Some of those Yellowstone cutthroat trout have apparently washed over the falls in the last few years, and the resulting hybridization is a cause of concern.

“Right now, we have a real gold mine of native westslope cutthroat trout above Painted Rocks,” Clancy said. “It amounts to very significant population of westslope trout that we would love to maintain for the long term if we can.”

FWP proposes to remove all the fish above the falls this summer with the use of the fish toxin rotenone.

“Our goal is eliminate that source of hybridization,” Clancy said.

No one is really sure how the Yellowstone cutthroat got there in the first place. With the falls as a barrier, Clancy guesses that upper reach of Overwhich was probably fishless before the cutthroat arrived.

“There are no real records of them being stocked up there,” he said. “There are Yellowstone cutthroats in Capri Lake in the upper Warm Springs drainage. It’s logical that whoever put those fish in that lake could have used the Overwhich trail.”

Capri Lake was probably stocked in 1950s or '60s.