Climate change expedites hybrid trout takeover


When two species mate, their offspring end up with undignified new names like ‘pizzly’ (a grizzly and polar bear pairing) or ‘sparred owl’ (for barred owl and spotted owl hybrids). But the more rare species in such couplings face a far worse fate – hybridization can be a path to extinction.

That’s why hybridization is a major concern for conservationists of the West’s 12 cutthroat trout species. For over a century, rainbow trout native to the Pacific Coast have been enthusiastically planted throughout the U.S., plus every continent besides Antarctica, for sport fishing. In the West, they interbreed with their ruby-jawed cousins to form “cuttbow” hybrids. Those offspring are often not as fit to survive and reproduce. And it turns out that our warming climate is setting the stage for these less adaptable mutts to prosper.

In a new study in the journal Nature Climate Change, a research group led by Clint Muhlfeld, an aquatic ecologist with the U.S. Geological Survey in Glacier National Park, connected warmer temperatures and decreased precipitation to accelerated hybridization of rainbow trout and the Northern Rocky’s Westslope cutthroat trout. Though scientists have long predicted that climate change will increase opportunities for species to hybridize, the new research results are the first to show it.

Westslope cutthroat trout in the Flathead River system, one of their last strongholds. Photo by Jonny Armstrong

The study centers on one of the Westslope cutthroat’s last strongholds, Northwest Montana and British Columbia's relatively pristine Flathead River watershed. Despite the millions of rainbows pumped into the river system before Montana stopped in 1969, and extensive hybridization in a low-elevation site in the 1970s and ‘80s, there was little evidence of rainbow trout taking over in the watershed then. But by the 2000s hybridization had expanded to 52 percent of all the sites sampled in the Flathead.

So what explains the late, yet seemingly sudden cuttbow explosion in the Flathead River region? According to Muhlfeld’s analysis, it’s related to lower May precipitation and higher summer stream temperatures recorded in recent decades. Rainbow trout spawn in the early spring when snowmelt runoff is still low, while cutthroat are adapted to later spawning and high streamflows. So decreased runoff in the droughty early 2000s most likely favored the invasive rainbows and gave them a window for expansion.

“It’s rapid, extreme changes like this that are critical tipping points over which the consequences are irreversible,” says Muhlfeld. That’s because hybridization is an evolutionary one-way street. Once cutthroat populations interbreed with rainbows, there’s no getting the native fish back.

For over 10,000 years Westslope cutthroat trout rode out drought, fire and glaciations in the cold streams, rivers and lakes of the Pacific Northwest. Today, thanks to dams and invasives, unhybridized Westslope cutthroats are confined to mountain headwaters in 10 percent of their former U.S. habitat, and 20 percent of their Canadian range, holding out mostly in Montana and Idaho, plus British Columbia and Alberta. Hybridization spurred on by the warming climate is another blow to the already-dwindling populations.

Yet there’s also a positive fish tale unfolding in the Flathead. State biologists began battling the rainbow invasion in the 2000s, removing rainbow trout and erecting barriers; and it appears to be working. The spread of rainbows and hybridization began slowing in 2004. That’s likely thanks to the state’s efforts, and may be related to cooler temperatures and higher streamflows compared to the early 2000s – a bit of temporary relief that will eventually be swamped by the larger warming trend.

Robert Al-Chokhachy, the Bozeman-based USGS fisheries biologists behind that analysis, says the removal success shows the value of fighting species invasions early on. Perhaps it’s not too late for cutthroats in the Flathead region to escape the dual threats of hybridization and the warming waters. “Our biggest mandate right now,” he says, “is to give fish a chance.”

Sarah Jane Keller is a High Country News correspondent based in Bozeman, Montana. She tweets @sjanekeller. Correction: The original version of this story said that the Flathead River watershed entered Montana and Alberta, but it is actually Montana and British Columbia.