Disease is the leading cause of fish mortality and welfare compromise in aquaculture, and the treatments used — chemicals, vaccines, and antibiotics — all carry their own welfare costs that must be weighed carefully against the diseases they address.
Disease management in aquaculture requires a genuine welfare cost-benefit analysis at every decision point. Leaving disease untreated allows fish to suffer and die from infection; treating with crowding, chemicals, or antibiotics causes its own welfare harm. The most successful welfare outcomes come from prevention rather than treatment — through vaccination, site management, selective breeding, and density reduction. Norway's dramatic reduction in antibiotic use through vaccination is a model showing that it is possible to produce large volumes of farmed fish while dramatically reducing both disease burden and treatment-related welfare costs simultaneously.