Rainbow trout are the most widely farmed freshwater salmonid globally. Their welfare science is more developed than for most farmed fish, creating an opportunity for evidence-based standard setting.
Rainbow trout welfare science represents a model for other farmed fish species. Validated welfare indicators now exist for both environmental and fish-based measures. The Trout Farm Welfare Assessment Protocol allows systematic welfare monitoring that enables benchmarking and improvement. Stocking density is the primary modifiable variable affecting welfare: above 30-40 kg/m3, crowding stress, fin damage, and behavioral restriction become significant. Slaughter welfare is improving as percussive and electrical stunning systems replace live CO2 or cold water stunning. Antibiotic reduction through improved vaccination protocols and biosecurity has reduced treatment stress. Rainbow trout welfare is where salmon welfare was 15 years ago — advancing, but with much still to achieve.