Native trout in hot water

Scientists say that on top of habitat loss, invasive species and habitat damaging silt deposits from roads and dams, western Montana's native fish are facing yet another obstacle: climate change.

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A recent study published by researchers at the Northern Rocky Mountain Science Center examined a century's worth of air temperature data and discovered that the lower portion of the Flathead River Basin could soon be too hot for native fish to survive. Fisheries Ecologist Clint Muhlfeld says the data shows that northwestern Montana's average temperatures are tracking with global trends, but increasing at a faster rate. Additionally, the data shows a three-fold increase in hot summer days and a loss of roughly one month of cold winter days. That lost month is divided between the start and end of winter, and the accompanying precipitation that once came as snow now comes as rain.

"The warming air temperature trends are causing the snowpack levels to be reduced and to run off earlier," says Muhlfeld. "That means decreased base flows in the summertime, and that leads to higher water temperatures."

Fish rely on the water around them to regulate their body temperature, but cutthroat and bull trout in particular are sensitive to those changes because they've evolved over thousands of years to survive in water below 55 degrees. For invasive species like brook, rainbow and brown trout—which aren't as sensitive—warm water is an opportunity to move in and outcompete the native fish, hybridize with them and even drive them from their habitat.

"On the southern limit of their habitats their range has contracted ... especially in the Bitterroot and Rock Creek," says Muhlfeld. "As the temperature warms up we can expect [native] trout populations to retreat to the headwaters streams—the coldest environments."

Bull trout are kind of like the canary in the coal mine. They have the coldest water requirements of any fish in the northwest, so their presence—or absence—is indicative of the water system's temperature.

Muhfeld says the recent research gives fisheries managers an opportunity to consider long-term plans if they want to protect native trout. But he warns that the managers don't have much time.

"Now's the time to act," he says. "The history, diversity and distribution is critical now as these trout face the challenge of a warming climate."

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