We evaluated hydrogen peroxide (H
2O
2) as an alternative anti-fungal agent of malachite green in the practical use at salmon hatcheries. Repeated exposure (2 times/week, just after fertilization to eyed-egg stage) to 1, 000 ppm of H
2O
2for 60 min showed efficacy comparable to malachite green for the inhibition of fungal infection in rainbow trout (Oncorhynchus mykiss) and brook charr (Salvelinus fontinalis) eggs. On the other hand, H
2O
2treated eggs from cherry salmon (O. masoumasou) and white-spotted charr (S. leucomaenis) showed a higher fungal infection rate and a lower eyed-egg rate than those of malachite green-treated eggs. The toxicity of H
2O
2to salmonid eggs varied with species, i.e. 1, 000 ppm of H
2O
2for 60 min was not toxic to eggs from brook charr and yamame salmon (land locked form of cherry salmon), but exposure to 750 ppm of H
2O
2appeared to be toxic to those from cherry salmon and white-spotted charr.
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