FHF prosjekt 901514. Effekter av behandling mot lakselus, amøbegjellesykdom (AGD), og notrengjøring på gjellehelse hos laks i oppdrett

The project investigated how common production-related stressors, including thermal delousing, freshwater treatment for Amoebic Gill Disease (AGD), and net cleaning, affect gill health in Atlantic salmon. Particular attention was given to gill pathology, pathogen load, immune responses, and changes in gill microbiota. Experimental studies showed that thermal delousing alone did not cause mortality or significant gill damage in pathogen-free salmon. However, field observations indicated that mortality risk increased when fish were simultaneously infected with certain gill pathogens (Paramoeba theridion, Candidatus Branchiomonas cysticola) or viruses such as SAV, PMCV, and PRV1. Pre-treatment pathogen screening is therefore recommended before thermal delousing. Freshwater treatment proved effective against AGD, significantly reducing Paramoeba perurans infection levels as confirmed by molecular, histological, and clinical assessments. Mortality associated with freshwater treatment was primarily linked to the presence of other diseases or compromised gill health. The results highlight the importance of assessing fish health status prior to treatment. Net cleaning was found to increase particle concentrations and turbidity within sea cages and released material containing various pathogens and parasites. Although only one study site experienced elevated mortality following net cleaning, the results suggest that cleaning activities may contribute to pathogen dispersal between cages and potentially between farms. Temporary reductions in oxygen levels were also observed during cleaning operations. All treatments, including delousing, AGD treatment, and net cleaning, significantly altered the composition of the gill microbiota. While the functional implications of these changes remain unclear, no major deterioration of gill health was observed following treatment under field conditions. Overall, the study demonstrates that production-related treatments can influence fish health directly and indirectly through interactions with pathogens, environmental conditions, and microbial communities. The findings provide important knowledge for improving fish welfare and developing safer management strategies in Norwegian salmon aquaculture.