Study: climate change has boosted fish cross-breeding

A recently published study asserts that climate warming accelerated hybrid breeding among native and non-native trout in the Flathead River Basin over the last 30 years, one of the first looks into how changing climatic conditions can have “evolutionary consequences.”

The study, published in the latest issue of Nature Climate Change, was led by Clint Muhlfeld, a research assistant professor with the University of Montana and a research ecologist with the U.S. Geological Survey based in Glacier National Park.

It was based on 30 years of research by scientists with UM, the Flathead Lake Biological Station and the Geological Survey.

The researchers used genetic monitoring data stretching back to the late 1970s, coupled with stream and climate temperature data.

Muhlfeld said roughly 20 million rainbow trout were stocked in the upper Flathead River system from the late 1800s to 1969. Rainbows existed in the river system along with native westslope cutthroat for most of that period without evidence of hybridization.

That started to change about 30 years ago.

“Over 30 years, we saw hybridization spread rapidly,” Muhlfeld said. “This was primarily driven by periods of reduced precipitation and increasing trends in stream temperatures.”

The research indicates that rainbow trout initially were held in check from hybrid breeding with cutthroats. Rainbows prefer to spawn in the early spring in ascending stream flows with lower temperatures while cutthroat tend to spawn later in the spring in descending stream flows.

But those conditions began to merge.

While Northwest Montana does have cooler weather than other parts of the country, the rate of increased air temperatures in the region tripled over the last 30 years relative to the rate of global average air temperature increases, Muhlfeld explained.

He emphasized that periods of drought over the last three decades, along with higher temperature trends, that triggered the conditions for rainbows and cutthroats in the river to interbreed in the spring, particularly at one well-documented major source: Abbot Creek in the Coram area just southwest of Glacier Park.

While the study on climate and hybrid breeding by itself may give the impression that hybridization is proliferating in the Flathead River system, that is not the case, largely because of rainbow trout suppression efforts led by Montana Fish, Wildlife and Parks, Muhlfeld said.

Starting with fish trapping and electro-fishing on Abbot Creek in 2000, the suppression effort was expanded to four other known rainbow spawning streams in the same part of the river system.

New research evaluating those efforts shows that efforts to suppress hybridization have been promising, Muhlfeld said.

“What we’re seeing is a stabilization and actually a significant reduction in the number of adult rainbows and hybrid rainbows in the system over time,” he said. “We know climate plays a role, but we also know that the suppression efforts seem to be working.”

Context matters, Muhlfeld added.

While suppression is proving to be worthwhile to preserve a relatively abundant population of westslope cutthroat in the Flathead, rivers east of the Continental Divide have cutthroat populations that have almost been entirely displaced by non-native species.

“Where they are nearly completely eliminated, that’s probably not where you want to put a lot of time and effort and money into those situations,” he said.

A significant point of the recently published research is that changing climatic conditions can influence hybridization.

“The evolutionary consequences of climate change are one of our greatest areas of uncertainty because empirical data addressing the issue are extraordinarily rare,” said Ryan Kovach, a post-doctoral scholar at the Flathead Lake Biological Station and one of Muhfeld’s co-authors. “This study is a tremendous step forward in our understanding of how climate change can influence evolutionary process and ultimately species biodiversity.”

Or as Muhlfeld explains it, westslope cutthroat trout have survived thousands of years in the region from glacial changes, catastrophic wildfires, flooding and a changing climate.

Research that came out in 2009 found that hybrid rainbow-cutthroat are considerably less capable.

“We know hybridization rapidly decreases fitness — their ability to reproduce and survive,” Muhlfeld said. “Fitness in a fish isn’t necessarily measured by what’s at the end of your fly line or how hard they fight.”

Online:

The article can be viewed online at http://dx.doi.org/10.1038/nclimate2252.

Reporter Jim Mann may be reached at 758-4407 or by email at jmann@dailyinterlake.com.