Disentangling the role of salmon lice on the marine survival of Atlantic salmon (BaseLice)

Salmon lice is one of the biggest challenges for the sustainable salmonid aquaculture in Norway and internationally. This has become particularly evident the last few years as a consequence of the newly ratified management system, that divide the coast into 13 production zones where environmental indicators will decide whether the fish farming is sustainable. The first «environmental indicator» that has been implemented is the impact of salmon lice from fish farms on the marine survival of Atlantic salmon. In the project BaseLice the goal was to understand how salmon lice can impact the survival of Atlantic salmon. The overarching hypothesis was that the effect of salmon lice cannot be seen as an isolated effect on the survival of salmon, but rather a factor that interacts with other effects to impact the recruitment and ultimate return of salmon to rivers in Norway. Releasing treated salmon smolts into the wild The first method that we used was experimental release and adult recapture of manipulated groups of salmon post-smolts. We initiated a large tagging study involving 36 000 hatchery reared salmon post-smolts measured and individually tagged. Before release, half of the fish in each group was treated against salmon lice, while half was left untreated as a control. Within each treatment, half was given a secondary stressor low pH (acute environmental acidosis) and exposure to aluminum. This has been shown to have a negative effect on survival and growth of salmon and is a potential stressor in the rivers and fjords of Norway. In total, there were six releases, three in a year with low infestation pressure from fish farms (2015), and another three in a year with high infestation pressure from fish farms (2016). The results demonstrate that the effect of aluminum and pH treatment is small and non-significant in both years. In contrast, the effect of treatment against salmon lice is non-significant in the year with low infestation pressure, and high in year with high infestation pressure (~30% reduction in survival). In conclusion, it seems that the small sublethal effect on physiology due to low pH has no effect of the impact of salmon lice, but that salmon lice alone can strongly reduce the returning number of salmon. Scales reveals patterns of marine growth A classical method to evaluate the growth conditions at sea for salmon is to study their scales. These deposits rings and can tell how the individual fish has grown similar to how rings are counted and measured in trees. The growth during the early life of salmon are particularly important for the survival of salmon. A logical question was therefore whether salmon lice can effect the early life growth of salmon. To study this we photographed and measured the scales of 800 salmon from the Vosso river from 2003-2014, and compared fish that had been treated or not treated against salmon lice. The results indicate that salmon lice had a strong affect on growth during a period of general low survival, but no effect during a subsequent period of high survival. The period of low survival corresponded to a period on the west coast of Norway with low survival for many wild salmon populations. The study thus corroborates the hypothesis that the effect of salmon lice on wild fish is context dependent and is worse during periods of low marine survival. Salmon lice impacts life history traits in salmon Salmon can return to their spawning river after 1, 2 or several years at sea. The reason for staying at sea can be many, men in general maturing later is associated with larger size (and higher fecundity) but also lower likelihood of survival. Salmon lice can affect this life history trait by affecting both survival and growth. In the BaseLice project we demonstrate both theoretically and empirically how salmon lice can affect the life history traits of salmon. Salmon lice increases the age at return of salmon. However, the effect of this delayed age at spawning may be both negative or positive depending on the growth and survival at sea. However, during the last few years marine growth and survival has been generally poor, and the impacts of delayed age at spawning have generally been negative. In sum, these studies demonstrate that salmon lice can be an important factor in the population dynamics of wild Atlantic salmon. The results confirm the hypothesis that the effect of salmon lice is context dependent. In years where marine growth and survival is poor, the effect of salmon lice can be detrimental, while in years when conditions are good, salmon lice may not have a large effect. A sustainable long-term conservation of Atlantic salmon populations thus must consider that Atlantic salmon should also be able to sustain themselves during period of poor marine survival and growth.

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Marine survival is one of the most debated topics in ecology of salmonids. Perhaps the most important debate in recent years is the role of the parasitic crustacean Lepoptheirus salmonis on the marine survival of Atlantic salmon and the role of fish farming in this host-parasite system. In many publications, the effects of salmon lice on marine survival are reported as an average percent reduction in spawners. In biological systems however, the answer is seldom as straightforward. Mortality is often compounded, meaning that certain combinations of environmental factors may be lethal while the presence of single factors by themselves may not be critical. To date, the best method to studying the effect of salmon lice on the marine survival of salmon is field experiments with release of parallel groups of smolt treated and not treated with anti-parasitic agents and subsequent recapture of fish as adults (randomized control trials (RCT)). The result from a recent metaanalysis of such studies in Norway suggests that the effect of treating smolt against salmon lice is highly dependent on baseline risk. This suggest that the effect of salmon lice cannot be interpreted independent of other mortality factors and marine conditions. These results imply that conclusion from earlier studies on population effects of salmon lice may be invalid or at least imprecise, and needs to be fully understood. We propose a project with an international collaboration between epidemiologist, salmon biologist and marine biologists that aim at disentangling the role of salmon lice on the marine survival of Atlantic salmon based on historic dataset of randomized control trials, field experiments, individual growth analysis and theoretical modelling.