Efficacy of Emamectin Benzoate Figure 3. Mean abundance of mobile Lepeophtheirus salmonis, pre and post emamectin benzoate treatment, between 2002 and 2006. Mean mobile L. salmonis abundance (6SE), pre-treatment and 0–83 days after commencement of treatment. Plots based on data from 108 treatment episodes at 47 Atlantic salmon farms in Scotland in the period 2002 to 2006. doi:10.1371/journal.pone.0001549.g003 associated 95% confidence intervals. Significant interaction factors are also listed and are further illustrated in Figure 4. The factors included in the model accounted for 66% of the variation observed amongst post-treatment sea lice abundance. Lice levels per fish subsequent to an ineffective treatment (7.01, CI [5.95–8.22]) were around 10 times higher than those observed following an effective episode (0.65, CI [0.55–0.75]) (Table 4). Overall, mean louse abundance was lowest between days 21 and 62 and levels recorded during this time were found to be significantly lower than those observed between day 7 and 13 (p,0.01). Post-treatment lice abundance was also found to be significantly higher in the second year of production when compared to the first (p,0.01). Three significant interactions were found amongst the variables studied. Figure 4A highlights the differences that were observed between the geographical region in which the treatment was applied and the year it was administered. Post-treatment abundance in the Western Isles in 2006 (5.34, CI [4.33–6.53]) was significantly higher (p,0.05) than in any other region and year combination, except the South in 2003 (4.55, CI [3.62–5.66]) and the South in 2005 (4.15, CI [3.55–4.84]). Post-treatment abundance in the North, for all years, was significantly lower (p,0.05) than these three region and year values, except for 2006 (2.40, CI [1.77–3.18]), when it did not differ significantly from the South in 2005. Analyses showed that ineffective spring treatments (11.77, CI [8.07–16.97]) performed significantly worse (p,0.01) than inef- fective treatments applied in autumn (4.56, CI [3.44–5.97]) (Figure 4B). However, only two of the 30 spring treatments analysed were categorised as ineffective and the wide confidence PLoS ONE | www.plosone.org 6 February 2008 | Volume 3 | Issue 2 | e1549