Sensitivity of salmon lice (Lepeophtheirus salmonis) in New Brunswick, Canada, to the organophosphate Salmosan® (w/w 50% azamethiphos) using bioassays

Bioassays have been used as a monitoring tool to determine changes in sensitivity of sea lice populations to various bath treatments during the Atlantic salmon production cycle. In this study we report on the results of bioassays conducted between 2009 and 2012 for L. salmonis with the objective of detecting changes in sea lice sensitivity to Salmosan® (w/w 50% azamethiphos), a delousing agent used in the Bay of Fundy region of New Brunswick, Canada. EC₅₀ values ranged from 4.6 ppb to 402 ppb. Although sea lice stage was not a significant factor influencing observed EC₅₀ values, there were significant differences among years, with 2009 being significantly lower than all other years, and 2011 being significantly higher than 2010 or 2012. Season was also found to be a significant predictor with EC₅₀ values in the winter/spring being lower than those predicted in the summer/fall. While sea lice resistance to Salmosan® (w/w 50% azamethiphos) has not been reported from Eastern Canada, variable EC₅₀ values indicate unmeasured influences on tolerance to Salmosan® (w/w 50% azamethiphos) in the populations of L. salmonis sampled from the Bay of Fundy during the 2009 to 2012 period. The possibility of more recent changes in sensitivity remains unknown due to the lack of a centralized repository of bioassay data or other measures that might reflect the emergence of resistant sea lice.