Evaluation of formalin and hydrogen peroxide treatment regimes on rainbow trout eyed eggs

This study examined the use of antifungal chemicals at nonstandard treatment concentrations or treatment intervals during the incubation of eyed eggs of rainbow trout Oncorhynchus mykiss until hatch in vertically stacked incubation trays. Daily chemical treatments of 500, 1,000, and 1,667 mg of commercial formalin (containing 37% formaldehyde)/L for 15 min and a nonchemical, dead-egg-removal control were included in three trials. The second and third trials added a true control (no antifungal measures) and the third trial also incorporated a treatment of formalin at 1,667 mg/L for 15 min every other day. In these three trials, formalin at 1,000 or 1,667 mg/L daily or 1,667 mg/L every other day significantly improved egg survival to hatch relative to the other treatments and completely controlled fungal growth. A fourth trial evaluated treatments of formalin at 750 mg/L for 15 min daily or every other day, hydrogen peroxide at 500 mg/ L for 15 min daily or every other day, and control groups for both treatments. Egg survival to hatch was significantly different among the treatments; the daily 750-mg/L formalin treatment produced the greatest number of hatched fry, followed by the treatment with 750 mg/L formalin every other day, the dead-egg-removal control, and daily hydrogen peroxide treatment. Survival in the every-other-day hydrogen peroxide treatment and the true control could only be estimated because of extreme fungal growth, and was substantially less than the other treatments. Only daily 750 mg/L formalin treatments and dead egg removal completely controlled fungal growth. To adequately control fungus and maximize egg survival, either daily 15-min treatments of formalin at concentrations of at least 750 mg/L or every-other-day treatments of formalin at 1,667 mg/L are recommended.