Beneath the surface of Horseshoe Lake west of Kalispell, a small force of planted predators has been going to work on unwanted nuisance fish.

The lake was stocked with 500 6-inch tiger muskellunge in the fall of 2006, and another 500 fish averaging nearly 9 inches in May 2007.

Since then, the only tiger muskies in Montana west of the Continental Divide have been quietly growing to a size where they can impact infestations of northern pike minnows and suckers.

"The initial indications are that they are having the kind of impact we were expecting," said Mike Hensler, a state fisheries management biologist.

Montana Fish, Wildlife and Parks established the experiment population to be a biological control and to have a unique warm-water trophy fishery in Western Montana.

It was a controversial pursuit, with the state hearing from many anglers who were concerned about the potential proliferation of an exotic predator fish into waters beyond Horseshoe Lake.

But Montana Fish, Wildlife and Parks officials felt confident in proceeding with the planting project largely because the tiger muskies are functionally sterile. Their numbers depend on stocking, and it appears the Horseshoe Lake tiger-muskie population won't be getting reinforcements anytime soon.

That's because a serious disease - viral hemorrhagic septicemia - turned up in Midwest waters, prompting Western states to establish a quarantine on all tiger muskie eggs and fish from Midwestern hatcheries.

"What happened is we lost our brood stock source in Wisconsin," said Jim Vashro, the state's fisheries manager for Northwest Montana. And there really aren't any other sources.

Some Western states have explored the potential for starting their own tiger-muskie brood stocks, but disease issues have gotten in the way, Vashro said.

So the 1,000 tiger muskies planted at Horseshoe Lake are on their own, but they have been growing rapidly and they have the protection of a fishing regulation that requires catch and release of all tiger muskies under 40 inches.

Limited gill netting in spring 2008 turned up 11 tiger muskies ranging from 15 to 23 inches long.

"I think at that length they should start to have an impact" on abundant populations of northern pike minnows and suckers, Hensler said.

Netting results are showing hints of an impact. The average length of northern pike minnows and suckers has increased, indicating that tiger muskies are "cropping off the smaller fish" in the 159-acre lake.

Anglers already have caught tiger muskies, a species that can be difficult to hook into.

"I know that people have gone out there and fished for them," Hensler said. "One guy called and said he caught one."

Hensler predicts that angler interest in the lake's tiger muskies will pick up once the fish get in the 36-inch range. Like northern pike, they can eventually exceed 40 inches.

A 49-inch, 35-pound tiger muskie that was caught in an Eastern Montana lake earlier this month would have set a state record, but the angler was disqualified because he violated fishing regulations.

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