Direct evidence of increased natural mortality of a wild fish caused by parasite spillback from domestic conspecifics

Abstract

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.

Prospects for the future of pink salmon in three oceans: From the native Pacific to the novel Arctic and Atlantic

“…Pink salmon are not, however, completely immune to salmon lice as studies from the north Pacific have shown that populations of pink salmon can be negatively impacted by salmon lice similar to Atlantic salmon (e.g. Vollset et al, 2023), with reduced returns in years with high infestation pressure during their smolt year (e.g. Krkošek & Hilborn, 2011).…”

“…may be less problematic for pink salmon, giving a competitive advantage to pink salmon compared to Atlantic salmon in environmental conditions that promote lice promulgation. Pink salmon are not, however, completely immune to salmon lice as studies from the north Pacific have shown that populations of pink salmon can be negatively impacted by salmon lice similar to Atlantic salmon (e.g Vollset et al, 2023),. with reduced returns in years with high infestation pressure during their…”

The paradox of predation studies

“…In this example, we present a simple ecosystem where juvenile Atlantic salmon ( S. salar ) have been documented to be impacted by predation by Atlantic cod ( Gadus morhua ) [35], sea lice ( Lepeophtheirus salmonis ) from fish farms [36] and warm summer water temperatures [37]. Each of these three component causes may lead to death by themselves or in combination, meaning there are many sufficient causes.…”

Premises for a digital twin of the Atlantic salmon in its world: Agency, robustness, subjectivity and prediction

“…Most Atlantic salmon Salmo salar now live in captivity (Food and Agriculture Organization [FAO], 2023), where living conditions have changed profoundly over only 20 generations. Atlantic salmon evolved in the North Atlantic Ocean and its rivers (Lennox et al., 2021), where most populations now are at risk (International Council for the Exploration of the Sea [ICES], 2022), predominantly due to anthropogenic changes in the rivers (Lenders et al., 2016) and, more recently, reduced survival in the ocean (Vollset et al., 2022, 2023). Artificial environments combined with artificial selection change the experiences of the fish and their life histories.…”

“…in the North Atlantic Ocean and its rivers (Lennox et al, 2021), where most populations now are at risk (International Council for the Exploration of the Sea [ICES], 2022), predominantly due to anthropogenic changes in the rivers (Lenders et al, 2016) and, more recently, reduced survival in the ocean (Vollset et al, 2022(Vollset et al, , 2023. Artificial environments combined with artificial selection change the experiences of the fish and their life histories.…”