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.
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