Oregon Department of Fish and Wildlife fish biologists and the state Water Resources Department said Wednesday their investigation so far has found "a suite of unusual conditions" may have contributed to the death of thousands of fish on a side channel of the Deschutes River southwest of Bend last week.
Those conditions included drought, better than average fall flows in recent years, and fish migrating out of Wickiup Reservoir.
The rest of their statement:
The dead fish were found in a side channel of the river near Lava Island southwest of Bend.
According to ODFW fish biologist Erik Moberly, the dead fish were reported last Thursday afternoon and on Friday morning volunteers and ODFW staff were able to rescue about 750 trout (a combination of redband rainbow and brown trout) and 500 sculpin that were still alive in a handful of water pools.
In addition to trout, about 1,220 mountain whitefish and a similar number of sculpin died when declining water levels left them stranded in the natural lava side channel, which normally has water only during higher flows.
Redband rainbow trout are listed as a sensitive species by the state of Oregon. No other listed fish, including bull trout, were found.
All the dead fish were found in a ½-mile stretch of the side channel.
Water levels in the Deschutes River normally decline this time of year as releases from Wickiup Reservoir are reduced by dam operators.
Every year, the ramp-down rate is conducted at a rate lower than the maximum ramp-down rate set by the U.S. Forest Service Upper Deschutes Wild and Scenic River Management Plan, which was developed in collaboration with ODFW, OWRD, and irrigation districts.
Consistent with that plan, this year’s ramp-down occurred at a slower rate than previous years.
"ODFW and OWRD will continue to investigate whether ramp-down levels were a factor and if necessary, work with partners to make adjustments in the future," the statement continued.
"However, a slower ramp down rate is considered to be better for fish, leading water and fishery managers to look for other explanations why so many fish were stranded in the channel."
ODFW said it "believes that two good years of water in 2011 and 2012, when the channel did not completely dry up, resulted in more fish in the side channel this year than would normally occur."
In addition, the agency said, "there appears to be more fish in the river that emigrated from Wickiup reservoir, which was low this year due to drought conditions. "