Gender-bending bass found in Colorado’s Yampa River |

Gender-bending bass found in Colorado’s Yampa River

Bruce Finley
The Denver Post

Male bass in Colorado rivers and other basins around the nation widely exhibit feminine sex traits, a federal fish study released Monday shows.

This gender-bending was most common in the southeastern U.S. as well as in western Colorado, in the Yampa River, where 70 percent of male bass had eggs developing alongside their testicular organs, the U.S. Geological Survey study found.

The causes aren’t clear, scientists said in the report in Aquatic Toxicology. Nor could they say whether “intersex” fish could reproduce.

But the extent of the intersex fish was startling, said Jo Ellen Hinck, the USGS biologist who led the project.

“When we see 70 percent, we don’t think that’s normal,” Hinck said, referring to a sampling along the Yampa about 18 miles west of Craig.

The researchers studied 16 species, collecting data from 1995 through 2004 (funding was cut in 2006), and documented intersex characteristics in three other species, including catfish.

Researchers with microscopes examined about 1,500 fish in nine river basins: the Apalachicola, Colorado, Columbia, Mobile, Mississippi, Pee Dee, Rio Grande, Savannah and Yukon. Only in the Yukon Basin in Alaska did researchers find exclusively male males.

The intersex condition was most common in bass, with about a third of male smallmouth bass and a fifth of male largemouth bass showing eggs growing alongside testicular organs.

Other experts tracking the feminization of fish said the findings raise further concerns about endocrine-disrupter chemicals – from human sources such as birth-control pills and other estrogen-rich medicines to detergents – that alter their reproductive capabilities.

USGS scientists now must verify, using museum samples, whether intersex bass occur naturally, said David Norris, a University of Colorado professor of integrated physiology who has documented fish- gender distortions in Boulder Creek, Fountain Creek and the South Platte River.

If not, “we’ve got a concern,” Norris said. “At these incredibly low levels of contamination, we’re starting to produce reproductive effects in animal populations.

“We’ve allowed these compounds to accumulate in our water,” he said. “We can anticipate even greater effects in the future, even to the extent that we will start to see effects in humans. A fetus is much more sensitive to these chemicals than an adult.”

Federal wildlife officials along the Yampa will consider possible sources of pollutants, said Tom Chart, director of the Upper Colorado River Endangered Fish Recovery Program for the U.S. Fish and Wildlife Service.

“If we’re having these endocrine disrupters showing up in bass, it’s very likely they’re affecting native and endangered fish as well,” Chart said. “This is out of the natural balance.”

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