Salmon lice--impact on wild salmonids and salmon aquaculture

Abstract

Salmon lice, Lepeophtheirus salmonis, are naturally occurring parasites of salmon in sea water. Intensive salmon farming provides better conditions for parasite growth and transmission compared with natural conditions, creating problems for both the salmon farming industry and, under certain conditions, wild salmonids. Salmon lice originating from farms negatively impact wild stocks of salmonids, although the extent of the impact is a matter of debate. Estimates from Ireland and Norway indicate an odds ratio of 1.1:1-1.2:1 for sea lice treated Atlantic salmon smolt to survive sea migration compared to untreated smolts. This is considered to have a moderate population regulatory effect. The development of resistance against drugs most commonly used to treat salmon lice is a serious concern for both wild and farmed fish. Several large initiatives have been taken to encourage the development of new strategies, such as vaccines and novel drugs, for the treatment or removal of salmon lice from farmed fish. The newly sequenced salmon louse genome will be an important tool in this work. The use of cleaner fish has emerged as a robust method for controlling salmon lice, and aquaculture production of wrasse is important towards this aim. Salmon lice have large economic consequences for the salmon industry, both as direct costs for the prevention and treatment, but also indirectly through negative public opinion.

Figure 1

Relative nominal catch of Atlantic salmon from 1960 to 2010 in ‘non-farming countries’ (USA, Russia, Iceland, Sweden, Denmark, The Faeroe Islands, Greenland the UK – except Scotland, France and Spain), ‘Norway, Scotland, Ireland and Canada’, and ‘Norway’(NASCO 2011). The Faroe Islands is included among ‘non-farming’ as their salmon fishery is a marine fishery.

Figure 2

Percentage survival of smolts in Ireland from 2001 to 2009 with 95% confidence limits (CL) fitted. Sea lice infestation was not implicated in the observed decline in survival common to both groups (Jackson et al. 2011a). The treated groups were given a prophylactic SLICE treatment giving protection against sea lice infestation for approximately 9 weeks (Jackson et al. 2011b).

Figure 3

Annual trend (May mean) (SE) of Lepeophtheirus salmonis on one-sea-winter salmon.

Figure 4

Catches of wrass in Norway for use as cleaner fish in salmon cages (Fiskeridirektoratet 2012).