Rainbow trout are one of the most widely farmed fish species globally. Their welfare is significantly affected by stocking density, water oxygen levels, and management of chronic stress in intensive systems.
Rainbow trout are among the most studied farmed fish in terms of welfare science. Evidence firmly establishes that they can experience pain, fear, and chronic stress. High stocking densities — prevalent in intensive systems for economic reasons — cause elevated cortisol levels, increased aggression, fin erosion, and suppressed immune function that increases disease susceptibility. Poor water quality, particularly low dissolved oxygen and high ammonia, causes acute distress and is a significant welfare risk in poorly managed systems. Feed deprivation before harvest — used to clear the gut — causes welfare harm and should be minimized. Pre-slaughter stunning is increasingly used and required in some jurisdictions, representing a significant welfare improvement over practices where fish are killed without stunning. Trout welfare certification schemes are emerging and provide a useful framework for producers committed to improvement.