Technical Notes: Evaluation of Three Candidate Fungicides for Treatment of Adult Spring Chinook Salmon

Abstract Use of malachite green as a fungicide in fish culture was terminated by the U.S. Food and Drug Administration (FDA) in 1991. Formalin has been the replacement antifungal agent, but the present FDA registration restricts its use to only eggs of salmonids and esocids: further, concerns about its safety for users and about formalin effluents in the environment persist. Three candidate chemicals (iodine, glutaraldehyde, and hydrogen peroxide) were tested along with the reference fungicides malachite green and formalin for antifungal activity on adult spring chinook salmon (Oncorhynchus tshawytscha). Compounds were added to flow-through systems without correction for flow dilution. Malachite green and formalin were effective in preventing prespawning mortality and in minimizing the incidence of fungal infection. Fish from iodine treatments at 0.1 or 0.02 mg/L had mortality levels like those of untreated controls. Most deaths of iodine-treated (and control) fish were accompanied by fungal infections, whereas fungal infection was almost absent among malachite greentreated fish. Glutaraldehyde at 31.2 or 6.2 mg/L was effective as a fungicide, but it was toxic at 62 mg/L, and regulatory restrictions on its use may be formidable. Hydrogen peroxide was the most promising candidate based on efficacy and regulatory considerations. None of the fish deaths in treatments with hydrogen peroxide at 100 mg/L were associated with fungal infections. Hydrogen peroxide treatment at 25 mg/L allowed only a very low incidence of fungal infection. Therefore, hydrogen peroxide is deemed the best candidate for development as an antifungal agent in spring chinook salmon culture.