Efficacy of Emamectin Benzoate Figure 2. Efficacy of emamectin benzoate treatments in controlling infestations of mobile Lepeophtheirus salmonis, between 2002 and 2006. Post-treatment mobile L. salmonis abundance as a percentage of pre-treatment abundance (6SE), 7–83 days after commencement of treatment. Plots based on data from 101 treatment episodes at 47 Atlantic salmon farms in Scotland in the period 2002 to 2006. doi:10.1371/journal.pone.0001549.g002 Statistical modelling of post-treatment lice abundance As was observed in Figures 2 and 3, efficacy appeared to vary amongst years. However, it was important to consider other factors that may have influenced post-treatment abundance such as geographical region, season and stage in the production cycle. As shown in Table 3, between 2002 and 2006 emamectin benzoate treatments were administered to different ages of fish, at various times of the calendar year and across the North, South and Western Isles regions of the west coast of Scotland. Some of the treatments were loch-wide while others were for single farms only. The statistical model that was developed as part of this study (Table 4) formally investigates the effect each of these factors had on the post-treatment abundance of mobile L. salmonis lice. To take account of differing levels of lice challenge, pre-treatment lice abundance was also included as a covariate in the model. In each of the seven-day time periods following treatment that were analysed, 42% to 68% of treated farms reported lice levels. Lice counts appeared to become slightly less frequent after day 62, however it should be noted that around 20% of emamectin benzoate episodes were followed by an additional sea lice treatment within the 12 week period and any lice counts recorded after such an event were discounted from the analysis. Table 4 gives the results of a GLM analysis of mean sea lice abundance following emamectin benzoate treatment intervention, based on all 108 episodes. All variables were found to be statistically significant (p,0.01) and are listed along with least squares estimates of post-treatment mobile abundances and their PLoS ONE | www.plosone.org 5 February 2008 | Volume 3 | Issue 2 | e1549