Improving welfare for the world's most widely farmed salmonid fish
Rainbow trout (Oncorhynchus mykiss) is the most widely farmed salmonid globally, with production exceeding 800,000 tonnes annually. Major producing countries include Iran, Turkey, Norway, Italy, Chile, and the USA. Trout are farmed in freshwater raceways, recirculating systems, and net pens. As sentient fish with well-documented pain responses, their welfare across these production systems is a significant concern.
Stocking Density: High densities cause chronic stress, fin damage, reduced growth, increased disease, and abnormal aggression. Research shows welfare-correlated impacts above 40 kg/m³; many operations stock at 70-100 kg/m³.
Water Quality: Low oxygen, high CO2, elevated ammonia, and temperature fluctuations cause physiological stress and tissue damage. Trout are oxygen-sensitive; dissolved oxygen below 7 mg/L causes stress responses.
Handling & Grading: Air exposure during grading causes hypoxia and physical injury. Anesthetic use during handling significantly reduces stress hormones.
Disease: Bacterial kidney disease, furunculosis, and sea lice (in sea-going populations) cause pain and mortality. Crowding stress increases disease susceptibility.
Trout slaughter methods vary dramatically in welfare outcomes. Carbon dioxide stunning is widely used but causes aversion, prolonged unconsciousness onset, and potential pain. Percussive stunning followed by spiking (ikejime) is the most welfare-positive method. Electrical stunning can be effective if properly calibrated. The RSPCA Assured standard and GlobalG.A.P. aquaculture certifications specify humane slaughter requirements.
The RSPCA Assured (UK) and Label Rouge (France) certification schemes set minimum welfare standards for trout including stocking density limits, water quality requirements, enrichment, and slaughter methods. The ASC (Aquaculture Stewardship Council) standard incorporates welfare indicators. Fish welfare science is advancing rapidly, and standards are expected to tighten in coming years as evidence accumulates.