Timing is everything: Survival of Atlantic salmon Salmo salar postsmolts during events of high salmon lice densities

Abstract

Atlantic salmon in aquaculture act as reservoir hosts and vectors of parasites like salmon lice and this parasite is shown to harm wild salmonid populations. In this study, n = 29,817 tagged Atlantic salmon were studied in four release trials. Half of the released fish were given prophylactic treatment against lice, the other half represented sham control fish. We used a nested design comparing years with low and high lice density and seasonal dynamics in infestation pressure. The released Atlantic salmon thus experienced highly variable lice infestation pressures, which we linked to survival and growth in returning fish. The fish were released in a protected ‘National Salmon Fjord’ and n = 559 Atlantic salmon were recaptured after spending 1–4 years at sea. In most experimental groups 1%–2.5% of the fish were recaptured at return. However, survival of unprotected fish was extremely low for the trial released at the highest density of lice: only 0.03% of these Atlantic salmon returned to the river, compared to 1.86% in the protected group. Synthesis and applications. We document that high lice density can cause more than 50 times higher mortality risk in Atlantic salmon on their sea migration, even in a fjord with protected status. Fine‐tuned and hard‐to‐predict year‐to‐year differences in timing, both for the wild smolt migration and the population build‐up of lice released from aquaculture, means life or death to wild salmon. Management actions such as spatial segregation of farmed fish and lice (e.g. closed farm pens), and/or moving farms away from vulnerable habitats for wild salmonids (fjords and coastal areas), may be needed to ensure sustainable coexistence of wild and farmed Atlantic salmon.

Synchrony and multimodality in the timing of Atlantic salmon smolt migration in two Norwegian fjords

“…As a result, smolts will likely be outside of our receiver network before any salmon lice-induced mortality can occur. This means that variation in migration timing will probably have a greater effect on return rates than fjord survival, especially in years where salmon lice densities are high 17 .…”

“…However, modelling and monitoring of salmon lice abundance has shown that the risk of infestation for a migrating smolt can vary significantly through time and space, as the density of fish farms can differ widely among regions and migration routes while salmon lice abundance increases substantially with increasing temperatures through spring 14 , 16 . As a result, models that estimate salmon lice induced mortality are especially sensitive to deviations from assumptions about when smolts begin their migration and how quickly they reach the open sea 16 , 17 . As such, there is a need for a better understanding of these phenological traits.…”

Validating timing of salmon smolt runs obtained by telemetry studies

“…For example, lice‐induced mortalities in post‐smolts depend on both migration timing and residency through fjords and coastal areas (Kristoffersen et al, 2018; Nilsen et al, 2017). Bøhn et al (2020) found that control smolts had 50 times higher mortality risk than smolts treated with lice prophylaxis when migration timing was late (June) and infestation pressure high. By contrast, control fish that migrated during lower infestation pressures (May) did not have a lower likelihood of survival than treated fish (Bøhn et al, 2020).…”

Roles of three putative salmon louse (Lepeophtheirus salmonis) prostaglandin E2 synthases in physiology and host–parasite interactions

“…Heavy infestations can cause severe problems to the host, including wounding, secondary infections and osmotic disturbances [ 5 , 6 ]. Infestations of farmed fish have been difficult to manage due to the development of resistance against parasiticides [ 7 ], and the negative impact on wild salmonid fish through transmission of lice from farmed to wild fish stocks is a cause of environmental concern [ 8 – 10 ].…”