Classification of Post-Delousing Mortality in Farmed Atlantic Salmon: A Case Study of Standardised Causal...

Area of Science:

• Aquaculture
• Fish health
• Veterinary pathology

Background:

• Mechanical delousing is a common practice to control sea lice in farmed Atlantic salmon.
• This procedure can cause stress and injury to fish.
• Accurate mortality cause determination is crucial for fish welfare and farm management.

Purpose of the Study:

• To investigate the cause-specific, fish-level mortality of farmed Atlantic salmon after mechanical delousing.
• To determine the duration of pathological effects and mortality following the procedure.
• To compare farmer-recorded mortality causes with necropsy findings.

Main Methods:

• Study of three Atlantic salmon populations across two marine sites.
• Data collection at four time points: 1 day before to 13 days after mechanical delousing.
• Necropsy of 453 dead fish to determine cause-specific mortality through macroscopic pathology and a uniform code system.

Main Results:

• Mechanical delousing can induce mortality and pathological changes up to 13 days post-procedure.
• Cerebral haemorrhages were identified in approximately 25% of peracute mortalities.
• Farmer-recorded mortality causes significantly underestimated injury/trauma compared to necropsy findings.

Conclusions:

• Mechanical delousing has prolonged negative impacts on Atlantic salmon health and survival.
• Current farmer reporting practices may not fully capture the extent of delousing-induced trauma.
• Revised guidelines for recording mortality causes are needed to improve data accuracy and fish welfare assessments.