Sea lice treatments cause significant welfare harm to farmed salmon. Developing less harmful control methods is one of the most important welfare priorities in salmon farming.
The sea lice welfare problem is a genuine double bind: untreated lice cause chronic tissue damage and stress to salmon, while available treatments cause acute welfare harm during their application. Crowding fish for bath treatments creates oxygen depletion and panic behaviour. Mechanical treatments using high-pressure water or heat cause physical injury — scale loss, bruising, and haemorrhage are documented outcomes. Cleaner fish deployed to eat lice suffer high mortality from starvation, inadequate habitat, salmon aggression, and disease. Genetic selection for sea lice resistance offers the most welfare-positive long-term solution — fish that naturally resist lice require fewer treatments. Vaccine development for lice resistance is ongoing. Closed containment systems (recirculating aquaculture systems) eliminate sea lice entirely but have higher energy costs and welfare risks from technical failures.