Sea lice population and sex differences in P-glycoprotein expression and emamectin benzoate resistance on salmon farms in the Bay of Fundy, New Brunswick, Canada.

Parasitic sea lice are a major challenge for salmon aquaculture. This is especially due to recent development of resistance to emamectin benzoate (EMB) in the parasite. We investigated 1) whether EMB treatment success in Grand Manan, Bay of Fundy, NB, can be explained through EMB bioassay and P-gp mRNA expression studies, 2) if other populations of sea lice not under EMB selective pressure possess similar EMB sensitivity as Grand Manan sea lice populations, and 3) heritability of EMB resistance in L. salmonis. EMB bioassay results indicated population, species, sex, and temporal differences in EMB EC50 values. RT-qPCR analyses revealed population and sex differences in P-gp mRNA levels, correlating with the bioassay results. Laboratory-reared sea lice maintained their EMB sensitivity status up to the F3 generation. Caligus elongatus, collected from Grand Manan showed >2 fold lower EMB EC50 values compared with L. salmonis collected from the same site. Concurrent exposure to EMB and verapamil yielded no increase in C. elongatus sensitivity to the parasiticide. Sea lice bioassay and P-gp mRNA studies can be used to track EMB resistance and sex differences in EMB sensitivity and P-gp mRNA levels exist in the parasite