Impact of parasites on salmon recruitment in the Northeast Atlantic Ocean

Parasites may have large effects on host population dynamics, marine fisheries and conservation, but a clear elucidation of their impact is limited by a lack of ecosystem-scale experimental data. We conducted a meta-analysis of replicated manipulative field experiments concerning the influence of parasitism by crustaceans on the marine survival of Atlantic salmon (Salmo salar L.). The data include 24 trials in which tagged smolts (totalling 283 347 fish; 1996-2008) were released as paired control and parasiticide-treated groups into 10 areas of Ireland and Norway. All experimental fish were infectionfree when released into freshwater, and a proportion of each group was recovered as adult recruits returning to coastal waters 1 or more years later. Treatment had a significant positive effect on survival to recruitment, with an overall effect size (odds ratio) of 1.29 that corresponds to an estimated loss of 39 per cent (95% CI: 18-55%) of adult salmon recruitment. The parasitic crustaceans were probably acquired during early marine migration in areas that host large aquaculture populations of domesticated salmon, which elevate local abundances of ectoparasitic copepods-particularly Lepeophtheirus salmonis. These results provide experimental evidence from a large marine ecosystem that parasites can have large impacts on fish recruitment, fisheries and conservation

The ecological profile of Atlantic salmon escapees entering a river throughout an entire season: diverse in escape history and genetic background, but frequently virus-infected

In 2014, 129 farmed salmon escapees captured in an upstream-migration trap located in the river Etne, western Norway, were investigated for viral infections, age at escape, size, and genetic composition. The frequency of escapees positive for salmonid alphavirus (SAV), piscine orthoreovirus (PRV), and infectious salmon anaemia virus (ISAV) was 12, 79, and <1%, respectively. Fatty acid analysis demonstrated that the individuals had escaped from farms at different stages of the production cycle, although the majority had probably escaped from farms in the same year as their capture in the river. Genetic analyses demonstrated that the escapees originated from multiple farms. This was also supported by the distribution of fish size and timing of entry into the river. A combination of genetic, fatty acid and viral infection analyses showed that in the river Etne in 2014: (i) most of the fish entering the river were infected with one or more viruses, (ii) the majority of them had escaped in the same year that they entered the river, (ii) they originated from multiple farm sources, and (iv) two of the identified genetic groups likely originated from two recent and distinct escape events. This is the first study to integrate results from multiple analytical methods in order to reveal the ecological and genetic diversity of escaped farmed fish entering a river with native salmon population throughout an entire season