In Colorado’s Yamp River, 70 percent of all male bass now have female characteristics — a phenomenon called “intersex” — according to a new study released Monday by the U.S. Geologic Survey.
Intersex fish have been found in watershed across the country, including Boulder Creek.
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.
Many scientists suspect that estrogen compounds — such as birth control and other pharmaceuticals — are to blame.