Rainbow trout are one of the most commonly farmed salmonid species, and stocking density is a primary determinant of their welfare in intensive raceway and cage systems.
Stocking density is perhaps the single most controllable factor affecting trout welfare in intensive systems. At high densities, trout cannot express normal swimming and resting behaviours, experience chronic social stress from inability to escape aggressive conspecifics, and show elevated cortisol indicating physiological distress. Fin and body damage from aggressive interactions cause direct pain and increase infection risk. The evidence base now clearly supports substantially lower stocking densities than legal maxima, and welfare-conscious producers are adopting these lower targets voluntarily through certification schemes.