Reduced sensitivity, and potential resistance, of sea lice to currently available medicines remains an important area for continued research. Further analysis regarding the structure of lice populations prior to and following treatment needs to be undertaken to assess whether it has any bearing on efficacy. The prevalence of sea lice following treatment intervention should also be examined to ascertain whether lice that persist post-treatment are present on many or few fish within a farm. Studies that compare alternative treatment strategies across the whole production cycle are also important [27] as product rotation will prevent over-dependence on one sea lice medicine and may discourage resistance in the long term. The ineffective treatments reported in this study may have lacked efficacy for a number of reasons relating to fish appetite and feeding rate, sub-therapeutic dosing due to underestimated biomass or variation in medicine inclusion level in the feed, amongst others. In vivo methods for assessing efficacy exist, but the techniques used for the bioassay of emamectin benzoate require further validation. Furthermore, the viability of lice post-treatment is often compromised and therefore improved methods for enumerating attached individuals, including characterisations such as ‘‘moribund lice’’ or ‘‘non-viable egg-strings’’, must be developed [35]. These concerns have led to the establishment of a scientific group to advise the Scottish salmon industry on a protocol for, References 1. International Council for the Exploration of the Sea (2007) Report of the Working Group on North Atlantic Salmon (WGNAS). ICES CM 2007/ACFM: 13, 8 p. 2. Johnson SC, Treasurer JW, Bravo S, Nagasawa K, Kabata Z (2004) A review of the impact of parasitic copepods on marine aquaculture. Zoological Studies 43: 229–243. 3. Pike AW, Wadsworth SL (1999) Sea lice on salmonids: their biology and control. Advanced Parasitology 44: 233–337. 4. 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Efficacy of Emamectin Benzoate ‘‘monitoring sea lice for resistance to approved treatments’’ (Scottish Salmon Producers Organisation, Minutes of the Integrated Sea Lice Management group, 30th August, 2007; Perth, Scotland). In summary, all concerned parties must continue to closely monitor treatment interventions before a definitive assessment can be made as to whether the efficacy of emamectin benzoate against infestations of sea lice on Atlantic salmon farms in Scotland is diminishing. Acknowledgments The industrial partner was Marine Harvest (Scotland). Thanks are due to all administrative and farm management staff who contributed to this project by collecting and providing access to production data. Particular thanks are due to the regional health managers; Carol Cox, Alisdair MacLennan and Chris Wallace. The authors wish to thank Gordon Ritchie of Marine Harvest for his comments and suggestions during the preparation of the manuscript. 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