on salmon under 2 kilograms. From Møre og Romsdal county southward the supply of wrasse caught in the wild is good, whereas in Trøndelag and northward, the supply is limited. Transfers of fish from southerly areas are therefore necessary. For salmon larger than 2 kilograms, the ballan wrasse is a more suitable species, but here the supply of fish caught wild is a much clearer stumbling block. Farming wrasse may therefore become a possibility (Kvenseth et al., 2002).The Institute of Marine Research has succeeded in producing ballan wrasse fry at the Aquaculture Station in Austevoll. Raising wrasse may make stable year-round delivery possible, and the wrasse will be able to be delivered ready-vaccinated with a health certificate. However, challenges remain in respect of spawning and mortality (Skiftesvik and Bjelland, 2003).

Costs of using wrasse

According to Kvenseth et al. (2002), the costs connected with this type of treatment are the same as for using feed with a delousing agent added. For large salmon, the numbers turn out very positive for wrasse. Purchasing ballan wrasse, at a 1 % mixture (ratio of wrasse to farmed fish in the cage) with large salmon, costs on the order of 1/10 of the cost of purchasing pharmaceuticals for equivalent louse control in large salmon 2 - 7 kilograms. Recent research (S. Øvretveit, 2003) shows that the number of delousings with pharmaceuticals has a negative impact on the feed conversion ratio (FCR). For an ordinary fish farm the extra costs will amount to between NOK 1 and 2 million for a generation. With the effective use of wrasse, these costs will be on the order of NOK 0.1 million in all.

A positive added effect of the use of wrasse is that they also nibble at the fouling of the fish farm nets. Fouling otherwise clogs the mesh so that water exchange is poor. To prevent fouling, impregnation using copper compounds has traditionally been used. This is discussed below in this chapter.

Potential problems

Transporting wrasse between parts of the country or between fish farm sites potentially spreads fish diseases. Contagiousness experiments were conducted on farmed goldsinny (Ctenolabrus rupestris) in Scotland. In various experiments the fish were exposed to the viral diseases infectious pancreatic necrosis (IPN) and pancreas disease (PD). Goldsinny do not appear to be able to spread the latter disease (PD) to salmon. The researchers found that goldsinny was likely as susceptible to the IPN virus as salmonids, but that the goldsinny had a greater ability to recover. The virus was also found in the faeces of goldsinny that had been exposed to high doses of the virus. Therefore, faeces can be a continuous source of infection in a salmon farm that is infected with IPN (Gibson and Sommerville, 1996). IPN was considered one of the most serious infectious diseases in Norwegian fish farming, causing annual economic losses of up to NOK 400 million in 1994 and 1995 (Biering,1999). Nevertheless, the goldsinny's potential as a source of infection for the IPN virus should not prevent it from being used for combating parasites, but transporting wrasse between different locations and the re-use of wrasse with different cohorts of salmon should be avoided (Gibson and Sommerville, 1996).

Diseases can also affect the wrasse themselves. Mortality in the wrasse can reduce their effectiveness on the louse infestation. In goldsinny and ballan wrasse the bacterial disease atypical furunculosis can cause problems. The illness is triggered by stress and may occur in the fish farm in connection with handling nets or fish. In corkwing wrasse bacterial infections like Vibrio splendidus and V. tapetis can cause problems. Such infections manifest themselves as ulcerations, listlessness and poor appetite. Viral infections are yet to be found in wrasse caught in the wild, but this may be due to a lack of a diagnostic method. While no parasites have been found in wrasse that can cause problems for salmon, there are parasites that can cause high mortality in wrasse (Kirkemo et al., 2003).

Several instances of high levels of mortality in wrasse in salmon farms have raised questions about whether the use of wrasse is an ethical use of animals. The Norwegian Council on Animal Ethics has considered the issue and concludes that the use of wrasse in salmon farming is desirable, but points out that fish farmers need to understand that the wrasse are living beings and not input factors on par with pharmaceuticals. The wrasse's biological needs have to be met in the form of access to food, availability of suitable hiding places and adequate wintering (Norwegian Council on Animal Ethics, 2000)