Rainbow trout are one of the most widely farmed fish globally. Stocking density is the single most studied welfare variable in trout aquaculture and has significant effects on behavior, physiology, and health.
Rainbow trout welfare is closely tied to stocking density, water quality, and handling practices. At densities above welfare thresholds, fish cannot swim freely, fin damage from contact with tank walls and other fish accumulates, and oxygen depletion creates chronic physiological stress. Trout are particularly sensitive to handling stress — netting, grading, and transfer events cause acute cortisol spikes that take hours to resolve. Anesthesia (using MS-222 or clove oil) during handling significantly reduces stress but is rarely used commercially due to cost and logistics. Slaughter practices are improving: percussive stunning (a sharp blow to the head) renders fish immediately insensible and is considered more humane than CO2 or asphyxiation. ASC certification requires percussive stunning for salmonids including rainbow trout.