Scientists: Climate Influencing Global Trout Populations

Researchers based in Glacier National Park publish first global review of trout ecology and impact of climate change

By Tristan Scott
Native westslope cutthroat trout and bull trout stranded in a pool in Glacier National Park's Ole Creek. Courtesy Jonny Armstrong | USGS

Temperature-sensitive trout thrive in water that is cold, clear and abundant – not exactly groundbreaking news. But a recent study tracking the relationship between warming climes and the adverse effects on global trout populations is the first to establish a scientific connection.

A team of researchers with the U.S. Geological Survey – including two researchers based at Glacier National Park – found that climate directly and consistently influences trout populations worldwide, and published their finding recently in the international quarterly journal “Reviews in Fish Biology and Fisheries.”

In the study, titled “Impacts of Climatic Variation on Trout: A Global Synthesis and Path Forward,” lead author Ryan Kovach and his colleagues provide the first global synthesis of trout responses to climate change over time. Despite the economic, cultural, and ecological value of trout, Kovach said long-term data comparing the health of trout populations to changes in streamflow and water temperature are surprisingly limited.

Still, several key patterns emerged from the existing data. In particular, lower summer streamflows were associated with reduced growth, survival, and abundance across trout species, locations, age classes, and even continents. This pattern is concerning, Kovach said, given the trend of diminished winter snowpack and subsequent summer streamflows throughout the western United States and elsewhere.

The team conducted the synthesis by tracking down and analyzing an exhaustive catalogue of trout ecology studies from all over North America, as well as in Spain, France, Norway, and the United Kingdom. They identified 42 studies in all that met their criteria, which reveals a dearth of research into how trout will fare in a warming world.

Kovach, who is based at the USGS Northern Rocky Mountain Science Center in West Glacier, said the relatively narrow scope of the available information reveals data limitations, underscoring the need to implement additional long-term monitoring programs.

But the trends that emerged from the data are clear – freshwater trout populations across the globe are healthier when the water is cold and the streamflows high.

“Any kid with a goldfish can tell you that fish need water, but it is pretty interesting to see such consistent patterns in our global review,” Kovach said. “Regardless of location, species and age class, there was a consistent correlation between years with higher streamflows and populations of trout that were stronger and more abundant.”

Climate change may be especially problematic for trout because they require cold-water habitats, which are increasingly fragmented by human development, and management actions like habitat restoration should be taken.

But water temperature and streamflow are the most powerful predictors of fish survival, Kovach said.

“The key message is that climate does have a strong influence on trout populations, and this is something we have observed over time. We’ve seen that year-to-year variations in streamflow and temperature can have an impact on trout populations. This highlights that climate and climate change is not speculative, it is realized. It is something that we need to confront now in terms of natural resource management, but also from a societal perspective.”

Clint Muhlfeld, a USGS aquatic research ecologist based in West Glacier, was a co-author on the study. The study’s other co-authors are Robert Al-Chokhachy, Jason Dunham, Benjamin Letcher and Jeffrey Kershner.

“This is the first global review of climate change impacts on freshwater trout, with interesting results and implications in Montana and beyond,” Muhlfeld said. “I think this reflects our current understanding of these impacts based on real data, which is surprisingly limited, but strong patterns were found that will shape future research and conservation worldwide.”

The study emphasizes that climate change will pose challenges for trout worldwide, and the best path forward will be defined by coordinated efforts that combine greater research, data collection and analyses with broad-scale policy and local management decision-making, “an approach clearly lacking in current research and management efforts.”