Parasite spillback from domestic animals can distort the balance between host and parasites in surrounding wildlife, with potential detrimental effects on wild populations. In aquatic environments, parasite spillback from aquaculture to wild salmon is one of the most contentious sustainability debates. In a 19 year time series of release group studies of Atlantic salmon, we demonstrated that (i) the effect of subjecting out-migrating salmon smolts to parasite treatment on marine survival has been reduced over a time, (ii) the relation between salmon lice levels in the out-migration route of the salmon and effect of treatment against the parasite is weak, but also (iii) the return rates in both treated and untreated groups of salmon are negatively correlated with salmon lice levels, and (iv) returns of wild salmon to the region are similarly negatively correlated with salmon lice levels during the out-migration year. Our study suggests that salmon lice can have a large effect on wild salmon populations that is not revealed with randomized control trials using antiparasitic drugs. This should be better accounted for when considering the impacts of farms on wild salmon populations.
Emamectin benzoate (EB) is a prophylactic pharmaceutical used to protect Atlantic salmon (Salmo salar) smolts migrating out of rivers and into the ocean against sea lice parasites.Randomized control trials comparing the marine survival of smolts treated with EB to a control group is used to calculate the fraction of marine mortality attributable to sea lice parasitism. However, it is assumed that there is no baseline difference in survival induced by the application of EB treatment. We used a combined laboratory and field study approach to investigate the potential impacts of EB treatment on behaviour and survival of hatchery-reared Atlantic salmon in western Norway. In aquaria experiments, EB-treated salmon smolts did not differ significantly in exploratory behaviour. Fish from treated groups responded similarly to simulated predator attack with spontaneous escape and elevated gill beat rate. Three rivers in the Osterfjord system of western Norway were selected for field experiments, Dale, Modalen, and Vosso. Dale River smolts were treated with intraperitoneal EB injections and had lower probability of detection in a wolf trap downstream of the release site than control smolts.Salmon smolts raised in the Vosso River hatchery were treated with EB delivered in their food and were detected on PIT antennas at the rivermouth at lower rates than control fish, but only when released at downstream sites. Calculation of risk ratios suggested that the bias in mortality caused by treatment with EB decreased the estimated survival of treated fish from an expected 18% to 46%, reducing the observable negative impact of sea lice on Atlantic salmon smolts.The results suggest that estimates of the fraction of mortality attributable to sea lice may be underestimated due to lower baseline survival of treated fish caused by treatment and bring urgent attention towards a potential systematic underestimation of the impacts of sea lice on wild salmon.
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