JOHN DAY, Ore—Due to recent high temperatures in eastern Oregon, 118 wild adult spring chinook salmon died in the Middle Fork of the John Day River.
The dead salmon were found near the mouth of Big Boulder Creek and the mouth of Vinegar Creek, with 41 and 62 carcasses counted respectively. Numerous resident rainbow trout and mountain whitefish mortalities were also observed.
Adult spring chinook salmon migrate into the John Day River every year in May seeking pools where they rest during the summer months until they spawn in September.
“It is common for some salmon to succumb to quickly rising water temperatures, but this year's heat wave seems to have taken an unusually heavy toll” said ODFW Fisheries Biologist Tim Unterwegner.
Most of the fish are estimated to have died during the first week of July when air temperatures in John Day peaked at 107 degrees and water temperatures in the river climbed 8 degrees between July 2 and July 5. Stream temperatures measured up to 84 degrees which exceeds lethal temperatures for chinook salmon. Additionally, flows in the Middle Fork were about one-third the average during this period, resulting in higher water temperature conditions. No observations, reports or evidence was found suggesting these fish died from pollutants, disease or fishing. Parasite and disease samples were not taken from the dead salmon because they were too decomposed for effective analysis.
ODFW biologists also conducted mortality surveys on portions of the North Fork John Day River above Desolation Creek up to Granite Creek. On those surveys, 16 dead salmon were found and at least 70 were found alive indicating the die-off is limited to the upper Middle Fork. The mortalities on the North Fork were limited to the reach between Ryder Creek and Horse Canyon. No dead salmon were observed in surveys closer to the headwaters of the North Fork or its tributaries.
While no systematic survey has been conducted on the Upper Mainstem John Day, a few dead fish have been observed above Prairie City by biologists from the Confederated Tribes of the Warm Springs Reservation. Chinook surveys in the Grande Ronde and Umatilla Rivers have recorded some mortality during the same time period but not nearly to the extent seen in the Middle Fork of the John Day, said Unterwegner.
This die-off will likely reduce the number of adult spring chinook salmon returning to the Middle Fork in future years. The full extent of this die-off is unknown until spawning ground surveys are conducted during September and smolts are counted during the spring of 2009. After those surveys are completed, the proportion of adults that died before being able to spawn and the impact it had on juvenile salmon and steelhead production can be estimated.