Sea trout (migratory brown trout) undertake estuarine and coastal migrations between freshwater spawning streams and marine feeding grounds. Their welfare is affected by sea lice from salmon farms and river barriers.
Sea trout welfare is affected by the dual stressors of sea lice infestation during coastal migration phases and river barrier obstruction during spawning migration. Sea lice infestations cause direct tissue damage from feeding and osmoregulatory compromise that can be fatal at high infestation levels. Wild sea trout in areas adjacent to salmon farms show higher lice burdens than those in areas without farms, and population declines in several Scottish and Irish sea trout fisheries have been attributed at least partly to farm-associated lice. River passage obstruction prevents spawning migration completion, representing welfare harm through the frustration of primary reproductive behaviour and practical denial of reproductive success. Effective sea trout conservation requires addressing both marine (lice management near farms) and freshwater (barrier removal or effective passage) components of the life cycle.