Libby Dam tests inconclusive on sturgeon spawning

The spill test at Libby Dam ended Thursday morning, but it’s still unclear whether there was any success in using higher flows to get white sturgeon to move into optimal spawning habitat on the Kootenai River.

The test started June 10, with an expectation of releasing up to 10,000 cubic feet per second over the Libby Dam spillway, in addition to running water through the dam’s turbines at powerhouse capacity of about 25,000 cfs.

But the spill was held at 7,000 cfs, on average, over most of the seven-day test period to keep total dissolved gas levels in the river from exceeding a threshold that’s about 23 percent higher than the gas levels that occur in the river when there is no spill.

Water plunging from the spillway can elevate gas levels to a point that they cause trauma to fish below the dam.

Jim Dunnigan, a Montana Fish, Wildlife and Parks biologist, said crews that regularly monitored the river found no evidence of fish mortality caused by elevated gas levels.

“We went out on a fairly rigorous schedule just to look for dead fish,” he said, adding that there was a handful of trout and many kokanee that were killed by going through the dam’s turbines or over the spillway.

Harm to the kokanee population in Lake Koocanusa was not a concern because kokanee are so abundant, he said.

Dunnigan said monitoring for gas bubble disease in fish below the dam will continue now that the river is receding and water clarity is improving.

Meanwhile, Idaho Fish and Game has been monitoring the movements of adult sturgeon to determine if the higher flows enticed sturgeon to move into optimal spawning habitat upstream from Bonners Ferry called the Braided Reach.

Four females were in that section prior to the spill. But as of Monday, five days into the spill, and by Thursday, only one other adult sturgeon had been detected in the Braided Reach, Idaho biologist Pete Rust said.

Rust said data has been downloaded from a series of receivers that detect the movements of female sturgeon that are tagged with sonic transmitters as they pass by.

He said data will continue to be downloaded over the next few weeks, so it is too early pass judgment on the spill test.

Interestingly, 13 tagged adults have moved into a section of river right in Bonners Ferry, about a mile from the Braided Reach. And so far this year, 19 tagged sturgeon have moved as far as a stretch just downstream from Bonners Ferry where the riverbed is not adequate for spawning.

“A lot of these tagged fish are getting close, but they just aren’t going up and we don’t understand why,” Rust said.

Cold spring conditions on the river may be having an effect this year, he said, and some of the spawners may move into the Braided Reach in coming weeks as the water temperature warms.

Biologists believe that sturgeon spawning success has been minimal to nonexistent since the construction of Libby Dam 38 years ago. Only about 500 adult sturgeon, an ancient species that can live for decades, are estimated to remain in the Kootenai River.

Reporter Jim Mann may be reached at 758-4407 or at jmann@dailyinterlake.com.