Use of Formalin during Incubation of Rainbow Trout Eyed Eggs

The influence of daily formalin treatments on eyed egg and fry mortality was evaluated during the incubation of eyed eggs of Cleghorn strain rainbow trout Oncorhynchus mykiss in vertical-flow incubators. The study was initiated in 1995 and replicated yearly through 1998. In 1997 and 1998, significant (P < 0.05) increases in survival were observed in trays of eggs treated with formalin at 1,667 mg/L for 15 min daily from egg eye-up until just before hatch compared with incubation trays where dead eggs were manually removed for control of Saprolegnia sp. In 1996, 1997, and 1998, significant mortality reductions were observed prehatch when using formalin. A significant improvement in survival from hatch to fry swim-up occurred only in 1998. No significant differences in survival occurred during any life stage in 1995. Yearly differences in formalin influences are attributed to changes in the age of broodfish, spawning frequency, and sample sizes. Both formalin treatments and hand picking of dead eggs adequately controlled visible fungal development. Daily formalin treatments from egg eye-up to hatch are recommended unless there are overriding safety or environmental concerns.