Microbial inhibition in response to treatments of hydrogen peroxide and formalin on landlocked fall chinook salmon eyed eggs, as determined by scanning electron microscopy

Scanning electron microscopy was used to compare microbial growth in eggs of landlocked fall chinook salmon Oncorhynchus tshawytscha receiving daily 15-min treatments of 1,667 mg/L formalin, 700 mg/L hydrogen peroxide, or no chemical treatments during incubation in vertical-flow incubators from egg eye-up to hatch. Eggs receiving no chemical treatment had bacterial densities at the end of the experiment that were significantly greater than those on eggs receiving either of the chemical treatments. In eggs not chemically treated, bacterial densities significantly increased from 2,413 bacteria/mm(2) on the external egg membrane surface at the start of the experiment to 69,598 bacteria/mm(2) 12 d later. Although the initial and final numbers were not significantly different, bacterial counts on eggs treated with formalin averaged 2,075 bacteria/mm(2) on day 1 and 3,764 bacteria/mm(2) by the end of the experiment, while those of eggs treated with hydrogen peroxide ranged from 5,406 bacteria/mm(2) at the start of the experiment to 14,431 bacteria/mm(2) after 12 d. The number of bacteria attached to the external egg membranes did not differ significantly between eggs treated with formalin and those treated with hydrogen peroxide throughout the study. Fungal growth was negligible and was only observed in control eggs. Additionally, the external membranes of the control eggs, but not the chemically treated eggs, were visibly degraded over the 12-d period. Egg survival was significantly lower in control eggs than in eggs receiving either of the chemical treatments and was significantly correlated with bacterial numbers.