Treatments for ich infestations in channel catfish evaluated under static and flow-through water conditions

In response to producer reports of poor efficacy using published treatments against Ichthyophthirius multifiliis (ich) infestations of channel catfish Ictalurus punctatus, we initiated a preliminary study to see which of the compounds that are permissible in food-fish aquaculture were efficacious un der what we judged to be ideal laboratory conditions. We planned to use the results of this work as a basis for further study in the field. In our stu dies we used fingerling catfish in glass aquariums with stable water qualit y and daily treatments in both static and flow-through water systems. In so me experiments, infested fish and healthy fish were stocked together so tha t the ability of the treatment to eliminate preexisting infestations could be examined separately from the treatment's ability to block transmission. Malachite green and methylene blue were used as positive controls, and untr eated fish were used as the negative control for efficacy. Treatments with sodium chloride, hydrogen peroxide, potassium permanganate, elevated temper atures, chloramine-T, povidone iodine, and high flow rates were ineffective under all conditions and concentrations tested. Alternate-day treatments w ith formalin at 25 and 50 mg/L in static water or 100 mg/L in flow-through water aquariums were not effective at eliminating infestations or preventin g fish-to-fish transmission. Daily treatments with 50 mg/L formalin in stat ic water or 100 mg/L formalin in flow-th-rough water aquariums were toxic t o channel catfish. Formalin at 25 mg/L in static water aquariums was effect ive at eliminating infestations and reducing fish-to-fish transmission when used as a daily treatment. Copper sulfate was effective in blocking the in festation of healthy fish, but it did not eliminate established infestation s on channel catfish, even after 20 daily treatments under static condition s. Malachite green and methylene blue eliminated established infestations o n fish, whereas copper only prevented transmission; this suggests that the dyes are able to penetrate deeply or act systemically, allowing them to kil l subcutaneous stages of ich. Further field trials were not pursued because none of the legal or economically viable treatments showed efficacy under presumably ideal laboratory conditions.