A bioassay for sea lice Lepeophtheirus salmonis sensitivity towards emamectin benzoate (EMB) was validated for field use. A probit regression model with natural responsiveness was used for the number of affected (moribund or dead) sea lice in bioassays involving different concentrations of EMB. Bioassay optimization included an evaluation of the inter-rater reliability of sea lice responsiveness to EMB and an evaluation of gender-related differences in susceptibility. Adoption of a set of bioassay response criteria improved the concordance (evaluated using the concordance correlation coefficient) between raters' assessments and the model estimation of EC50 values (the 'effective concentration' leading to a response of 50% of the lice not prone to natural response). An evaluation of gender-related differences in EMB susceptibility indicated that preadult stage female sea lice exhibited a significantly larger sensitivity towards EMB in 12 of 19 bioassays compared to preadult males. In order to evaluate sea lice sensitivity to EMB in eastern Canada, the intensive salmon farming area in the Bay of Fundy in southwestern New Brunswick was divided into 4 distinct regions based on industry health management practices and hydrographics. A total of 38 bioassays were completed from 2002 to 2005 using populations of preadult stage sea lice collected from Atlantic salmon Salmo salar farms within the 4 described regions. There was no significant overall effect of region or year on EC50 values; however, analysis of variance indicated a significant effect of time of year on EC50 values in 2002 and a potential effect in 2004 to 2005. Although the range of EC50 values obtained in this 3 yr study did not appear sufficient to affect current clinical success in the control of sea lice, the results suggest a seasonal- or temperature-associated variation in sensitivity to EMB. This will need to be considered if changes in EMB efficacy occur in the future.
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Infections with bacteria of the genus Pasteurella have increased in occurrence in Atlantic salmon (Salmo salar) farms in Norway since 2018. This increase coincides with increased use of non-medicinal treatments against the parasitic salmon louse, Lepeophtheirus salmonis, in the farms. Here, we analysed the statistical association between the use of non-medicinal delousing methods and pasteurellosis in salmon farming in western Norway, from 2018 to 2023.
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Department of Health Management and Centre for Veterinary Epidemiological Research, Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, Prince Edward Island, Canada.
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Salmon lice (Lepeophtheirus salmonis) are parasites on salmonid fish and a density-dependent constraint to the sustainable farming of salmonids in open net pens. To control the parasites, fish farmers in Norway are required to count the number of salmon lice in different developmental stages on a subset of the fish each week. Furthermore, they must ensure that the number of adult female lice per fish does not increase beyond a specified threshold level.
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Institute of Aquaculture, University of Stirling, Stirling, Scotland, UK.
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