Lake Koocanusa fish kill investigated

They don't know how it happened, but officials with Montana Fish, Wildlife and Parks intend to sleuth their way to answers behind a another fish kill on Lake Koocanusa.

"We've seen identical types of kills at least twice before," said Jim Vashro, the department's regional fisheries manager.

The difference this time was a timely report and a quick response that allowed the department's fish health specialist to collect samples of the dead kokanee salmon before they decomposed.

"Obviously, as the fish decompose, any evidence is degraded," Vashro said. "It appears the bulk of the fish died on Sunday and Monday and it was reported late Monday. Our fish health specialist from Great Falls was able to find fish that were still dying on Tuesday, so he got very good samples.

The kill involved "thousands" of kokanee salmon, with some odd circumstances.

"What's interesting is every dead fish is an adult kokanee that was getting ready to spawn in about a month," Vashro said. "No other fish species are affected and no young kokanee are affected, so that's kind of peculiar."

Vashro stressed that the fish kill, which appeared to be concentrated in an 11-mile stretch of water from Libby Dam to the Five-Mile area, amounted to an extremely small percentage of the overall population on the lake. He predicted it would also have very little influence on kokanee reproduction this year.

Vashro said some conclusions can be drawn from the fish kill.

Because it did not impact young kokanee or other species, "that tells us it's not likely a chemical or bacterial" problem, he said.

"There could be a stress factor for the maturing fish, for the onset of spawning," he said. "We're also going to investigate the weather in that area for the last four or five days."

Biologists will try to determine if the fish were "staging" for spawning in particular areas, and whether any weather events could have played a part.

"There are no signs of trauma or infection. The only notable factor is that they all have a distended air bladder," he said, explaining that the dead fish have a "bloated appearance."

That has prompted consideration of upwellings in the lake, but Vashro is suspicious of that theory.

"Fish were popping up over three days. I've never seen an upwelling last that long," he said. "It is a mystery. But luckily, we got in on the ground floor on this one … and that gives us a chance at tracking down the cause.

Laboratory testing on the fish samples could take two weeks to complete, Vashro said.

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