Effect of Water Flow Rates in Hatching Jars to Control Fungal Infections of Rainbow Trout Eggs

Abstract

Abstract The successful culture of fish eggs often depends on chemical treatment to control fungus. Attempts to control fungus without chemicals have centered on the use of elevated water flow to roll fish eggs in hatching jars. We cultured uninfected and fungusinfected eggs of rainbow trout (Oncorhynchus mykiss) at various flow rates. Eggs that were cultured at 300- and 600-mL/min flow rates exhibited no egg movement, higher rates of fungal infection, and reduced hatching success. Eggs cultured at a 1,200-mL/min flow were lifted into the water column and rolled moderately; this flow rate significantly increased the percent hatch due to control of fungus. A flow of 1,800 mL/min vigorously rolled the eggs and controlled fungus, but it also resulted in increased egg mortality. The use of water flow to roll eggs controlled fungal infections and may reduce, or in some circumstances eliminate the need for chemical treatment. The success of this physical method of treatment is dependent on hatchery personnel maintaining flow rates at levels that induce a moderate rolling of the eggs, which inhibits fungal infections without damaging eggs. A major concern in fish culture is the infection of eggs by aquatic fungi (Saprolegnia spp.), which are ubiquitous in hatchery water supplies (Schreck et al. 1993). The occurrence and severity of a fungal outbreak depends on the water source, water temperature, organic load, and length of time required for the eggs to hatch. Once a fungal infection starts, it can spread rapidly from infected to healthy eggs (Piper et al. 1982). Hatchery personnel often use chemicals to control fungus on fish eggs. However, chemical treatment is costly and can itself cause mortality. There is a need to develop alternative culture methods that minimize the necessity for chemical treatment.