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🐟 Farmed Fish Welfare Indicators

The Challenge of Fish Welfare Assessment

Assessing welfare in fish presents unique challenges compared to terrestrial animals:

• Fish are underwater — direct observation is limited
• Fish mask illness and injury (prey species behaviour) — signs are subtle
• Very large numbers make individual assessment impractical
• Understanding of fish sentience and pain was limited until recently

Despite these challenges, robust welfare assessment frameworks have been developed, particularly for Atlantic salmon, seabass, seabream, tilapia, and rainbow trout.

Categories of Welfare Indicators

Operational Welfare Indicators (OWIs)

Practical measures that can be applied on-farm without specialist equipment:

• Mortality rate — daily and cumulative; sudden increases signal welfare problems
• Feed intake — reduced intake is an early stress/disease indicator
• Abnormal swimming behaviour — surface swimming, spiral swimming, loss of balance
• External injuries — fin damage, wounds, scale loss, eye damage
• Gill score — assessment of gill colour, shape, and presence of lesions
• Eye haemorrhage — redness or bleeding in/around the eye
• Skin condition — loss of mucus, discolouration, scale damage

Physiological Indicators

• Plasma cortisol — primary acute stress response measure; requires blood sampling
• Glucose and lactate — indicators of metabolic stress
• Plasma chloride — indicator of ionoregulatory stress (relevant in seawater species)
• Heart rate — monitoring possible via biologgers in research contexts

Behavioural Indicators

• Crowding behaviour — fish packed at the surface or corners indicate hypoxia or stress
• Startle response — appropriate response to sudden stimuli indicates neurological health
• Avoidance behaviour — aversion of harmful stimuli indicates nociceptive capacity
• Aggression and fin damage — indicators of social stress and high density
• Abnormal swimming — circling, jumping, loss of schooling behaviour

Welfare Assessment Frameworks

SWIMWAY (EU-funded)

The SWIMWAY project developed operational welfare indicators for Atlantic salmon, European seabass, and gilthead seabream. Indicators were validated across commercial farms and are increasingly used in certification and regulatory contexts.

Salmon Welfare Index

Used in Norwegian salmon aquaculture, combines mortality rates, wound scoring, gill health, treatment records, and other measures into a composite welfare index.

ASC Welfare Requirements

Aquaculture Stewardship Council certification increasingly requires welfare monitoring plans and reporting of welfare outcome indicators alongside environmental standards.

Species-Specific Indicators

Atlantic Salmon

• Cataract score (eye opacity) — indicator of dietary deficiency and welfare compromise
• Sea lice burden — counts of Lepeophtheirus salmonis per fish
• Spinal deformity score
• External wound score

Rainbow Trout

• Fin erosion score — particularly dorsal fin
• Dark colouration — indicator of stress
• Skin discolouration and ulceration