Net Pen Aquaculture: Welfare Considerations for Salmon and Trout

Net Pen (Cage) Aquaculture: Welfare Science and Practice

Net pen aquaculture — large floating cages in sea lochs, fjords, and coastal waters — is the dominant production method for Atlantic salmon globally, accounting for the majority of the 2.6 million tonnes of farmed salmon produced annually. Norway leads production, followed by Chile, the UK (primarily Scotland), and Canada. The welfare of fish in net pens is influenced by a complex array of factors including stocking density, water quality, sea lice management, predator interactions, and weather-related stress events. Understanding these factors is essential for welfare improvement in this high-volume sector.

Net Pen Environment: Welfare Opportunities and Challenges

Advantages Over Land-Based Systems

Generally lower stocking densities than intensive land-based RAS
Natural water quality management (tidal exchange)
Natural light cycles
Space for normal schooling behaviour

Welfare Challenges

No escape from adverse environmental conditions (temperature extremes, harmful algal blooms, jellyfish)
Exposure to sea lice (Lepeophtheirus salmonis) — a major welfare and production problem
Predator stress (seals, sea birds, cetaceans)
Net fouling reduces water flow and oxygen availability
Crowding stress during harvest operations
Disease transmission within densely populated cages

Sea Lice: The Primary Welfare Challenge

Sea lice are ectoparasitic copepods that feed on the skin, mucus, and blood of salmon. They are the leading welfare and economic problem in sea-cage salmon production worldwide:

Heavy infestations cause open wounds around the head and dorsal region
Lesions become infected; secondary bacterial infections can be fatal
Pain from skin damage is well-supported by evidence — physiological and behavioural responses consistent with nociception
Treatment methods (thermolicer, hydrolicer, freshwater bathing, cleaner fish, medicinal treatments) each carry their own welfare costs

Treatment Welfare Impacts

Thermolicer (hot water treatment): Thermal shock kills lice but causes scale loss, fin damage, and mortality in treated fish if water temperature is too high
Hydrolicer: Pressurised water removes lice; crowding and handling stress; scale loss
Freshwater bathing: Osmotic shock kills lice; can stress fish if duration or flow is incorrect
Cleaner fish (wrasse, lumpsuckers): Natural predators of lice; themselves require welfare consideration — mortality rates in cleaner fish are very high in many operations

Stocking Density and Behaviour

Scottish regulations specify maximum 25 kg/m³ for Atlantic salmon. Evidence suggests lower densities (15–20 kg/m³) produce better welfare outcomes:

Space to school naturally and establish dominance hierarchies with less aggression
Reduced transmission of sea lice and infectious pathogens
Better oxygen availability across the pen
Reduced injury from crowding during operations

Slaughter and Harvest Welfare

Net pen harvest involves crowding fish to one end of the cage using a collecting net — a highly stressful process. Best practice:

Minimise time between crowding and stunning
Use electrical stunners calibrated for salmon (commercially available RSW-based stunning systems)
Monitor stunning efficacy — check for loss of opercular movement, tonic-clonic spasms, absence of righting reflex
Rapid bleeding after stunning

Welfare Monitoring in Net Pens

Daily mortality counts — sudden spikes trigger investigation
Sea lice counting (weekly minimum; regulatory thresholds trigger compulsory treatment)
Appetite and feeding behaviour (underwater cameras increasingly used)
Behavioural observation: surface behaviour, schooling patterns, response to feeding
Harvest welfare assessment: record stunning efficacy, crowding mortality, processing line injury