LAUSR

Abstract Salmon lice, Lepeophtheirus salmonis (Kroyer, 1837), are ectoparasites of the Atlantic salmon, their presence on the fish host is often linked to increased economic losses to the aquaculture industry in the northern hemisphere. Initial attempts to control parasite numbers primarily involved chemical treatment methods, however, there has been a gradual shift towards what are considered more environmentally friendly methods including thermal treatment. This method has been in use for several years; however, few studies have been conducted on its effect on the salmon lice. This study aimed to determine the effect of 2 min exposure to increased water temperature on the copepodid and pre-adult II stages from seven L. salmonis populations and develop in vitro bioassay methods for on-site testing. Seven geographically distinct populations (population A-G) originating from northern-, mid-, and southern-Norway were included in this study. Treatment temperatures were calculated as Δ°T, the difference between the maintenance temperature (12 °C) and treatment temperature. For each population, parasites were exposed for 2 min to a range of water temperatures (Δ°T, 18 ‐ 28°C), after which they were returned to 12 °C seawater. Parasites were observed and classified as unaffected/affected at two timepoints, immediately following exposure and 24 h post-treatment. Differences between populations were observed in the copepodid stage immediately following exposure. All copepodids in populations B and E were immobilised at Δ20°C, compared to population C with unaffected copepodids present at Δ26°C. However, 24 h later all populations exhibited normal behaviour at Δ22°C. Population A and E were all affected at Δ25°C, while unaffected copepodids were observed in population C (16.3%) at Δ26°C. The pre-adult II stage showed a similar trend when observed immediately following exposure, all parasites affected in population A and E at Δ19°C, whereas population C had 20% unaffected at Δ22°C. At the 24 h observation parasites had generally recovered from exposure to Δ23°C with the lowest being population F (33%) and the highest being population A (83.3%). This study illustrated that all stages of L. salmonis had an immediate reaction to thermal treatment, yet the majority recovered over time. Differences in baseline sensitivity were observed between populations, to ensure high treatment efficacy, routine in vitro bioassays to determine whether thermal treatment should be implemented.