Abstract
Salmon lice Lepeophtheirus salmonis from aquaculture can cause negative impacts on sea trout Salmo trutta and other wild salmonids. Long-term records from 5 Irish rivers were used to explore relationships between annual sea trout runs and estimated total number of lice on nearby salmon farms. It was hypothesised that local environmental conditions may result in system-specific differences in realised louse pressure on sea trout. Louse count was thus tested as an absolute number and as a relative pressure, i.e. standardised by farm. When the standardised total number of mobile lice on a given salmon farm in April was above baseline level (50th percentile of observed annual values on that farm), there was a high probability of a below-average sea trout run in the local river. Absolute louse counts did not show an important effect on runs. This finding suggests that salmon farm louse production in spring can have a strong system-specific regulating effect on wild sea trout populations. Total number of lice on a farm was most strongly driven by changes in individual infestation rate, with a lesser effect of stocking density. Thresholds for number of mobile lice per farmed salmon required to maintain total louse count below the baseline varied with stocking density and among systems: greater density required lower infestation rate. Regulations relying on a generic louse threshold to trigger treatment are not sufficient to protect sea trout populations—stocking density and site characteristics must be considered to evaluate system-specific infestation pressure and impacts on wild salmonids.
Highlights
Salmon lice Lepeophtheirus salmonis impose widely reported negative impacts on wild sea trout Salmo trutta populations in areas with Atlantic salmon Salmo salar aquaculture (Taranger et al 2015, Thorstad et al 2015)
The simplest tested model to include standardised louse pressure as a covariate (Model 12) had the lowest Watanabe Akaike information criterion (WAIC) overall, with the corresponding model for absolute numbers having close to lowest WAIC in that set (Table 2)
Annual returns of sea trout to the study rivers fluctuated around average when standardised louse infestation was below a system-specific baseline level (< 50th percentile of observed annual values) at local farm sites
Summary
Salmon lice Lepeophtheirus salmonis impose widely reported negative impacts on wild sea trout Salmo trutta populations in areas with Atlantic salmon Salmo salar aquaculture (Taranger et al 2015, Thorstad et al 2015). Sea trout are known to inhabit inshore areas for considerable periods, often in the vicinity of salmon farms (Pemberton 1976, Thorstad et al 2007, Middlemas et al 2009). This behaviour imposes sustained vulnerability to infection, with potential for trout to encounter lice of farm origin over an extended period (Thorstad et al 2015). The heaviest infestations and greatest variation in infestation are seen on sea trout captured close to salmon
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