Rainbow trout are the most commonly farmed freshwater fish in Europe, with welfare challenges from crowding, disease, and handling in intensive production systems.
Rainbow trout in overcrowded raceways show reduced growth, increased aggression, and immune suppression reflecting chronic welfare compromise. Fin damage from aggression is a visible welfare indicator of inappropriate stocking density. Handling during grading causes acute stress measurable through cortisol elevation. Infectious disease outbreaks cause systemic illness with pain, lethargy, and mortality. Pre-slaughter electrical or percussive stunning is used in some facilities but not universally, leaving carbon dioxide narcosis the dominant method with its associated aversive characteristics.