Rainbow trout (Oncorhynchus mykiss) is one of the most widely farmed salmonids globally. Welfare across the production cycle — from rearing density to slaughter — determines quality of life for billions of fish.
Rainbow trout welfare science is more advanced than for many farmed fish species, largely because trout were early subjects of fish pain research. The demonstration that trout respond to noxious stimuli in ways consistent with pain experience — including learned avoidance, analgesic-responsive behavior, and trade-offs that suggest subjective experience — provided a foundation for welfare standards. European welfare regulations increasingly apply to farmed trout, requiring effective stunning before slaughter. Crowding remains the primary chronic welfare challenge: high-density raceways and cages prevent natural spatial distribution, cause competitive feeding stress, and increase fin erosion from contact injuries. Genetic improvement for disease resistance and selective breeding for welfare-relevant traits such as fin integrity and stress response are important long-term tools.