Native westslope cutthroat trout and bull trout stranded in a pool in Ole Creek in September 2012 due to the low water levels. Courtesy Jonny Armstrong | USGS

By TRISTAN SCOTT for the Flathead Beacon

To anglers, trout streams are characterized by free-flowing water that’s clear and, most importantly, cold.

But in the years ahead, the chilly streams that sustain Northwest Montana’s prized native fish could grow increasingly tepid as a result of rising global temperatures. This summer, which has been marked by historic high temperatures and record-low flows due to a scant snowpack and an early spring runoff, researchers say trout will endure stresses that could adversely affect habitat, genetic biodiversity and migratory patterns, while promoting hybridization with nonnative species.

For anglers, the specter of state-imposed fishing restrictions could shut down the cherished Montana pastime on rivers and streams throughout the Crown of the Continent Ecosystem. So-called “hoot-owl” restrictions went into effect July 3 on the Bitterroot, Blackfoot and Clark Fork rivers.

Water temperature strongly regulates the distribution, abundance and physiology of stream-dwelling fish. This is particularly true for cold-water species like bull trout and westslope cutthroat, which are ectothermic (cold-blooded) and constrained to stream and lake environments, said Clint Muhlfeld, a Flathead-based aquatic ecologist with the U.S. Geological Survey in Glacier National Park.

“Native bull trout and westslope cutthroat trout have among the coldest temperature requirements of North American trout, so increases in stream temperatures and reductions in flow like the region is already experiencing this summer can be very stressful to these prized fish,” Muhlfeld said.

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