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Montana fly fishing guide and shop owner Jason Lanier hooks a feisty rainbow trout almost every day he hits the waters in the lower valley of the Flathead River system. From an angler's perspective, the catch is a thrill. Rainbows put up a good fight, much better than the one offered by the state's native westslope cutthroat trout.
"And cutthroats that have some rainbow genetics in them typically fight harder for sure," the owner of the Bigfork Anglers Fly Shop told NBC News.
About 20 million rainbows were stocked in the river system that spans Montana and southern British Columbia, Canada, from the late 1800s to 1969. The fish can, and do, mate with cutthroats. This hybridization may drive the genetically pure natives to extinction, according to Clint Muhlfeld, a research ecologist with the U.S. Geological Survey in West Glacier, Mont.
What's more, climate change is accelerating the hybridization process, according to new research led by Muhlfeld. "This is the first example we are aware of that has shown how invasive hybridization has probably spread due to climate warming," he told NBC News.
Time bomb waiting to go off
Relative to cutthroat trout, rainbows prefer warmer waters with lower spring flows and earlier spring runoff. Over most of the past century, these preferences confined rainbow populations to the lower portions of the Flathead River system that are sheltered from the spring flooding and chilly waters that typically prevailed elsewhere.
As a result, historical genetic samples revealed, hybridization between the two fish was largely restricted to one downstream Flathead River population — a population that Muhlfeld called "a time bomb waiting to go off given the right environmental conditions." Those conditions appear to have arrived in the form of drought in the early 2000s, which produced lower spring flows and warmer waters.
These conditions "allowed rainbow trout to grow and survive really well and they just kind of massively expanded and spread out everywhere and corrupted the native genomes that had evolved over millennia," he said.
According to the paper published Sunday in the journal Nature Climate Change, between 1978 and 2008, the rate of warming nearly tripled in the Flathead basin. Muhlfeld suspects the extreme years — the drought years in the early 2000s, for example — are likely responsible for most of the rainbow expansion.
If fisheries managers allow the rainbow trout populations to persist, "over time you're going to lose the native cutthroat trout," Muhlfeld said. "The writing is on the wall."
His earlier research found that hybrids with as little as 20 percent rainbow DNA are about 50 percent less fit than genetically pure cutthroats. In the natural world where survival of the fittest rules, "This has important consequences," he added. "If our native fish are mostly hybridized and their fitness is lower, then their potential to be resilient and adapt in the face of climate change is going to be reduced."
As part of a bid to reduce the threat of hybridization, Montana Fish, Wildlife and Parks is more than a decade into a program of intentional removal of rainbow and hybrid trout from portions of the Flathead River system.
According to Lanier, the fishing guide and fly shop owner, the program is controversial. Removal of the rainbow and hybrids means there are fewer fish to catch, at least for the short term. "You have to take into consideration the fishery and the economics," he said.
'Wild trout' fisheries
Most people that come to Montana to fish do so to catch wild trout — a trout that reproduced naturally in one of the state's idyllic waterways. Introduced rainbow fit this definition. Many of the state's rivers including the Madison are managed to promote rainbow populations for the benefit of anglers and the tourism revenue they generate.
But continued hybridization, too, could have economically damaging consequences, according to Bruce Farling, the executive director of Trout Unlimited Montana, a conservation organization that supports efforts to conserve and expand the native populations.
"People do come here to fish for westslope cutthroat trout," he told NBC News. "They are a little easier to catch, so that is one of the attractions for them. Besides that, there is a cultural legacy issue here. These are the fish that we inherited and these are the first fish that a lot of us caught as kids and learned how to fish with."
Perhaps the compromise, Lanier said, is to focus westslope cutthroat conservation efforts on streams that have already been plugged with dams to prevent the spread of invasive rainbows. Elsewhere, he said, "we are just beyond the tipping point where we have to manage it as a fishery rather than trying to preserve the westslope cutthroat."