While Libby Dam operators were able to keep flooding at bay in the Bonners Ferry area this year, the extraordinary runoff on the Kootenai River presented a different environment for white sturgeon spawning.
The U.S. Army Corps of Engineers operated the dam for pure flood control purposes this year, a change from past years when flows and temperatures have been adjusted in efforts to test and improve sturgeon spawning conditions.
This year, the dam was scheduled to conduct a “spill test,” releasing water over its spillway to boost flows in order to coax sturgeon into moving into prime spawning habitat called the braided reach just upstream from Bonners Ferry.
But the spill test wasn’t necessary. Unprecedented low elevation snowpack feeding Kootenai River tributaries led to high flows for weeks on the river, requiring restraint at Libby Dam.
“The main thing was to keep Bonners Ferry from getting wet,” said Brian Marotz, a Montana Fish, Wildlife and Parks biologist and a member of the interagency White Sturgeon Recovery Team. “Just the way it came off, the water came to just below flood stage (at Bonners Ferry) and stayed there for a month and that allowed us to achieve the conditions that the spill test was going to produce without actually having to spill.”
But the sheer duration of high flows was a bit different than spill would have produced and there were sustained cold water temperatures in the river, both of which were different for white sturgeon, an ancient species that has not had any detectable success in natural spawning in the river since Libby Dam was built in the 1970s.
Pete Rust, a biologist for Idaho Fish and Game, and others were watching spawning behavior closely during this year’s unusual conditions.
“The spawning season was a bit delayed,” he said, noting that the first eggs collected on adhesive mats on the river bottom didn’t happen until May 31, when they are usually first picked up during the middle of the month.
But even with delayed season, Rust said he anticipated more spawners would move into the braided reach than actually did.
Four female sturgeon tagged with sonic transmitters were detected moving into the reach, which may be regarded as being below established spawning success criteria.
Last year, six adult females went upstream from Bonners Ferry. But Rust noted that one year can’t be compared to the next because the number of spawners varies from year to year.
Sturgeon spawners in the Kootenai range from 50 to 100 years old, varying in length from four to seven feet this year. Because females spawn only every five to seven years, the number of spawners will change from year to year, Rust said.
This year there were 28 fish, mostly females, that were tagged and monitored, while there were 36 fish in last year’s spawning group.
But the number of fish that are monitored also depends on success in catching them and fitting them with transmitters, and this year conditions were difficult for doing that.
“If you have high, cold, dirty water it’s hard to catch them and that’s what we had for a while,” Rust said.
Idaho Fish and Game and the Kootenai Tribe of Idaho cooperate in the annual catching effort also to provide the tribes’ unique hatchery on the river with eggs and milt. The hatchery’s abundant production of white sturgeon may be the only near future reproduction because spawning success has been so poor for wild sturgeon, which are listed as endangered under the Endangered Species Act.