The latest broadside in the global warming discussion hits close to home. A new study shows that climate disruption and warming has increased the likelihood that native westslope cutthroat trout in the upper Flathead River will breed with introduced rainbows. The result: more hybrid cuttbows and fewer native trout.
Rainbows were introduced into the Flathead more than 100 years ago, but the rate of hybridization has increased significantly in the last 30 years. The study suggests why. Rainbows prefer to spawn earlier in the spring than cutthroat, but climate change has compressed snowmelt and runoff into a shorter period, leading to more commingling on the redds.
A cuttbow is fine sport on the end of your line, but it’s a lost fish when it comes to genetic diversity. To make matters worse, these hybrids can form zombie-like swarms that swamp aquatic systems, pushing native fish closer to the brink.
We’re lucky that in the upper Flathead many parts of our native fisheries remain intact despite multiple threats. Bull trout still run from the mainstem as far north as the Canadian North Fork on epic spawning migrations. And anglers still catch westslope cutthroat, often dozens in a summer day. They may not be the biggest fish, but they are plentiful and eager to eat dry flies.
I’ve assumed the cutthroat in the Flathead were sheltered relics, protected by barrier falls that prevented later arriving, marauding rainbows from taking over the joint. If you buy that theory, the hybrid cuttbow swarms that now threaten native populations are just a human-created version of what would have happened if the falls had been low enough to allow rainbows to migrate upstream.
Maybe. It’s easy to find examples of rainbow trout “dominance” all across Montana where we’ve stocked the fish on top of native populations. The introduced fish take over, while the native cutthroat are relegated to remnant populations in the headwaters.
But Clint Muhlfeld, the U.S. Geological Survey biologist who authored the climate study published in the journal “Nature,” reminded me that stocked rainbows are not the same fish as those that might have migrated to the Flathead if the downstream geology had been different. There remain examples where wild rainbows and cutthroats coevolved and persist as separate species occupying different niches in the same habitat.
Those zombie swarms are likely a symptom of farming rainbow trout. Hatchery managers select for different characteristics in fish than nature does. We raise trout that are comfortable living in close quarters, eating feed pellets, and staying alive long enough to be caught despite being stocked into a variety of habitat types. Raise a few generations selecting for these characteristics and you’ve got something very different from a wild fish. It may still look like a rainbow, but its not going to act like its wild ancestors when it’s unleashed on an aquatic ecosystem.
If rainbows had migrated to the Flathead on their own there probably would have been some sorting out between that species and cutthroats — at least that’s what happened elsewhere. In this alternate biological universe, the first European anglers to wet a line in the Flathead might have found both fish, either living side-by-side or occupying different portions of the watershed. The hybrid cuttbows sometimes common in rivers where rainbows have been introduced aren’t the usual byproduct of wild fish interactions.
I’m not resigned to defeat and the loss of our native cutthroat. Rainbows may yet take over the upper Flathead, but I don’t think it’s inevitable and there are steps we can take to save native fish. One promising approach: identifying where rainbows spawn and capturing them before they can reproduce.
But the bigger message is that Flathead cutthroat are just another canary reminding us its time to get our act together on climate change. Most of the things we need to do to slow human-caused warming of our planet — such as using less fossil fuel, embracing wind and solar power, walking more and driving less — are things we should be doing anyway.
It turns out that keeping the planet habitable for species also has side benefits for anglers.
This is the first of three columns on protecting Montana’s native trout.
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