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Aquaculture Environment Interactions

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AEI 13:377-388 (2021)  -  DOI: https://doi.org/10.3354/aei00413

System-specific salmon louse infestation thresholds for salmon farms to minimize impacts on wild sea trout populations

Samuel Shephard*, Patrick Gargan

Inland Fisheries Ireland, 3044 Lake Drive, CityWest Business Campus, Dublin 24 Y265, Ireland
*Corresponding author:

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.


KEY WORDS: Salmon louse thresholds · Salmo trutta · Salmon aquaculture · West of Ireland rivers


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Cite this article as: Shephard S, Gargan P (2021) System-specific salmon louse infestation thresholds for salmon farms to minimize impacts on wild sea trout populations. Aquacult Environ Interact 13:377-388. https://doi.org/10.3354/aei00413

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