Soft-Egg Disease in Landlocked Fall Chinook Salmon Eggs: Possible Causes and Therapeutic Treatments

Soft-egg disease was observed in the spawn from 5 of 15 landlocked fall chinook salmon Oncorhynchus tshawytscha from Lake Oahe, South Dakota, with the percentage of prematurely hatching fry per spawn ranging up to 42.0%. Soft-egg disease was only identified in eggs collected from presumed 4-year-old salmon, and its occurrence is hypothesized to be at least partially caused by maternal dietary stress during egg development. Egg mortality after the appearance of soft-egg symptoms was significantly less in the incubation trays receiving daily hydrogen peroxide treatments at 1,000 or 2,000 mg/L for 15 min and trays receiving either daily or twice-daily formalin treatments at 1,667 mg/L for 15 min, compared with the trays that received either iodophor treatments or no therapeutic treatment at all.