*Corresponding author: niccole_wandelear@fws.gov Copepods are the most numerous of the parasitic crustaceans affecting a wide range of marine and freshwater finfish worldwide (Woo 2006). Species of the genus Salmincola are parasitic copepods of major concern in wild and cultured salmonids. The incidence and spread of Salmincola spp. can be exacerbated by degraded environmental conditions, e.g., increased water temperatures, high organic loading, low dissolved oxygen, and overcrowding. Such parasitic crustaceans often open portals of entry for secondary, opportunistic pathogens, thereby enhancing morbidity and mortality of fish (Bandilla et al. 2006). Fish heavily infested with Salmincola spp. will flash, jump, or rub along hard surfaces as they try to rid themselves of the parasites. Fish may become darker in color, produce excessive mucus, exhibit fatigue in flowing water, and go off feed. As a result, reduced growth and productivity of fish populations may occur.

Salmincola spp. are ectoparasites of major concern in wild and cultured salmonids. These parasites can cause respiratory distress and facilitate the entry of secondary pathogens. Of particular concern in the United States is S. californiensis, which can infest all Oncorhynchus spp. and is restricted largely to freshwater. Bath treatments with formalin and hydrogen peroxide have traditionally been used to control infestations of some parasitic copepods in cultured salmonids; however, these treatments can be difficult to apply, expensive, and stressful to fish and have not been shown to be effective against S. californiensis. A more effective and efficient treatment method needs to be developed. SLICE (0.2% emamectin benzoate [EB]) is a commercial in-feed treatment that has been shown to be effective for the control of sea lice infestations in seawater-reared farmed salmon and trout. We postulated that EB might also be efficacious for the control of parasitic copepods such as S. californiensis on freshwater-reared salmonids. Four trials were conducted to evaluate the effectiveness of SLICE-medicated feed administered at a target dosage of 50 mu g EB.kg fish(-1).d(-1) for seven consecutive days to control infestations of S. californiensis on freshwater-reared rainbow trout. At the end of each trial (after either a 30- or 42-d posttreatment period), copepod prevalence in treated tanks was substantially reduced compared with the pretrial prevalence recorded in the reference populations. Additionally, a significant difference was detected in mean abundance between treated and control groups, with a 79-96% reduction in mean abundance among fish offered the EB-medicated feed. Based on the results of these trials, it was concluded that SLICE was efficacious in reducing infestations of S. californiensis on freshwater-reared rainbow trout.