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RENEE MONTAGNE, HOST:

It's a classic image of the American West: snow-capped mountains with cool, clear streams teaming with trout. More than just an image, trout is also big business, attracting fishing enthusiasts from around the world. Now rivers and streams are getting warmer, and there's often less water in them, and that means Western trout may be in trouble.

NPR's Christopher Joyce recently visited Montana with scientists who suspect a changing climate is to blame.

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CHRISTOPHER JOYCE, BYLINE: I am headed up a mountain road with two scientists from the U.S. Geological Survey. We're in the Northern Rockies, in dense stands of Douglas fir and aspen trees. Here, wildlife is part of daily conversation.

CLINT MUHLFELD: I shot an elk maybe like three miles up that drainage.

ROBERT AL CHOKHACHY: Really?

MUHLFELD: Yeah.

JOYCE: No elk today, though. Biologists Clint Muhlfeld and Robert Al Chokhachy are on a fishing expedition: trout fishing. They're studying the trout's world which is changing radically as the climate warms. Record droughts, declining winter snow pack which means measly stream flow after the spring melt, warmer water in the streams.

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JOYCE: To understand what that means to a trout, you have to get in the water with them. We struggle into hip boots as Al Chokhachy explains the day's work.

CHOKHACHY: So we're going to be using electricity to shock fish. And then we capture them and we put in these little individual tags.

JOYCE: The tag is a tiny transmitter about half the size of a match-stick. They put them inside each fish they catch. Then the fish go back into the stream.

CHOKHACHY: What we're trying to do is look at the specific effects of climate on cutthroat trout in one of their northernmost strongholds.

JOYCE: It's a speckled, cold water fish with iridescent gold and pink skin. The young ones here are just three inches long, but they'll grow to a foot or more. Fishing in Montana generates about $250 million a year, and the majority of that is from trout. And yes, they taste great, mild and a little earthy.

These scientists will track and re-catch these fish for years, follow their lives, their growth rate, mortality rate, even how they die.

CHOKHACHY: You'll find your tags in coyote scat, things like that off...

JOYCE: Coyote scat.

CHOKHACHY: Yeah, off the side of the river, and kind of cool.

MUHLFELD: We've actually...

CHOKHACHY: Oh, yeah...

MUHLFELD: ...found them in people's freezers. So, we'll find them.

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JOYCE: It's a foot-deep stream, cold and clear as glass. Native trout begin life here.

Brad Shepard of the Wildlife Conservation Society wears a power pack on his back that's wired to metal pole.

BRAD SHEPARD: We put a current in the water. It actually draws fish. Many people think it kills them. It doesn't.

JOYCE: Their attraction to the current draws them out of hiding and makes them easier to net.

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SHEPARD: All right, ready to rock and roll?

JOYCE: Yeah,

Shepard sticks the pole in the water. The power pack beeps.

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JOYCE: Do I need to get out of the water or...

SHEPARD: No.

JOYCE: Or do I get shocked?

SHEPARD: If you're wearing boots you're fine.

JOYCE: Yeah, likely story.

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SHEPARD: But I'll let you put your hand in if you want to experience the full experience.

JOYCE: And the fish do come as if called. The crew nets them and identifies the species.

SHEPARD: Cutthroat.

CHOKHACHY: That's what we like to see.

JOYCE: Once they've filled three buckets with fish, the team sits down to do a little ichthyological surgery. Shepard measures and weighs each fish, then cuts a tiny slit in its belly and inserts a tag. Each tag has a numerical code that can be read electronically. They place sensors in the stream bed that will recognize each fish as it swims by.

SHEPARD: You ready?

UNIDENTIFIED MAN: Small tag. 121.

SHEPARD: One two one?

UNIDENTIFIED MAN: One two one.

SHEPARD: Yep . Nineteen.

UNIDENTIFIED MAN: One nine.

JOYCE: Once the fish are back in the stream, the incision heals quickly and the fish are no worse for wear. Over time, the team will watch to see if changes in stream flow and water temperature affect these fish.

SHEPARD: Flow and temperature control essentially everything the trout do. It cues when they spawn, so when they hatch, when they emerge, when they come out, how fast they grow, where they go. But as you shift things in one direction, you can in fact lose part of the population.

JOYCE: Understanding how this will all work out will take time. But already, changes have cropped up.

For example, non-native fish that have been introduced in the West - like rainbow trout - normally live farther downstream where the water is warmer. Now they're moving up into higher streams. When they get there, they breed with the native cutthroat.

Biologist Clint Muhlfeld says new research he's done suggests that making hybrids like this may not be a good thing.

MUHLFELD: What we found is that as you allow hybridization to progress in fish, you actually see a rapid decrease in fitness. That's not a good thing for trout.

JOYCE: And if things aren't good for trout, they're not good for the trout business. Tomorrow we'll take a look at what people are doing to protect the trout in a warming world.

Christopher Joyce, NPR News.

SHEPARD: This is a pork chop.

UNIDENTIFIED MAN: Holy Cow.

SHEPARD: Ninety-four. Look at the chunk of that, 94.

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MONTAGNE: You're listening to MORNING EDITION from NPR News.

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