LAUSR

Abstract Sea lice, Lepeophtheirus salmonis (Kroyer, 1837), are ectoparasitic crustacean parasites responsible for economic losses in the Atlantic salmon aquaculture industry in the northern hemisphere. Numerous chemical and non-chemical control methods have been developed, including freshwater bathing. Freshwater bathing is regarded as an environmentally friendly treatment; however, reports of variable treatment efficacies have raised concerns regarding the general applicability of this treatment method. Our study aimed to determine the salinity in parts per thousand (‰) at which median survival (EC50) was obtained at 24 h for the copepodid and pre-adult II stages of L. salmonis from geographically separated populations and to develop a bioassay method for on-site pre-treatment tests. Parasites were separated into four different geographically separated populations which are referred to as population A, B, C, and D. Using a stepwise method, parasites were exposed to a range of salinities (35‰ to 0‰) for 24 h, survival was observed after 24 h and results were classified as unaffected/affected. L. salmonis copepodids illustrated significant differences in tolerance among populations. Population A was the least tolerant, with an EC50 of 17.5‰, whereas population C had an EC50 of 11.3‰. No significant population difference in tolerance was observed among the pre-adult II stages. Pooling the data from pre-adult II L. salmonis from three populations yielded an EC50 of 2.8‰ for both sexes combined, 2.4‰ for males, and 2.6‰ for females. All stages of L. salmonis examined in this study exhibited higher tolerances to low salinity than observed in previous studies. We suggest regular monitoring of freshwater sensitivity in areas where this control option is regularly used to detect possible deviations from baseline sensitivity at an early stage.