Effect of water temperature and exposure duration on detachment rate of salmon lice (Lepeophtheirus salmonis); testing the relevant thermal spectrum used for delousing

Thermal delousing has become the most applied method for treatment against salmon lice. However, the tem-perature range used is strongly aversive for salmonids, and the method is associated with increased mortality. Treatment temperature x duration combinations should be tailored to maximise delousing efficiency and minimize welfare impacts on the host fish. We tested the detachment rate of sessile, pre-adult and adult male and female salmon lice (Lepeophtheirus salmonis) as a function of ambient temperature (11-16 degrees C), exposure tem-perature (28-36 degrees C), and exposure duration (0-120 s). Dead Atlantic salmon (Salmo salar) hosts were used to avoid negative fish welfare and detachment of lice due to fish behaviour. Within the range tested, higher exposure temperatures were associated with higher detachment rates among pre-adult and adult lice, while no sessile lice detached at any temperature. Moreover, no treatment combination detached 100% of lice of any stage, and at 28 degrees C, detachment of adult females was negligible. Most detachments occurred within the first 30 s of exposure. We conclude that for a given delousing efficiency, lower temperatures must be compensated for by considerably longer exposure durations. This may be a higher risk for the welfare of the host fish than higher temperatures and shorter exposure durations.

Automated summary

Thermal delousing is the most commonly used method to treat salmon lice, but the temperature range used is aversive for salmonids and leads to increased mortality. Higher exposure temperatures are associated with higher detachment rates, primarily occurring within the first 30 seconds of exposure. Lower temperatures and longer exposure durations are needed to achieve the same delousing efficiency.