Salmon lice from aquaculture reduce marine survival of Atlantic salmon.

Salmon lice from marine salmon aquaculture have been shown to impact the survival of wild salmon migrating through aquaculture areas, but quantifying the level of impact has proven difficult. This difficulty has led to the use of randomised control trials. These trials comprise paired releases of typically hatchery-origin groups of untreated salmon smolts and those with a prophylactic treatment against salmon lice, to provide an estimate of the level of increased marine survival due to the treatment, as a proxy to inform such effects on wild salmon. Here, we use data from previously published Irish studies together with 15 unpublished paired release trials standardised using actual recapture data (raw data) to examine differences in survival. Results from meta-analysis of 43 paired releases spanning the period 2001 to 2019 show a significant treatment effect against lice with a risk ratio of 1.22, equating to 18% less return of untreated adult hatchery salmon. Meta-regression further demonstrated that the risk increased with lice infestation pressure from salmon farms. Infestation pressure was also significantly associated with declining return rates in both the treated and untreated groups, corroborating earlier findings that the chemical treatment against lice may not completely shield the post-smolt salmon against the virulent effect of lice. Overall, these results indicate that the effect of salmon lice is larger than the average effects across years reported in earlier randomised control trial studies. Policy implications. The meta-analysis points to salmon lice from salmon aquaculture as a mechanistic threat to wild Atlantic salmon. It emphasises the need for the sector to have effective sea lice management such that there is no increase in lice-induced mortality of wild salmonids as per the international goal of the North Atlantic Salmon Conservation Organisation. The findings are also relevant for the conservation goals set for Atlantic salmon under the EU Habitats Directive in river stocks potentially susceptible to this pressure.