Thousands of illegally introduced northern pike will end up high and dry in Milltown Reservoir during the next two weeks.
The plan calls for the reservoir level to be lowered one foot a day for the first four days, and then six inches a day for the next four days, according to David Schmetterling, a fisheries biologist for Montana Fish, Wildlife and Parks.
That would bring the reservoir level down six feet in eight days. The drawdown could stop short of that maximum depth, however, if biologists determine that the desired effect on the pike population has been reached beforehand.
The goal, Schmetterling said, is to kill young-of-the-year and 1-year-old pike in Milltown's extensive shallow backwaters, where they will be stranded when the water level drops.
That's exactly what happened in late August of 1999, when the reservoir was drawn down six feet for repairs, according to Schmetterling. Monitoring pike populations in the reservoir since then, FWP biologists found that young-of-the-year and 1-year-old pike from that year are absent.
Once the drawdown is complete, the reservoir will be slowly refilled at the same rate, which will be completed by Labor Day. During the drawdown, and until the reservoir is refilled, it will be closed to recreation.
The closure is a safety precaution, Schmetterling said. During the 1999 drawdown, anglers fishing from boats in the reservoir approached dangerously close to the dam, he said. The deep mud exposed by the drawdown, and sloughing banks, also pose a hazard. And contact with the reservoir's toxic sediments could be a health risk.
During the entire drawdown process, Montana Power will monitor water quality upstream in the Blackfoot and Clark Fork rivers, as well as immediately below the dam in the Clark Fork to make sure that toxic sediments in the reservoir aren't flushed downstream.
"That's a big concern," said Schmetterling. "The sediments make all management of the reservoir very difficult. The concern is that in doing these manipulations we don't want to damage the habitat downstream."
The gradual drawdown also should have minimal effects on downstream flows.
Some of the young pike displaced by the drawdown probably will be washed through the dam into the river below, he said, which will also have the desired effect of reducing the pike population in the reservoir.
The survival rate of those small pike isn't expected to be high in the river, Schmetterling added, because the shallow rearing habitat they require is not plentiful in the river.
In addition to meeting their fate by stranding in the shallows, or being washed through the dam, many of the small pike displaced by the drawdown will simply be eaten by larger pike.
The drawdown probably won't have much effect on adult pike in the reservoir, which will find sanctuary in the two deep-water channels formed by the two rivers. The reservoir is teeming with thousands of adult pike, averaging 2 1/2 pounds, Schmetterling said.
Angling pressure, which has increased substantially in the past couple of years, can help reduce the population of adult pike in the reservoir, he said. But by itself, he added, angling won't be enough to eradicate the alien predators.
Starting next year, Schmetterling said, FWP plans to begin trapping adult pike to further slice pike numbers.
During the drawdown, FWP biologists will monitor its progress by looking for dead fish in the exposed shallows, and by using gill nets. Next year, biologists will gauge the effectiveness of the drawdown by surveying numbers of young pike remaining in the population.
If the drawdown is successful, Schmetterling said, it will become an annual event.
"This is the kind of thing that needs to happen every year," he said. "So there isn't a continued supply of fish into the population. Pike are long-lived, so we'll have to do it for a long time to eradicate them. It's just like controlling weeds. We need to use a variety of management tools to get a handle on the pike situation, which has been out of control for several years."
"There are no easy answers," Schmetterling added. "If there were, we probably would have gotten rid of the pike already."
The hoards of pike in Milltown reservoir compound an already complex problem for native and game fisheries of the Clark Fork River system caused by the presence of Milltown Dam.
For the past several years, Schmetterling has studied the dam's effects on fisheries. While fish can move downstream over the dam, their upstream spawning migrations are obstructed. Thousands of fish of all species futilely attempt that migration every year.
Cutthroat and bull trout that Schmetterling has transported upstream over the dam and implanted with radio transmitters have traveled many miles upstream to spawn in the streams of their origin.
A passage for fish, from the lower Clark Fork to their historic spawning tributaries on the Blackfoot and upper Clark Fork, is considered essential for restoring the endangered bull trout, cutthroat and other native fish populations in the drainage.
But any native fish now trying to negotiate the waters of Milltown Reservoir is at grave risk of being devoured by pike that have found ideal habitat in its warm, shallow waters. In his studies, most of the bull trout Schmetterling has found in the reservoir have been in the stomachs of pike.
"The pike threat changes the way we view fish passage," he said.
And the pike also have an effect on FWP's other fisheries projects in the drainage.
"There's been so much hard work put in to protect fish and wildlife resources in this area," said Schmetterling. "All the restoration projects done in the Blackfoot, and cutthroats and bull trout transported over the dam, just mean more food for pike. And one illegal introduction of pike has the ability to tremendously undermine all the work done improving water quality and quality of habitat."
Reporter Daryl Gadbow can be reached at 523-5264 or at dgadbow@missoulian.com.
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