Efficacy of SLICE Premix (0.2% Emamectin Benzoate) for Reducing Infestations of Salmincola spp. on Freshwater‐Reared Rainbow Trout

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 μg 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.