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FWP proposes non-native fish kill above Overwhich Falls

FWP is looking for public comment on a plan to use toxins to remove non-native and hybrid cutthroat trout from Overwhich stream above the falls (pictured here). The falls are located about 15 miles up Overwhich Creek from where it flows into the West Fork of the Bitterroot River.

By Michael Howell

Montana Fish, Wildlife & Parks (FWP) has recently released an Environmental Assessment (EA) for public review of its proposal to remove non-native and hybridized cutthroat trout from the upper portion of Overwhich Creek and its tributaries in the southwest Bitterroot Valley, above Painted Rocks Dam.
According to FWP fisheries biologist Chris Clancy, Westslope Cutthroat Trout are native to Montana west of the Continental Divide and in the Upper Missouri drainage to the east. But in the Yellowstone drainage, Yellowstone Cutthroat Trout are a genetically distinct subspecies and not native to the Bitterroot watershed. Under the proposal, Yellowstone and hybridized cutthroats would be removed from a ten-mile stretch in the upper reaches of the creek immediately above Overwhich Falls. The falls are located in a remote area about 15 miles upstream of where Overwhich Creek flows into the West Fork of the Bitterroot River.
The aim of the project is to protect the large population of genetically pure Westslope Cutthroat Trout that inhabit most of the streams above Painted Rocks Reservoir from hybridization with the Yellowstone Cutthroat that occupy the far reaches of Overwhich Creek above the falls.
One reason this is important, says Clancy, is that it is one of the largest documented repositories of native Westslope Cutthroat Trout in the state. But aside from that it also has a good chance of resisting hybridization in the future due in large part to the dam that prevents passage of fish upstream. The greatest threat to that population of pure Westslope Cutthroat Trout right now is the population of Yellowstone Cutthroat Trout and hybridized trout in the upper reaches of the Overwhich that may find their way downstream.
“It’s not often that you find large intact populations of pure Westslope Cutthroat Trout that have a good chance of staying that way, with little threat from non-natives,” Clancy said. “Removing non-native cutthroats from about 10 miles of water gives the basin a unique opportunity to maintain native cutthroats.”
According to Clancy, the upper portions of Overwhich Creek were probably fishless historically. He said that historically fish migrated into high lakes from the river and could not have migrated up Overwhich Falls. This is not an uncommon phenomenon on the forest. He surmised that the most likely scenario was that the fish were carried in by the bucket load on the way to Capri Lake. Stocking records indicate that Overwhich Creek was stocked with undesignated cutthroat trout between 1931 and 1954. The records do not indicate the exact location of the stocking. Capri Lake, which is about one mile from Overwhich Falls but is in the Warm Springs Creek drainage, was stocked with Yellowstone Cutthroat Trout in 1967, as well as undesignated Cutthroat Trout in 1952 and Westslope Cutthroat Trout in 1979. It is possible that some fish were also stocked in Overwhich Creek upstream of Overwhich Falls at the same time because Capri Lake is most easily accessed from the Forest Service trail that parallels Overwhich Creek upstream of the falls.
The aim, according to the EA, would be to use the fish toxin rotenone to exterminate the fish population above the falls. From the falls to the headwaters of Overwhich Creek is about four stream miles. Portions of Colter and Shields Creeks, tributaries to Overwhich, would also be treated. Each of these streams is between two and three miles long.
No fish would be re-stocked above falls after the removal is complete unless all the fish cannot be removed. In that case, FWP may stock pure strain Westslope Cutthroat Trout from nearby streams to “swamp” the remaining Yellowstone Cutthroat Trout that manage to survive the piscicide treatment.
Complicating the project is the presence of Bull Trout, an endangered species, in the stream below the falls. FWP recognizes that there is some likelihood that some Bull Trout may be killed and is working with the U.S. Fish and Wildlife Service to mitigate that potential impact in a few was, including moving the toxin drip stations upstream from the falls so that the estimated reach of impact below the falls will be minimized. The waters below the falls will also be electro-shocked before the toxin is used and any Bull Trout found will be relocated further downstream. People working with nets will also patrol the stream below the falls after the toxin is released and pluck any Bull Trout found floating. If picked up in time and placed in fresh water the fish can be resuscitated and survive.
Clancy said that FWP will have a professional team come in to do the poisoning. Dye is used in the water to determine how far the toxin reaches before becoming diluted and ineffective. This information is used to locate the site for the drip stations. The toxin is only active for about four hours and the entire project will be completed in one day. But it does take more than one treatment. Small fry, many no bigger than an inch, often hang out near the banks and in backwaters where the toxin does not often mix well, allowing many of the small fish to live. By coming back the following year and repeating the process, those fish are generally killed in the second application.
According to Clancy, the use of toxin to remove certain populations of fish is not common in these parts. This will be the first time on the Bitterroot National Forest. But the process has been used successfully in 103 places around the state.
Following the use of the toxin, which does break down naturally, an oxidizing agent, potassium permanganate, can also be added to the water to detoxify the rotenone more rapidly.
Although the toxin may kill animals other than fish, such as tadpoles, and invertebrates, these creatures are expected to readily re-colonize the area from upstream after the poisoning event.
Clancy said that some effort will be made to save some of these amphibians and invertebrates in the affected portion of the stream by placing them in buckets of fresh water overnight and releasing them the next day.
“No body likes killing fish,” said Clancy, “and the tadpoles and invertebrates are collateral damage. So we do this to make ourselves feel a little bit better.” He said it was like the guy picking up starfish on the beach and throwing them back into the ocean.
“Somebody walks up to the guy and says what he is doing is futile since there are so many starfish and they will probably just wash back up again after being thrown in. He tells the guy that what he is doing won’t make any difference. But the guy just throws another one back, and says, ‘It makes a difference to that one’.”
The EA can be found online at, under “News,” and “Recent Public Notices”; by mail from Region 2 FWP, 3201 Spurgin Road, Missoula 59804; phone 406-542-5540; or email
A public hearing is scheduled in Hamilton on Tuesday, May 23, at 7 p.m. at the Bitterroot National Forest Headquarters, 1801 North First Street, to discuss the proposal, answer questions, and take public comment.
Comments may also be submitted online or directed to Sharon Rose by mail to the address above, phone to 406-542-5540, or email to by 5 p.m. on June 9.

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