New case study: thermal delousing - effectiveness and fish welfare in Scottish salmon farming

Evaluating the efficacy and welfare implications of thermal delousing under commercial conditions

Sea lice are a challenge for salmon farming, causing stress, physical damage, and increasing the risk of disease. Traditional control methods include chemical treatments and biological approaches, but these can have environmental impacts.

Thermal delousing - exposing fish to heated water for a short period to remove sea lice - has gained traction as a non-medicinal, environmentally friendly alternative. Despite the benefits, concerns about the potential welfare implications of thermal delousing have been raised.

This project assessed thermal delousing under real farm conditions in Scotland. Valued at over£290k, the partners were Mowi Scotland, the University of the West of Scotland, Scottish Sea Farms, Bakkafrost Scotland and the University of Aberdeen.

Researchers analysed historical farm data, conducted field sampling of over 1,100 fish at multiple treatment stages, and performed laboratory tests on health indicators. Key findings:

• Effectiveness: thermal delousing reduced lice burdens, confirming its value as part of an integrated pest management strategy.
• Mortality: a slight increase in mortality was observed post-treatment (average 0.03% over 14 days), varying by equipment and fish size. Smaller fish showed a marginally higher risk.
• Health impacts: short-term changes were detected in blood chemistry and biomarkers, indicating temporary stress and mild gill and tissue irritation. These effects resolved within seven days, with no evidence of long-term harm.
• Welfare: while thermal delousing causes brief discomfort, its overall welfare impact is manageable compared to chemical alternatives, especially when best practices are followed.

Researchers reviewed existing mitigation measures and developed recommendations to optimise thermal delousing, including careful handling, monitoring, and seasonal adjustments.

The project showed that thermal delousing is an effective, environmentally friendly method for controlling sea lice in Scottish aquaculture. Although it causes short-term stress, impacts are reversible, and welfare can be safeguarded through proper protocols. Continued research and refinement will further improve outcomes, supporting sustainable fish farming and reducing reliance on chemical treatments.

The full title of this project is ‘Efficacy & welfare implications of thermal delousing in Scottish salmonid aquaculture’.

Read the case study