Sea lice management in salmon aquaculture involves treatments that cause significant acute stress to farmed fish, creating complex welfare tradeoffs between lice burden and treatment-associated harm.
Sea lice management creates a genuine welfare dilemma for farmed salmon. Untreated lice burdens cause chronic tissue damage, open wounds, osmotic stress, and can be fatal at high levels. However, treatment methods used to reduce lice numbers also cause significant acute welfare impacts. Hydrogen peroxide bath treatments cause disorientation, surface-breaking behaviour, and mortality, particularly at elevated water temperatures. Thermal treatments using warm water can cause thermal shock. Mechanical delousing systems physically abrade fish and cause scale loss and injury. Cleaner fish used as biological control agents are often raised in conditions that fail to meet their own welfare needs and experience high mortality. The best currently available option — rigorous monitoring, early intervention at low lice thresholds, and use of the least aversive effective treatment — reduces but does not eliminate welfare harm.