The Environmental Protection Agency stopped the drawdown two weeks ago after rainbow trout fingerlings in two monitoring cages downstream from the dam went belly up.
Montana Fish, Wildlife and Parks sent fish collected after the event to the U.S. Fish and Wildlife Service's Fish Health Center in Bozeman for a histological examination. The exam showed deterioration to internal organs consistent with poor water quality.
It's likely wild trout were impacted as well by the event, said Montana Fish, Wildlife and Parks Biologist David Schmetterling.
Schmetterling is the lead state biologist studying the impacts of the Milltown Dam removal. As part of that study, Schmetterling implanted 40 wild trout just below the dam with radio transmitters earlier this year in an effort to see how the fish would respond during the reservoir drawdown.
About 30 percent of those fish died during the drawdown - most several days before the caged smaller fish succumbed.
“That's not what I expected to happen,” Schmetterling said.
Normally, Schmetterling said there's very little mortality of fish implanted with a transmitter. The surgery occurred well ahead of the drawdown and it's unlikely it impacted the trout's survival rates, he said.
“Zero mortality is the norm,” Schmetterling said.
It appears wild trout were more sensitive to environmental changes than the smaller fingerlings placed in the cages, he said.
The residual water quality in that stretch of the river may be to blame.
The trout fingerlings placed in the cages didn't fare well despite the fact they were fed on a regular basis and died with stomachs full of insects.
The fingerlings had no fatty deposits or muscle mass which indicates systemic stress and poor condition, Schmetterling said.
The wild trout implanted with radio transmitters didn't move downstream following the drawdown.
Officials expected tons of sediment in the reservoir to flush downstream during the drawdown, but lower than anticipated river levels kept sediment levels relatively low.
“The drawdown was ineffectual in that regard,” Schmetterling said. “It scoured less than 10 percent of what was expected.”
By mid-June the river had dropped to levels normally seen in mid-July and water temperatures spiked near the 70-degree mark.
“We didn't see a tremendous amount of sediment. We didn't see an increase of metals in the water Š we did see mortality,” Schmetterling said. “Water temperatures were higher than normal and that may have been the ultimate cause. Š Unfortunately, there's no one obvious answer. It may have been a culmination of different stresses.”
The final impact to wild trout won't be revealed until next spring when Schmetterling carries out annual population studies. The reach below the dam normally holds anywhere between 300 to 700 trout over 7-inches long.
“That's a pretty low number and is probably due to a number of factors, including water quality and the fact the dam limits migration,” Schmetterling said.
Forba said he expects scouring will continue over the next few years mostly from the Blackfoot River. The EPA will do what it can to limit the impact.
After the drawdown was stopped two weeks ago, the water level in the reservoir was raised 6 inches to slow any scouring, Forba said. There was a drop in the turbidity of the river downstream from the dam, he said.
“We're going to do what we can to keep the stresses down,” he said. “These high temperatures are hard on fish and not just on the Clark Fork River. We knew we would have some impact on aquatic life. We're going to try and minimize that.”
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