Impact of parasites on salmon recruitment in the Northeast Atlantic Ocean

Abstract

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.

Lessons from sea louse and salmon epidemiology

“…This means that, by necessity, manipulative studies are rare compared with correlative studies. Even when manipulative studies have been possible [3,69], debate and controversy have persisted [70,71]. Another major challenge is the fact that a negative statistical result does not necessarily translate to a negative biological conclusion.…”

“…Half of the fish were treated with a parasiticide that provided temporary protection from infection by sea lice in early marine life, while the other half received a control treatment. A meta-analysis of 24 paired-release experiments (totalling 283 347 fish) showed that lack of treatment (a surrogate of parasite exposure) decreased survival of returning adults by 39% [3]. Although there was variation among trials in effectiveness of treatments, with some trials showing no evidence of effect and others showing a strong effect, collective examination revealed clear evidence of a positive effect of the parasiticide treatment on salmon recruitment (figure 5d).…”

“…The 1.5-to 2-year seawater production cycles for farmed salmon add additional variability in host density in coastal regions. [3,59] explain underlying mechanisms [37] project behaviours of systems [20,28,43] predict outcomes of interventions [15,27] raise questions and generate hypotheses [66] propose models [49] present definition functions and goals timescale find possible explanations for patterns and phenomena [18] generate hypotheses [4] specify new data needs [27] [20] regression models -statistical -descriptive/correlational -can include spatial effects GLM, GLMM, random effects, logistic regression -identifying epidemiological factors effecting sea lice abundance on salmon farms [21] -associations between aquaculture and sea louse infections on sea trout [22] survival functions and hazard functions -statistical -descriptive/correlational survival analysis -impacts of sea lice on salmon survival in the NE Atlantic [3] -effects of salinity on sea louse survival on juvenile salmon [23] stochastic processes -discrete-time or continuous-time dynamics [29] rstb.royalsocietypublishing.org Phil. Trans.…”

“…(d ) A random effect meta-analysis of chemotherapeutant treatment on the likelihood of adult Atlantic salmon returning to spawn. Lines are the 95% CIs of the effect size for each trial with the closed diamond representing the overall meta-analytic effect across all studies [3].…”

“…At the same time, the expansion of industrial aquaculture over the past 50 years has created new pathways and reservoirs for pathogens [3,4]. Farmed fish are often exposed to pathogens from sympatric wild fish populations, but are sheltered from many of the natural processes (e.g.…”