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Montana is home to a big chunk of American wilderness and to animals that need it to survive. To help preserve the wilderness, biologists have to balance the needs of wildlife with cattle ranching and tourism, and even one species against another. As NPR's Christopher Joyce reports in the second part of our series on Centennial Valley, managing wildlife is a tricky science, just like picking and choosing what makes it onto the ark.

CHRISTOPHER JOYCE, BYLINE: Trumpeter swans have a seven foot wingspan. They're ivory-white, curvaceous, elegant and 70 years ago they were almost extinct. Too many people, and especially, too many hunters. I found a flock of these swans lounging on a lake in the Centennial Valley. Biologists built ponds for them here, even fed them and the swans' numbers recovered.

Unfortunately, what was good for the swans was not so good for the Arctic grayling trout, another rare species in the valley. What happened was biologists wanted to keep swans in the valley. It was safe from hunters. So they made ponds by damming streams. When they did that, the grayling trout apparently lost streambeds they lay eggs and reproduce. Steams like this one.

NATHAN KORB: So is Red Rock Creek. This is where the majority of the Arctic grayling in the Centennial Valley spawn.

JOYCE: Ecologist Nathan Korb is with The Nature Conservancy. Scientists, he says, sometimes make mistakes.

KORB: They did everything that they knew was best for those birds, but now we have a much broader perspective, so we are thinking about all species. There are lots of examples where we try something that sounds like a good idea, turns out not to be that good of an idea, and then remedy it and hopefully never try it again.

JOYCE: And now, they're trying to fix things for the grayling in Montana. Korb is a compact, athletic guy with shoulder-length blond hair and a quick smile. He pushes through tall grass to show me a weir that straddles the narrow stream. It's a trap to catch trout, especially an aggressive intruder, cutthroat trout. The cutthroat appears to be pushing the grayling out.

Well, what an elaborate-looking contraption that is.

KORB: Yeah, basically a pole fence that goes across the whole length so fish can't move up or downstream unless they go through the one opening.

JOYCE: The cutthroat is not native here. The grayling is. The cutthroat is widespread and aggressive, the grayling is not. In fact, this is the only population of the lake-dwelling Arctic grayling left in the Lower 48. It's distinctive with a large and colorful dorsal fin. The federal government may add it to the endangered species list.

We drive up a muddy road to see the next step in the grayling experiment at the headquarters of the Red Rock Lakes National Wildlife Refuge. Refuge manager Bill West is gutting and cleaning a bucket-load of cutthroat trout that have come from the stream trap.

BILL WEST: Some of the things that the refuge might have done to try to enhance swans in the past, build ponds, blocked these fish from their native spawning grounds.

JOYCE: Now, they want the streams under those ponds back and the plan to revive the trout population.

WEST: I want to pass on, definitively, to the next manager whether the cutthroat are part of the problem or they are just benign.

JOYCE: So biologists have to figure out how to figure that out. That's not easy. When it comes to managing wildlife, each situation is unique. This fish, in this valley, in these waters is unlike any other. One way to get to know it is to cut it open.

KYLE CUTTING: Just make one slit in the belly. You hit the spine and break it through, and we just remove the insides.

JOYCE: Biologist Kyle Cutting works for the U.S. Fish and Wildlife Service here. He's cutting open cutthroats to find out what they're eating. Is it the same stuff the grayling eat? Are they pushing the grayling out?

CUTTING: In the spring and late fall, their stomach were just packed full, and oftentimes these cutthroat had fish hanging out of their gills on both sides, you know, seven-inch-long fish.

JOYCE: The cutthroat trout here end up in local food banks. But the information they provide will help biologists decide the best way to help the grayling and avoid tipping the ecological balance against yet another native species. Biologists in Montana's Centennial Valley acknowledge that they have to micromanage wildlife this way and that it's an imperfect science. And in case they forget, they have the Trumpeter Swan Society to remind them. The society has already warned them that the new focus on trout should not jeopardize the swan's own tenuous comeback.

Christopher Joyce, NPR News.

GREENE: And you can see the wildlife in Montana's Centennial Valley at NPR.org. The images in our photo gallery were taken by NPR's John Poole.

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