About 360,000 juvenile summer chinook salmon were lost this month to disease at the Washington Department of Fish and Wildlife's (WDFW) Similkameen rearing pond near Oroville in Okanogan County.
The disease, known as "ich," is caused when the free-swimming stages of a parasitic protozoan (Ichthyophthirius multifilis) infect the skin and gills of the fish. The parasite came from naturally spawning adult chinook salmon in the river above the Similkameen rearing pond's water intake structure.
Bob Rogers, a WDFW fish health specialist, said the four-inch fish were transferred to the Similkameen rearing pond from the Eastbank Hatchery between Sept. 29 and Oct. 2. Exams on Oct. 4 showed very few parasites on the fish, but by Oct. 10 fish were dying and treatments were initiated. Losses continued to increase, despite increased treatments, and by Oct. 25 nearly all the 360,000 juvenile salmon were dead.
"The cause of the problem is a combination of factors," said Rogers, who noted the problem resulted from a high return of fish coupled with low water flows.
The Similkameen River flow is less than half normal levels; currently it measures about 336 cubic feet per second (cfs) compared to a 73-year mean for this time of 712 cfs, according to Rogers.
"Low flows make water temperatures higher and perfect for reproduction of this parasite," Rogers said. River water has been measured as high as 59 degrees this month, compared to past years' 47-54-degree range at this time.
The fish lost are Methow/Okanogan stock that would have been released into the Okanogan river system in northcentral Washington. Approximately 250,000 of the same stock fish remain at WDFW's Eastbank Hatchery and are scheduled for release in the Methow river system.
WDFW personnel are reviewing the incident to determine whether measures can be put in place to prevent a re-occurrence.