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Too much intermingling puts native trout in trouble

Mixing with introduced rainbow trout could reduce offspring for the troubled westslope cutthroat

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5:39am, March 18, 2009
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Even a little hybridization may hurt native cutthroat trout, suggests an unusual study of wild fish genetics.

Trout are none too precise in their mating habits, and natives often make viable babies upon encountering certain other trout species.

Rainbow trout, beloved for sports fishing, have been introduced widely into new waters, and biologists worry that the mixing of the rainbows’ genes into native trout populations will undermine the natives’ long-standing adaptations to local conditions.

For North America’s 12 remaining subspecies of the native cutthroat trout in the West, “hybridization is one of the biggest threats,” says Clint Muhlfeld of the U.S. Geological Survey’s Northern Rocky Mountain Science Center field office in Montana’s Glacier National Park.

Now he and his colleagues have begun to quantify that threat. A mildly hybrid fish — 20 percent rainbow DNA mixed into native westslope cutthroat — has only half the offspring of a purebred westslope, he and his colleagues report online March 17 in Biology Letters.

For wild trout, “this is the first time someone has looked at how different levels of hybridization affect fitness,” Muhlfeld says.

“I think it’s a really good first step,” says fisheries biologist Brad Shepard, based in Livingston, Mont., with the state’s department of Fish, Wildlife and Parks. Regulators haven’t had studies like this for wild trout, he says, and now he wants to see whether the deleterious genes get purged or retained over time and whether similar effects show up in other populations.

That “20 percent” hybrid resonates against the history of more than a decade of government decisions and conservationists’ law suits over whether to list westslope cutthroat trout as threatened under the U.S. Endangered Species Act. In 1997, conservation groups first petitioned for listing, and hybridization ranked high as a concern. The U.S. Fish and Wildlife Service has denied the petition. In its deliberations, the Service cited earlier observations that a westslope cutthroat can have as much as 20 percent rainbow trout DNA and still look like a westslope cutthroat.

Westslope trout with only 20 percent rainbow DNA typically do look just like the original westslope fish. But looks aren’t everything, so Muhlfeld began studying the vital problem of how well the hybrids reproduce.

From 2003 to 2007, he worked with biologists from two Montana universities as well as the state of Montana to sample westslope trout moving through the small Langford Creek in northwestern Montana. Researchers snipped a bit of fin from adults (61 females and 124 males) arriving to spawn during those years. When the offspring of spawning adults swam back down the creek, researchers nipped tissue from 648 young too.

Looking at 16 distinctive stretches of highly repetitive DNA in the fish, researchers figured out the parents of most of the offspring and about how much rainbow DNA each parent and youngster carried.

The first encounter of the two kinds of trout — a pure westslope trout mating with a rainbow — produced offspring capable of reproducing almost as well as the purebreds. But further hybridization led to generations with sagging reproduction. Those 20 percent-rainbow trout may still look much like, but certainly aren’t reproducing like, purebreds, Muhlfeld reports.

His data are “compelling,” says conservation biologist Noah Greenwald, in Portland, Ore., with the Center for Biological Diversity. The organization has spent years suing for westslope federal listing. At first glance, Greenwald says he’s not going to advise rushing back into court with a new lawsuit because he sees the issues that have preventing listing as more political than scientific.

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