Impacts of parasites on marine survival of Atlantic salmon: a meta‐analysis

Abstract

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

Salmon lice (Lepeophtheirus salmonis) development times, body size, and reproductive outputs follow universal models of temperature dependence

“…Salmon lice are external parasites that cause substantial economic losses to the salmon industry (Costello 2009a), and farmassisted parasitic outbreaks can reduce return rates of spawning salmon in sensitive rivers (Vollset et al 2014(Vollset et al , 2015, posing a potential threat to wild salmonid populations (Costello 2009b;Krkošek et al 2011Krkošek et al , 2013. The salmon louse has a direct life cycle that involves three planktonic larval stages that hatch from a pair of egg strings produced by an adult female: two noninfective naupliar stages and an infective copepodid stage (Hamre et al 2013).…”

Quantifying the contribution of sea lice from aquaculture to declining annual returns in a wild Atlantic salmon population

“…Wild salmon returns were strongly reduced (> 50%) following years when there had been high lice levels on the salmon farm during the smolt out-migration. This result accounts for the effect of unexplained amongyear variation in returns, which probably reflects how marine survival varies naturally independent of lice-induced mortality (Vollset et al 2016). 'Correcting' for the estimated lice effect predicted that Erriff salmon returns might now be twice as large without observed anthropogenic sea lice impacts, but would probably show a similar long-term decline.…”

“…A longer-term reduction in survival may be associated with impacts that impair on-going fitness during migration. The impact of sea lice seems to vary with baseline survival of salmon; a meta-analysis of studies using anti-parasite treatments on salmon smolts found that in groups with low recapture in the control group (low baseline survival), the effect of treatment was high, while in groups with high recapture in the control group (high baseline survival), there was no effect of treatment (Vollset et al 2016). This result implies that the detrimental effect of lice is exacerbated in situations when the salmon smolts also have to cope with increased pressure from other causes of mortality, e.g.…”

“…Some trials indicate that base line survival of smolts has an important influence on the success of lice treatment, with poorer environmental conditions increasing vulnerability to sea lice impacts. Hence, population-level effects of sea lice on wild salmon cannot be estimated independently of the other factors that affect marine survival (Vollset et al 2016). The contribution of sea lice to overall marine survival of wild Atlantic salmon re mains an important knowledge gap, particularly in the context of changing oceanographic conditions and the long-term decline of many populations.…”

In situ measurement of salinity during seaward migration of Atlantic salmon post-smolts using acoustic transmitters with data-storage capabilities and conventional acoustic transmitters

“…The utilization of salinities below 20.0 psu may reduce infestation risk by salmon lice [22,23], and the probability of infestation is generally shown to decrease with increasing freshwater influence [35]. Salmon louse is a marine parasite, which can severely reduce Atlantic salmon populations in farm-intensive areas [36,37]. Whether the fish actively preferred lower salinity waters or unintentionally experienced low salinity because they were swimming close to the surface for other reasons (for instance, avoiding predation) is not known, but nevertheless, this behaviour might reduce the salmon lice infestation risk.…”