Parasites are a major welfare and production challenge in fish farming. Treatment methods vary widely in their welfare impacts on both target fish and non-target species.
Parasite management in aquaculture creates genuine welfare tensions between treating parasites that cause chronic welfare harm and the welfare impacts of treatment itself. Chemical treatments for sea lice — including hydrogen peroxide baths and pesticide treatments — cause stress to fish during administration and have variable efficacy depending on resistance levels. Mechanical treatments using water jets or temperature shocks cause acute stress. Biological control using cleaner fish has lower direct welfare impact on salmon but raises welfare concerns about the cleaner fish themselves, which are held at high density and may experience significant stress. No current sea lice management approach is welfare-free; the goal is minimizing net welfare harm while achieving parasite control adequate to prevent the chronic suffering caused by unmanaged infestation.