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 infection-free 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.

Figure 1.

Map of the study regions showing rivers where salmon smolts were released and salmon farm locations (black circles). Interpretation of farming activities over the course of the study period should be made with caution, because the distributions of farms, stocking levels, parasite levels and management approaches have varied over the years.

Figure 2.

Forest plot displaying a random effect meta-analysis of the effect of treatment on the likelihood of a one sea-winter (1SW) adult salmon returning. Horizontal lines represent the 95% CIs of the effect size in each trial, and the relative sizes of solid squares reflect the percentage weighting (based on s.e. of effect sizes) of each trial in the meta-analysis (range 0.42–8.12%). The open diamond shows the overall meta-analytic effect across all studies, with its width corresponding to the 95% CI. Results are given by trial, identified by location (country and river of release of smolts), year (year when smolts were released) and ref. (the reference where the data were published).