The efficacy of emamectin benzoate against infestations of sea lice, Lepeophtheirus salmonis, on farmed Atlantic salmon, Salmo salar L., in British Columbia.

Sea lice are a naturally occurring ectoparasite of wild salmon (Nagasawa 2001; Beamish, Neville, Sweeting & Ambers 2005). There is also clear evidence that these parasites are seldom a production or fish health concern on farms in British Columbia (Saksida, Constantine, Karreman & Donald 2007), in direct contrast to most other salmon-producing regions (Heuch, Revie & Gettinby 2003; O Donohoe, Kane, Kelly, Nixon, Power, Naughton & Jackson 2008; Lees, Gettinby & Revie 2008a). Nevertheless, owing to concerns regarding the potential impact of sea lice originating from farmed Atlantic salmon, Salmo salar L., on wild Pacific salmon species, Oncorhynchus spp., in BC, their effective control continues to be a subject of considerable interest (Morton, Routledge, Peet & Ladwig 2004; Krkošek, Ford, Morton, Lele, Myers & Lewis 2007). Indeed, it has been suggested that the recent reductions in sea lice infestations on wild salmonids in the Broughton Archipelago (an area of major research focus over the past 5 years) are a consequence of improved lice management actions on salmon farms (Harvey 2009). While a number of management practices, such as single year-class production and between-cycle fallowing, can have a positive effect on lice control within farms, the most direct effects are associated with the use of medicines to control sea lice. This is particularly the case if a goal is to minimize lice numbers at a specific point in the production cycle, for example during the period when wild smolts are most likely to be migrating past farms. In 2003, BC regulatory authorities established requirements that farms maintain lice abundance below a threshold of three motile stage Lepeophtheirus salmonis between March and June (Saksida et al. 2007). In 2004, these same authorities commenced a sea lice surveillance programme where between 25% and 50% of active Atlantic salmon farms were assessed by government biologists for sea lice during each quarter to verify reported levels (Saksida et al. 2007). These regulations have not changed. In BC, the only product that is currently available to treat sea lice on salmon farms is SLICE (Intervet Schering-Plough Animal Health). SLICE is an oral formulation of emamectin benzoate, which is added to fish feed and delivered at a dosage of 0.5 lg kg fish for 7 days. There have been a number of reports indicating reduced efficacy of emamectin benzoate when used on farmed fish in a range of other salmonproducing countries. These include Chile (Bravo, Sevatdal & Horsberg 2008) and Scotland (Lees, Baillie, Gettinby & Revie 2008b), with less welldocumented reports in Ireland (O Donohoe et al. 2008 – non-specific report of reduced sensitivities , Journal of Fish Diseases 2010, 33, 913–917 doi:10.1111/j.1365-2761.2010.01192.x