Intervening in efficacy trials in this way is sometimes necessary for the welfare of fish, but it also complicates the comparability of subsequent efficacy parameters [28]. Despite the fact that production data are rarely as complete or balanced as that collected within a prospective clinical study it can give a better indication of medicinal efficacy under production conditions and over a longer period of time, providing sufficient treatment episodes are available for analysis. While the lice counts in this study were performed by various individuals on numerous farms, it should be noted that they were conducted by qualified personnel who had been given training in this procedure. However, as pre- treatment lice counts were based on records taken from up to 16 days prior to emamectin benzoate intervention, it is likely that lice levels were sometimes higher than the baseline figures used in the efficacy calculations. All partial site, staggered and mixed treatment episodes were screened out of this study, but it was important to further categorise the episodes that were included. Strategic sea lice treatments form part of the integrated health management programme adopted by Scottish salmon producers and not all treatment episodes are administered in response to a substantial sea lice challenge. It should also be noted that treatment-trigger guidelines are not uniform amongst countries. Despite the challenges encountered in classifying treatments as strategic or ineffective, their inclusion in this study was important as they were evident throughout the five year period. For the purposes of analysing loch-wide treatments it was important to ascertain whether other farm operators treated their respective sites for lice at the same time as the industrial partner in a coordinated manner. This information was not always available. Alternative strategies for assessing the efficacy of emamectin benzoate were considered, but none was found to be as robust as the methodology adopted. Counting the number of treatment interventions in each production cycle or calculating the mean number of days between treatments gives an indication of treatment efficacy and lice challenge within a site. However, the former method is better suited to regions where only one sea lice medicine is available for use [23] as it is difficult to compare in- feed emamectin benzoate treatments alone with emamectin benzoate and topical cypermethrin treatments. Assessing efficacy over the entire production cycle can also become complicated if stocks are split between sites for the second year of production or when the length of time that fish are at sea varies between sites and years. While useful, these assessments are better suited to macro level analyses and may be explored in a more general risk factors study. The purpose of this study was to focus on specific issues relating to emamectin benzoate interventions in Scotland since regular treatment began in 2002. In common with results observed in laboratory and field based trials conducted in the period 1997 to 2002 [20–22,26,27,33,34], the majority of emamectin benzoate treatments administered in this study significantly reduced infestations of mobile L. salmonis. However, the efficacy of treatments was not uniform between years or geographical regions. In particular the frequency with which interventions appeared to be ineffective increased toward the end of the study period. Time to maximum efficacy varied widely across episodes, but generally it was reached 28–34 days post treatment initiation and overall levels of lice were lowest between days 21 and 62. This is broadly in line with findings from a recent study on the Maine coast by Gustafson and colleagues that also used pre-treatment lice loads to calculate the efficacy of emamectin benzoate [28]. However, it should be noted that of the 19 treatments analysed in the Gustafson study, all were found to be more than 60% Efficacy of Emamectin Benzoate efficacious and pre-treatment abundance was generally lower, particularly when compared to the levels observed in Scotland in 2003 and 2004. Duration of efficacy has been reported to be as long as five months in the Broughton Archipelago region of British Columbia [23] although some lice re-infection occurred after three months. In the current analysis, post-treatment lice levels in Scotland generally remained below pre-treatment abundance for the full 83 days examined, but began to rise around week nine. It is interesting to note that, on average, the levels of lice observed prior to intervention were similar in both studies, but that farms in British Columbia were re-treated less often than those in Scotland. However, it is likely that the large populations of migratory Pacific salmon found in the waters of British Columbia create very different dynamics in terms of sea lice challenge than those in Scotland. The differences found between the present study and that conducted in Scotland in 2002 [27] are of particular interest. In the earlier study [27], zero levels of lice were attained for 12– 14 weeks post-treatment, however it should be noted that this small scale trial involved only two farms and three emamectin benzoate treatment episodes. Nevertheless, the results presented herein may indicate that emamectin benzoate is not as effective on Scottish salmon farms as it once was. Direct comparisons with other studies are less straightforward, either because efficacy was monitored over a shorter period of time [22,26,33] or because untreated cohorts provided a potential source of re-infestation, that may have extended the time taken to reach maximum efficacy or reduced the duration of efficacy [21,34]. Previous analysis has shown that since 2002 L. salmonis mobile abundance tends to vary between geographic regions in Scotland [19], with farms in the North generally experiencing lower levels of infestation. The present study shows that post-treatment lice abundance also differs between regions. Again, levels in the North were found to be lower and increases in post-treatment abundance in this region in 2004, and in the Western Isles in 2006, match similar trends observed in overall mobile lice levels [19]. The peaks and troughs observed in post-treatment abundance in the South did not closely match patterns of mobile infestation in this region, confirming that variables other than year and region are important and should be considered in a fuller analysis of all risk factors. It is known that seasonal variation occurs in lice infestation on Scottish farms and that abundance is generally higher in the second year of production [13,14,19]. It is perhaps then unsurprising that post-treatment lice levels also varied throughout the production cycle in the present study. The univariable analyses carried out by Gustafson et al [28] found that treatments administered to larger fish required more time to reach maximum efficacy. In this multivariable study no significant interaction was found between production stage and days after treatment, but post-treatment abundance levels were higher in second year fish. While the relative performance of emamectin benzoate treatments across the seasons varied, it appears that post-treatment levels were highest following winter (Nov–Jan) and spring (Feb–Apr) treat- ments. Given that all but one of the winter treatments occurred in the first production year and nearly all spring treatments occurred in the second, it appears that treatments applied in the middle six months of the production cycle may not be as efficacious as those administered during other periods. However, it should be noted that the distribution of treatments throughout the production cycle was not balanced. Furthermore, significant interactions were found between season and effective treatments as well as between season and loch-wide treatments, making it difficult to conclude what effect season has on treatment efficacy. PLoS ONE | www.plosone.org 9 February 2008 | Volume 3 | Issue 2 | e1549