The density of planktonic stages of salmon lice (Lepeophtheirus salmonis Køyer) in relation to biofouling on net structures, and the use of louse skirts in Norwegian salmon farms

The ectoparasitic salmon lice (Lepeophtheirus salmonis, Køyer) is a major problem for aquaculture of Atlantic salmon in Norway, affecting fish health, economy and the public opinion of the industry (Costello, 2006, Liu and Bjelland, 2014). The nauplii stages of salmon lice have limited swimming capacity, but can be transported for long distances (Salama et al., 2016), which makes salmon lice able to spread from one farm to another in adjacent areas. Attempts to limit infection includes preventive measures like enclosing the upper part of the fish cage in a tarpaulin (louse skirt) (Lien and Høy, 2011). Densities of salmon lice nauplii have been shown to be higher on the inside of fish cages (Gravil, 1996), and Costelloe et al. (1996) mentioned that this might be due to biofouling on the net structures. If biofouling on net structures can retain planktonic salmon lice inside the cages, a louse skirt might have the ability to do the same, causing a delay in dispersal. A delayed dispersal could give the lice time to develop into the infective copepodite stage closer to the salmon farm, possible even in time to infect the same locality or cage. This study aims to evaluate how the density of the planktonic stage of salmon lice, inside and outside the salmon cage, are affected by the presence of a louse skirt, and the amount of biofouling on the net structures of the salmon cage.