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.

Parasites may have large effects on host population dynamics, marine fisheries and conservation, but a clear elucidation of their impact is limited by a lack of ecosystem-scale experimental data. We conducted a meta-analysis of replicated manipulative field experiments concerning the influence of parasitism by crustaceans on the marine survival of Atlantic salmon (Salmo salar L.). The data include 24 trials in which tagged smolts (totalling 283 347 fish; 1996-2008) were released as paired control and parasiticide-treated groups into 10 areas of Ireland and Norway. All experimental fish were infectionfree when released into freshwater, and a proportion of each group was recovered as adult recruits returning to coastal waters 1 or more years later. Treatment had a significant positive effect on survival to recruitment, with an overall effect size (odds ratio) of 1.29 that corresponds to an estimated loss of 39 per cent (95% CI: 18-55%) of adult salmon recruitment. The parasitic crustaceans were probably acquired during early marine migration in areas that host large aquaculture populations of domesticated salmon, which elevate local abundances of ectoparasitic copepods-particularly Lepeophtheirus salmonis. These results provide experimental evidence from a large marine ecosystem that parasites can have large impacts on fish recruitment, fisheries and conservation.

Glover, K. A., Skilbrei, O. T., and Skaala, Ø. 2008. Genetic assignment identifies farm of origin for Atlantic salmon Salmo salar escapees in a Norwegian fjord. – ICES Journal of Marine Science, 65: 912–920. This study represents the first example of genetic assignment being used to identify the farm of origin for escaped Atlantic salmon. Following reports by local fishers of escaped farmed salmon in Romsdalfjord, western Norway, baseline samples were collected from 16 cages located on seven operational farms. The baseline samples, in addition to 29 escapees, were screened for 15 microsatellite loci. Pairwise FST values between baseline samples varied from <0.001 to 0.154, indicating variable but significant genetic differences among them. Direct assignment of the escapees (data from 13 informative loci) demonstrated that the most likely origin of 21 of them was from a single baseline sample (5I) collected at one farm. At a probability of 0.01, between 25 and 29 of the escapees were rejected in 12 baseline samples, 19–21 escapees were rejected from another three baseline samples, and only seven of the escapees were rejected from baseline sample 5I. Consequently, these data demonstrate that most of the escapees most likely originated from a single farm, and importantly, that 15 of the 16 baseline samples could with high probability be excluded as donors for most of the escapees recaptured in the area.

such Norwegian studies on Salmo salar provided a dataset for the time period 1996 to 2011 on 25 118 release groups comprising 657 624 fish released and 3 989 recaptured. The overall risk ratio 26 (RR), calculated as the probability of being recaptured in the treated group divided by the 27 probability of being recaptured in the control group, was estimated to be 1.18 (95 % CI: 1.07-281.30). The effect varied strongly between groups, quantified by Higgins measure of heterogeneity 29 (I 2 = 40.1%). Over 70% of this heterogeneity could be explained by the release location, time 30 period and baseline survival. The most important predictor variable was baseline survival. In 31 groups with low recapture in the control group (low baseline survival), the effect of treatment 32 was high (RR = 1.7), while in groups with high recapture in the control group (high baseline 33 survival), there was no effect of treatment (RR ~ 1.00). The most prevalent parasite in the region 34 affected by the drugs administered was Lepeophtheirus salmonis. Hence, the meta-analysis 35 supports the hypothesis that anti-parasitic treatment protects S. salar smolts from L. salmonis 36 during outward migration. However, the effect of treatment was not consistent, but was evidently 37 strongly modulated by other risk factors. The results suggest that the population level effects of 38 parasites cannot be estimated independently of other factors affecting the marine survival of 39Salmo salar. 40

The impact of salmon lice on the survival of migrating Atlantic salmon smolts was studied by comparing the adult returns of sea-ranched smolts treated for sea lice using emamectin benzoate or substance EX with untreated control groups in the River Dale in western Norway. A total of 143 500 smolts were released in 35 release groups in freshwater from 1997 to 2009 and in the fjord system from 2007 to 2009. The adult recaptures declined gradually with release year and reached minimum levels in 2007. This development corresponded with poor marine growth and increased age at maturity of ranched salmon and in three monitored salmon populations and indicated unfavourable conditions in the Norwegian Sea. The recapture rate of treated smolts was significantly higher than the controls in three of the releases performed: the only release in 1997, one of three in 2002 and the only group released in sea water in 2007. The effect of treating the smolts against salmon lice was smaller than the variability in return rates between release groups, and much smaller that variability between release years, but its overall contribution was still significant (P < 0.05) and equivalent to an odds ratio of the probability of being recaptured of 1.17 in favour of the treated smolts. Control fish also tended to be smaller as grilse (P = 0.057), possibly due to a sublethal effect of salmon lice.