Metals Blamed For Trout Deaths

Heavy metals that surged down the Clark Fork River from Milltown Dam during an ice floe and flood last February may have decimated rainbow and brown trout populations, fisheries biologists said.

It’s estimated the number of catchable-sized rainbow trout dropped by an estimated 62 percent from 1995 to 1996 and the brown trout population is down an estimated 56 percent. And a report by biologists Dennis Workman and Rod Berg of the Montana Department of Fish, Wildlife and Parks said juvenile trout took an even greater hit, with estimates showing a 71 percent decrease in the population of young rainbows and brown trout down 86 percent.

“I have never seen a one-year decline like this on any of the big rivers in Montana: the Clark Fork, Yellowstone or Missouri,” said Berg, who has used electrofishing to estimate trout populations for the past 23 years.

“It is obvious that a fish kill occurred between June of 1995 and June of 1996,” said Berg, noting he had trouble finding the fish needed to make a sound population estimate.

And the fish Berg and his crew did find were in poor health, when compared to fish shocked, measured and released in years past. “They were skinny,” Workman said. “They weren’t as lively.”

Workman, the lead fisheries biologist in the region, said he and Berg considered drought, angler harvest, ice scouring and the release of metals-polluted sediments from Milltown Reservoir as possible explanations for the population loss.

All were eliminated except the metals-laden downwash.

Water samples taken by the U.S. Geological Survey and the Missoula City-County Health Department showed total recoverable copper of 400 parts per billion below Milltown Dam on Feb. 9, just before an ice jam pushed down the Blackfoot River and forced the dam to open wide its flood gate.

The state standard, intended to protect aquatic life, is 18 parts per billion.

On Feb. 10, samples taken downriver showed copper levels of 630 to 770 parts per billion. Zinc hit 1,140 to 1,310 parts per billion. And arsenic jumped to 97 parts per billion.

Milltown Reservoir is a federal Superfund cleanup site because of the metals that washed down Silver Bow Creek and into the Clark Fork River during a century of copper mining and smelting in Butte and Anaconda.

The problem in explaining the drop in trout numbers, Workman said, is that there was no observable fish kill during or after February’s flood. “We don’t have any physical evidence,” he said. “We don’t have the smoking gun - the dead fish.”

With the reservoir releasing 14,000 cubic feet per second of chocolate-colored, iceberg-loaded water, the dead fish likely were washed downstream in a hurry - and without notice, Workman said.

Workman said he hopes Superfund managers and state officials learn something from the dieoff and use it in selecting a cleanup plan for Milltown Reservoir. It also should be considered in assessing a request by Atlantic Richfield Co. to weaken the standard for copper in the upper Clark Fork, he said.