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Warming’s impact: Biologists study data on cutthroat trout and water temperatures

Fisheries biologists are looking at data suggesting native cutthroat trout are being stressed by warmer water temperatures. The findings have prompted FWP to consider changing the temperature which triggers a shutdown of fishing on the Bitterroot.

Two years of finding dead cutthroat trout in the upper reaches of the Bitterroot River during the hottest part of summer have biologists rethinking strategies to protect the native species during low water years.

For the last two summers, Montana Fish, Wildlife and Parks fisheries biologist Chris Clancy has floated the lower three miles of the West Fork and 12 miles of the mainstem to specifically look for dead trout.

The floats started after the department received reports from anglers of dead fish on the river.

In both years, Clancy found dead fish when water temperatures rose to 66 degrees during the heat of the day and didn’t drop below 59 degrees at night.

Under the current drought policy, the state begins considering closing the river to angling during the heat of the day when water temperatures hit 73 degrees on three consecutive days.

The majority of the dead trout that Clancy found over the past two summers were cutthroat. Most of the dead fish were found on the mainstem of the river, where water temperatures were higher.

“Somewhere between 60 to 70 percent of the dead trout we found were cutthroats,” Clancy said. “The cutthroat population in that stretch of river is about 20 percent of the total number of trout.”

Since cutthroat trout represent a small proportion of the total trout population in those two reaches, Clancy said the state may consider changing the temperatures that trigger decisions to manage angling pressure.

“We will probably review the overall criteria that we’re using right now,” he said. “Cutthroat fisheries probably should have a lower temperature trigger than rainbows and brown trout.”

There is still a good deal of work that needs to occur before the state makes any changes in drought management policy for the Bitterroot River.

Clancy will review research literature this winter and consider population dynamics before making any recommendations.

“We will have to eventually make a judgment call,” he said. “We know there is higher mortality occurring for cutthroat populations when water temperatures rise beyond a certain point.”

The challenge is deciding if the number of dead cutthroat represent a significant percentage of the total population in those reaches, Clancy said.

There could be a variety of actions the department could take.

Besides being more sensitive to warmer water temperatures, cutthroat trout are also much easier to catch.

The way people handle the cutthroat trout they catch and release during the warmer times could be having an impact, he said.

“Getting that picture when water temperatures are high might be something you don’t want to do in order to protect the cutthroat fishery,” Clancy said. “We do want to feel comfortable that we’re not killing fish unnecessarily.”

The department is reaching out to other fisheries biologists with cutthroat populations to see if there are similar situations occurring around the state, said FWP regional fisheries manager Pat Saffel.

“We are pretty confident that we have an issue in the Bitterroot,” Saffel said. “We don’t want to wait for information to be gathered in other places in the state before we do the right thing in the Bitterroot.”

Saffel said there is a statewide committee of fisheries managers now considering the issue of water temperature triggers in drought years.

“We are gathering information from other cutthroat streams to see if there is an issue out there statewide,” Saffel said. “Cutthroat trout are a very valuable native species. There is a lot of reason to get this right.”

Last year presented the perfect storm for what could occur in a climate change-driven drought in the Bitterroot, he said. Warmer than normal temperature in May resulted in the snowpack coming off early, which resulted in low water flows during the hottest part of the summer.

Since trout are a cold water species, Saffel said lots of cold water in Montana’s rivers and streams makes life a lot easier for them.

“After a few good water years, last year reminded us of what happens in a low water year,” he said. “I heard there’s a blizzard coming soon. I hope that’s true.”

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Reporter for The Ravalli Republic.