Optimising delousing strategies: developing best practice recommendations for maximal efficacy and positive welfare - Final Report | Optimising delousing strategies: developing best practice recommendations for maximal efficacy and positive welfare - Final Report

The salmon aquaculture industry currently uses a range of strategies to combat salmon louse epidemics, however acute delousing methods are limited due to poor efficacy, poor welfare and environmental impacts, or potential resistance to medicinal treatments. The three delousing methods most commonly used (thermal, mechanical, and freshwater), used alone or in combination, have not been systematically studied or compared so that the use of these strategies can be properly optimised. This project generated experimental data on delousing efficacy and welfare impacts of single and combined treatments using these three methods, in a tank setting, and found that single freshwater bath treatments and freshwater baths combined with other treatments had the greatest delousing efficacy. The welfare of fish declined in all treatment groups throughout the experiments, and statistical analysis revealed no consistent difference between procedural controls and the individual or combined treatments. The findings suggest that any treatment applied alone or in combination has a negative impact on fish welfare but the handling rather than the treatment itself is the major contributor to poor outcomes. However, the treatments involving a 34 °C warm water bath resulted in the worst welfare outcomes, the lowest condition factors, and lowest growth rates, which suggests that the 34 °C warm water bath is particularly detrimental to fish welfare. The project also investigated the welfare impact of delousing at commercial sites using data acquired from sites over a production cycle. While the management strategies depended more on production goals and regulatory actions, the data also showed that mortality increases after all handling events and delousing operations. The greatest mortality increases were associated with well boat operations including thermal and mechanical treatments. Lastly, the temperature and low salinity tolerance levels, and the ability for these tolerance levels to shift upon exposure, were investigated in larval salmon lice. Three strains were investigated and while their temperature tolerance levels were similar, significant differences in salinity tolerance levels between strains were found. A more detailed investigation at the epigenetic level (specifically targeting 5-mC DNA epimethylation) revealed significant differences between groups, and indicated the potential for salinity tolerance markers to be found in older louse stages. This project provided comparative data on delousing methods can be used to generate knowledge on efficacy and welfare impact on single and combination methods, with various factors taken into account, which should be further verified with procedures at commercial scale.

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