Atlantic salmon (Salmo salar) are neurologically complex fish with sophisticated sensory systems. Evidence of pain-related responses, stress physiology, and learning ability in salmon strongly supports sentience. Norway and Scotland have legally recognised farmed fish as sentient, and welfare standards for salmon farming are among the most developed in aquaculture.
Parasitic copepods (Lepeophtheirus salmonis and Caligus spp.) are the most pressing welfare and production challenge in salmon farming. Heavy infestations cause tissue damage, haemorrhage, and osmoregulatory disruption. Treatments themselves — including hydrogen peroxide baths, thermolicer (warm water) treatments, and medicinal baths — cause acute stress and mortality. The treatment burden on welfare is substantial.
High stocking densities are associated with elevated cortisol, aggression, fin damage, and reduced feeding motivation. Norwegian regulations cap density at 25 kg/m³, with evidence that lower densities significantly improve welfare outcomes. Crowding for treatments (lice inspection, grading, vaccination) creates acute stress events that compound the effects of routine high density.
Gill disease (amoebic gill disease, complex gill disorder) is a major welfare and mortality cause. Gill pathology impairs respiration and causes chronic suffering. Environmental factors (jellyfish, algae blooms, poor water quality) interact with infectious agents to drive gill health problems in sea cage conditions.
Cataracts are extremely common in farmed Atlantic salmon (up to 90% prevalence in some populations), associated with nutritional deficiencies (histidine). Fin damage from conspecific aggression and equipment contact impairs welfare. Spinal deformities affect a proportion of fish and are associated with pain and reduced swimming ability.
The OptiMel and SWIM (Salmon Welfare Index Metric) welfare assessment tools provide standardised on-farm welfare measurement. Key indicators include: mortality rates, sea lice counts, fin damage scores, gill health scoring, swim bladder inflation success, cataract prevalence, and wound scores. Operational welfare indicators (OWIs) are being integrated into regulatory frameworks in Norway and Scotland.
Current best practice requires effective stunning prior to slaughter, typically electrical stunning or percussive stunning at the point of harvest. CO2 narcosis (common in some operations) is associated with significant welfare costs and is being phased out in favour of more effective alternatives. The Scottish Salmon Producers Organisation and Norwegian industry have made commitments to improved slaughter welfare.