Invasive species take toll on native fish

A low fog hung over Swan Lake and the golden spires of tamarack shot up through the mist on the far shoreline.

On the nearer shoreline, a trawling boat moved slowly, its nets splayed out alongside the boat. Workers on deck reeled in the nets and removed the fish caught it in it.

This is an annual rite on Swan Lake, where each fall thousands of nonnative lake trout are netted out of the lake to make room for native species like bull trout. In a normal year the nets reveal the biology of the lake, with lake trout and bull trout being in the nets. But this year another species was added to the catch: walleye. The netting operation a few weeks ago brought in two walleye, a nonnative species that were introduced to Swan Lake illegally.

Fisheries biologists will now have to deal with the illegal introduction of walleye in Swan Lake, while another illegally introduced species — black crappie — is making a stronghold in Flathead Lake. A recent fishing report from an avid Flathead Lake angler extolled the attributes and the abundance of fishing for black crappie. These fish are growing well in the south end of Flathead Lake, where Polson Bay offers shallow, warm water.

While some anglers enjoy fishing for warmwater species like crappie and bass, these illegally introduced fish create a world of new worries for fisheries biologists. The illegal introductions don’t always take a permanent hold, but when they do — such as pike, bass or perch — they can have long-lasting implications.

Bucket biologists — the term for people who dump species illegally — fly under the radar of the law. “We have no sense of who does it,” Mark Deleray, fisheries manager for Montana Fish, Wildlife and Parks in Kalispell, said. “We have made some cases but it’s very difficult to catch someone in the act of doing it.”

With help from sportsmens groups and organizations like Trout Unlimited, rewards have greatly increased for turning in people who bring in illegal fish. Rewards are as high as $15,000. One person was caught who put perch in Lake Mary Ronan, but that was many years ago, and the legal deterrent was not that high, Deleray said. Perch have now proliferated through Lake Mary Ronan, causing problems for other fish like kokanee salmon. Kokanee are also nonnative, but they are a sport fish that are a managed species.

Deleray said the state manages for two goals: sport fishing and preserving native species. It’s a difficult balancing act. “You can’t do everything everywhere,” he said.

Some of the best fishing on the west side of the Continental Divide, in terms of abundance and sporting, are nonnative species. Pike, bass, lake trout and perch are all nonnative species, but they are managed as a sport fish. Lake Superior whitefish, which are now very abundant in Flathead Lake, were not introduced illegally; they were planted by the government in the 1800s.

“There are advocates for many of these species,” Deleray said. “The problem is we lose the resources that are already there. You can’t have them everywhere. They degrade or compromise the existing fisheries.”

Now that crappie have taken hold in Flathead Lake, it will be difficult to get rid of them, Deleray said. Crappie are shaped like a plate. When they grow to adult size, prey fish have a difficulty foraging on them. Crappie will now compete with the other species for a limited food supply on Flathead Lake. The lake is cold and deep, so it’s “not very productive,” for food, Deleray said. “As you add new species that pie gets cut up into smaller pieces.”

Deleray said he was skeptical about the future of crappie in Flathead Lake. They will do well on the south end, “but most of the lake is not suitable for them,” he said. Eradicating illegally introduced fish is easier on smaller lakes where they can be poisoned and removed. High-mountain lakes in the South Fork of the Flathead River have been treated with a fish toxicant to kill nonnative rainbow trout and Yellowstone cutthroat trout. The lakes have been replanted with native Montana cutthroat trout from the state’s Sekokini Springs hatchery in West Glacier. Only two more lakes remain on that project. “That project has been very successful,” Deleray said.

The walleye that were netted on Swan Lake have been analyzed to see if they were born in the lake or were planted. “In a few months we’ll know,” Deleray said. The other uncertainty is how abundant walleye have become in Swan Lake. “The million dollar question now,” Deleray said, “is ‘will there be a lot of them?”

The netting operation on Swan Lake is in its eighth year. The effort puts out 27 miles of net throughout the fall season. The state has a plan for the species that it manages. The problem with illegal introductions, Deleray said, “is that people who make illegal introductions aren’t working off the same plan. “It’s what they personally want.”Adding new species to the tight food supply tends to homogenize the fishery. “You end up with a lot of mediocre fisheries,” Deleray said.

Lake Mary Ronan is a very productive lake, with perch, salmon, bass and trout. Lake Mary Ronan is also the state’s single source to obtain kokanee salmon eggs for the state hatchery in Somers. Those eggs are grown into juvenile fry, which are planted around Montana. 

“That’s why we get concerned when people starting moving things around,” Deleray said.

Now anglers are finding pike and crappie in there. “If all these species end up establishing themselves, it will be much different than what it is now,” Deleray said. “It may affect our ability to stock salmon.”