Monitoring effectiveness of emamectin benzoate for the treatment of sea lice (Lepeophtheirus salmonis) on farmed Atlantic salmon (Salmo salar)

Abstract

Resistance development to parasiticides has been a problem in both terrestrial and aquatic environments. A major challenge exists in early detection of resistance emergence due to the low frequency at which resistant organisms can occur initially within a population and the difficulty in detecting these organisms. Emamectin benzoate is an avermectin compound which has been used effectively in the treatment of sea lice on farmed salmonids around the world. The main purpose of this thesis was to examine field collected sea lice abundance data and treatment records to estimate changes in the effectiveness of emamectin benzoate for treatment of sea lice (Lepeophtheirus salmonis) infestations on farmed Atlantic salmon (Salmo salar). The objectives of this thesis were: (1) to review and describe methods for the evaluation of sea lice treatment effectiveness, (2) to evaluate and establish if changes in effectiveness of emamectin benzoate for the treatment of sea lice (L. salmonis) on farmed Atlantic salmon in New Brunswick, Canada, had occurred from 2004 to 2008, and (3) to determine if there was evidence of differences in the temporal development of resistance to emamectin benzoate across L. salmonis life stages using data collected in both Canada and Scotland. Field collected data from New Brunswick salmon farms (2004 to 2008) were examined for temporal and spatial changes in treatment effectiveness in populations of L. salmonis. Data analysis was carried out in two parts: examination of trends in treatment effectiveness and L. salmonis abundance and an examination of multiple factors influencing post-treatment L. salmonis abundance and treatment outcome through the use of statistical models (linear and logistic regression). A reduction in treatment effectiveness was found from 2004 to 2008 and treatment effectiveness was found to vary by location. Further analysis was carried out to examine for possible differences in treatment effectiveness between different sea lice life stages. Life stages were categorized into two groups: adult females (gravid and non-gravid) and other mobile stages (pre-adult female, pre-adult male, and adult male). Sea lice abundance records following emamectin benzoate treatments from the west coast of Scotland (2002 to 2006) and from New Brunswick, Canada (2004 to 2008) were examined. Differences in treatment effectiveness were found between the two groups by year and location. Changes in sea lice sensitivity to emamectin benzoate were not synchronized in all life stages. A difference in the rates of resistance development between locations was noted, with resistance developing more rapidly in New Brunswick than in Scotland. In summary, this research examined methods for the evaluation of emamectin benzoate effectiveness for the control of sea lice. A reduction in emamectin benzoate effectiveness occurred over time and varied by location. In addition, resistance development was not synchronized in all sea lice life stages. These methods could help aid in the detection of resistance development in parasites. However, no single method is likely to suffice for monitoring changes in sea lice sensitivity. In future, coordinated analyses from both laboratory and field studies will likely yield the best results