EVALUATION OF ANTIFUNGAL AGENTS FOR FISH CULTURE

The antifungal activities of 21 chemicals against species of Saprolegn ia, a ubiquitous genus of aquatic fungi frequently found in fish hatch eries, were evaluated with pure fungal culture and with Saprolegnia-in fected eggs of rainbow trout (Oncorhynchus mykiss). Fourteen compounds were ineffective for control of fungus on rainbow trout eggs or were toxic to the eggs. The seven compounds that effectively controlled fun gus on infected eggs and provided a reasonable margin of safety were A bbott A-73336, amorolfine, formalin, glutaraldehyde, hydrogen peroxide , melaleuca, and salt (sodium chloride). Only formalin, hydrogen perox ide, and salt appeared completely suitable for registration. Formalin, the replacement antifungal agent, is restricted to use with the eggs of salmonids and esocids. User safety and the effect of effluents on t he environment are also concerns related to formalin treatments. Forma lin effectively prevented fungal infections on eggs at concentrations as low as 250 ppm. A 1,000-ppm treatment of formalin not only prevente d infection but also decreased existing infection and increased hatchi ng rates at exposures of 15, 30, or 60 min. Hydrogen peroxide at conce ntrations of 500 and 1,000 ppm controlled fungus on infected eggs and increased hatching rates of treated eggs. Hydrogen peroxide was recent ly classified by the U.S. Food and Drug Administration as a low regula tory compound when used to control fungi on all species and life stage s of fish, including eggs, and it is the antifungal agent of choice fo r further development. Salt decreased infection level and increased ha tching rate of infected eggs at a concentration of 30,000 ppm. Salt is readily available and listed as a low regulatory compound, but the la rge quantities required limit its applicability.