Rainbow trout are one of the world's most farmed fish, raised in flow-through raceways where water quality, stocking density, and handling practices critically determine their welfare.
Rainbow trout welfare research has advanced substantially in recent decades. Studies show trout can learn to avoid areas where they received noxious stimuli and show physiological stress responses that parallel those of vertebrates whose sentience is not questioned. In raceway culture, chronic stress from overcrowding suppresses immune function and increases susceptibility to disease. Poor water quality - low oxygen, high ammonia, or elevated temperature - causes acute physiological stress. Handling during grading, treatment, and harvest causes significant acute stress that humane stunning practices can substantially mitigate.