In western Colorado, a rising number of intersex fish

A new study released Monday finds surprisingly high incidences of male fish carrying immature eggs in their testicles. The “intersex” fish are appearing throughout the nation, including in western Colorado.

The study did not examine why the aquatic hermaphrodites were so prevalent in the nation’s waterways, though lead author Jo Ellen Hinck suggested it was unlikely any one human activity or contaminant was responsible. She added that far more research would have to be conducted before any reliable assertions could be made on the matter.

The United States Geological Survey study, published in Acquatic Toxicology, follows on the heels of a blockbuster New York Times series about the Environmental Protection Agency’s failure to enforce the Clean Water Act. The Times series noted that there has been little public outcry about today’s pollutants, largely because many are invisible to the naked eye or nostrils—undetectable until they cause problems.

From 1995 to 2004, USGS scientists involved in this study looked for intersex fish among 16 species in nine river basins throughout the United States.

Intersex fish were found everywhere except the Yukon basin, and the most common occurrences were among smallmouth and largemouth bass: a third of all male smallmouth bass were intersex, and a fifth of all male largemouth bass.

The Yampa River at Lay, Colo., had some of the highest numbers:  70 percent of smallmouth bass tested were intersex.

Those involved say that while scientists have long been aware of the existence of intersex fish, this study demonstrated just how prevalent they are in the nation’s waterways.

An interesting note about the Yampa River finding:  scientists didn’t find an obvious sources of endocrine-active compounds in that waterway. Such compounds include pesticides, pharmaceuticals, PCBs, heavy metals, or household products such as laundry detergent or shampoo—and they are usually thought to be a cause of hermaphrodite fish.

Nonetheless, reports the Denver Post, federal wildlife officials in the area plan to look into the matter further:

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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Katie Redding

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