Researcher Draws Connection Between Climate Change and Harmful Hybridization of Native Trout

Local scientist Clint Muhlfeld publishes study in prominent academic journal ‘Nature,’ receives international attention

In his published research, aquatics biologist Clint Muhlfeld has detailed the plight of an obscure stonefly endemic to Glacier National Park’s high-elevation streams and revealed how a trout’s ear bone contains a geochemical diary of its liquid migrations.

But his most recent study will appeal to his largest audience yet, not only by virtue of the scope of the revelation, but also the size of the platform.

Muhlfeld, an aquatic ecologist at the U.S. Geological Survey’s Glacier Park field office, is the project leader of a study that links the rapid hybridization between a native Montana trout species and an invasive species in the Flathead River system to climate change.

It was published in the prominent academic journal “Nature,” a lofty benchmark of scientific prestige.

Muhlfeld is thrilled by the prospect that the international scientific community will absorb his research results, but he’s more excited that the general public will learn about the dramatic consequences of climate change already occurring.

“We publish all these papers and scientific literature, but when you get something in ‘Nature,’ which is one of the world’s leading journals, it gets out there to the public and that is what we are trying to do,” he said.

In the past three decades, hybridization between native westslope cutthroat trout and invasive rainbow trout has rapidly accelerated in the Flathead River system, a range-wide stronghold.

The article, published May 25 in “Nature Climate Change” and titled “Invasive Hybridization in a Threatened Species is Accelerated by Climate Change,” was based on 30 years of research by scientists with the U.S. Geological Survey, University of Montana, and Montana Fish, Wildlife and Parks.

Since news of the achievement began to trickle out, Muhlfeld has been fielding requests from international media outlets, and said while the attention has been a bit overwhelming he’s eager for the information to be so widely dispersed.

According to the study, stream temperature warming over the past several decades, coupled with decreases in spring flow over the same time period, contributed to the spread of hybridization between native westslope cutthroat trout and introduced rainbow trout – the world’s most widely introduced invasive fish species, some 20 million of which were planted throughout the Flathead River system in Montana and British Columbia.

Still, hybridization between the rainbows and the native cutthroat trout did not promulgate until the past three decades, when temperatures rose steadily, Muhlfeld said.

“This is the first study of its kind to show that climate warming can promote or accelerate invasive hybridization in nature, and in some species lead to genomic extinction,” Muhlfeld said. “It’s one more consequence of climate change and another example of what we are up against. It’s not all gloom and doom, but it is going to have irreversible consequences when we talk about native species.”

Hybridization has contributed to the decline and extinction of native fish populations worldwide, including all subspecies of cutthroat trout in western North America, which hold ecological and socioeconomic value, Muhlfeld said.

Genetically pure populations of westslope cutthroat trout are known to occupy less than 10 percent of their historical range.

The researchers used long-term genetic monitoring data coupled with high-resolution climate and stream temperature predictions to assess whether climate warming enhances interactions between native and nonnative species through hybridization.

Westslope cutthroat trout and rainbow trout both spawn in the spring and can produce fertile offspring when they interbreed. Over time, a mating population of native and nonnative fish will result in only hybrid individuals with substantially reduced fitness because their genomes are diluted by nonnative genes, which are maladapted to the local environment.

“Since the last Wisconsin Glacial Episode 14,000 years ago, these cutthroat in our system have survived cataclysmic circumstances, they have evolved to survive and adapt to tremendous change, wildfire, stream temperatures warming, glaciation advancing, and when humans get into the mix and start introducing invasive species like rainbow trout, species that aren’t locally adapted, you get an increase in hybridization and a dramatic decline in fitness,” Muhlfeld said. “Genetic diversity and evolutionary potential is what builds resilience in species and allows them to adapt in a changing world. These hybrids don’t have that genetic integrity.”

Historical genetic samples revealed that hybridization between the two fish species was confined to one downstream Flathead River population. However, the study noted, over the past 30 years hybridization rapidly spread upstream, irreversibly reducing the genetic integrity of native westslope cutthroat trout populations.

The rapid increase in hybridization was highly associated with climatic changes in the region, Muhlfeld said, drawing the correlation from data culled between 1978 and 2008, a period during which the rate of warming nearly tripled in the Flathead Basin.

This resulted in earlier spring runoff, lower spring flooding and flows, and warming summer stream temperatures. The locations with the greatest changes in stream flow and temperature experienced the greatest increases in hybridization.

As opposed to cutthroat trout, rainbow trout thrive in these climate-induced changes, and tolerate greater environmental disturbance. These conditions have likely enhanced rainbow trout spawning and population numbers, leading to massive expansion of hybridization with westslope cutthroat trout.

“We saw this rapid increase in hybridization across the entire network of the Flathead Basin,” Muhlfeld said. “We think that hybridization was essentially a time bomb ready to go off under the right set of environmental conditions, and climate change created that perfect storm.”

In addition to Muhlfeld, the study’s other authors are: Ryan Kovach, University of Montana; Leslie Jones, U.S. Geological Survey; Robert Al-Chokhachy, U.S. Geological Survey; Matthew Boyer, Montana Fish, Wildlife and Parks; Robb Leary, Montana Fish, Wildlife and Parks; Winsor Lowe, University of Montana; Gordon Luikart, University of Montana; and Fred Allendorf, University of Montana.

The study was supported by the Great Northern Landscape Conservation Cooperative, the Interior Department’s Northwest Climate Science Center, the National Climate Change and Wildlife Science Center, National Science Foundation, and Bonneville Power Administration.