Rainbow trout are the most widely farmed salmonid in Europe. Their welfare in recirculating aquaculture systems and raceways is significantly affected by stocking density and the aggressive hierarchical interactions that high density provokes.
Trout welfare in aquaculture is compromised by the same fundamental tension as salmon welfare: economic pressure to maximise production per unit volume conflicts with what fish need for welfare. Trout in high-density conditions experience chronic stress from constant hierarchical conflict, reduced ability to avoid aggressive interactions, and compromised water quality. Fin erosion - visible on most farmed trout at commercial densities - is a direct welfare indicator that is normalised within the industry but represents ongoing tissue damage from fighting and abrasion. Reducing stocking density, improving tank design to provide more structural complexity and flow variation, and careful monitoring of welfare indicators are achievable improvements.