Atlantic salmon (Salmo salar) is the most economically significant farmed marine fish species globally, with Norway, Chile, Scotland, and Canada as major producers. The welfare challenges of salmon farming are extensive and have attracted significant regulatory and scientific attention.
Global Atlantic salmon aquaculture produces approximately 2.5 million tonnes annually. The welfare of these fish—hundreds of millions of individuals capable of experiencing stress and pain—represents a major animal welfare frontier. Norway has the most developed regulatory framework for salmon welfare; other producing nations are at varying stages of regulatory development.
Sea lice (Lepeophtheirus salmonis, Caligus elongatus) are ectoparasitic copepods causing skin erosion, haemorrhage, secondary infection, and chronic stress in farmed salmon. Heavy infestations cause tissue damage to the head capsule and flanks, exposing underlying tissue. Lice management requires a multi-pronged integrated pest management approach: biological control using cleaner fish (wrasse, lumpfish); mechanical removal (Hydrolicer, Thermolicer—water jets or warm water treatment); and chemical treatments (medicinal bath treatments, in-feed medications). Each treatment method carries welfare costs: thermal treatments can cause thermal shock; mechanical treatments cause handling stress; chemical treatments may cause skin irritation.
Amoebic gill disease (AGD, caused by Neoparamoeba perurans) causes progressive gill pathology, compromising oxygen uptake and causing chronic welfare compromise. Salmon alphavirus (SAV) causing pancreas disease (PD) and heart and skeletal muscle inflammation (HSMI) are widespread viral diseases. Bacterial kidney disease (BKD), vibriosis, and furunculosis cause variable but significant welfare impacts. The disease burden in intensive salmon farming represents significant welfare cost that management improvements and better vaccines can reduce.
Atlantic salmon experience multiple handling events throughout their production cycle: vaccination (typically at smolt stage), grading, well boat transfers, crowding for treatments, and harvest. Each event causes acute stress measurable through cortisol, haematological changes, and behavioural indicators. Minimising unnecessary handling, using appropriate equipment, conducting procedures in cooler water to reduce metabolic demand, and ensuring adequate recovery time between handling events reduces cumulative stress burden.
Optimal stocking densities for Atlantic salmon welfare are debated, with regulatory limits in Norway set at 25 kg/mÂł. Evidence suggests that some behavioural welfare indicators deteriorate at densities above 20-22 kg/mÂł. Salmon require schooling conditions but face chronic competition and social stress at high densities. Monitoring dorsal fin damage, individual fish condition, and feeding behaviour provides welfare density indicators beyond simple weight-per-volume measurements.
The transition from freshwater parr to seawater smolt involves profound physiological changes and represents a vulnerable welfare period. Premature transfer before physiological readiness causes osmoregulatory stress and disease. Environmental manipulation to trigger smoltification (photoperiod changes) aligns physiological readiness with farm transfer schedules. Post-transfer monitoring for smoltification failure identifies at-risk fish before significant welfare deterioration.