for National Geographic News
Ever since the discovery of gender-bending fish in the Potomac River, scientists have wondered what could be changing the sex of large numbers of fish in the waterway outside Washington, D.C.
They may not have to wonder much longer.
A recent U.S. government study has found large quantities of chemicals in the river and its tributaries—pollutants that are known to cause sex change in animals.
These chemicals, from both residential and industrial sources, may be linked to the unnatural fish, says the U.S. Geological Survey (USGS) report, which was released late last month.
Males With Eggs
The discovery of the abnormal fish "was largely accidental," said Douglas Chambers, a USGS scientist who led the study.
"In 2002 we were looking at stream-water chemistry to understand the large fish die-offs at these sites. It was then that we found smallmouth bass with intersex, a condition where male fish develop premature egg cells."
(See "Male Fish Producing Eggs in Potomac River" [November 3, 2004].)
During a 2003 survey of the Potomac River and the Cacapon River of West Virginia, Chambers and his colleagues found large numbers of intersex fish.
The researchers also found chemicals from pesticides and flame retardants as well as fragrances commonly found in products such as soaps, antiperspirants, and deodorants.
"We analyzed blood plasma of 30 smallmouth bass from six sites," Chambers said. "All the fish contained at least one of the polluting chemicals, including fish that were not intersex."
However, Chambers said, the study has so far not turned up a single case of "imposex"—the condition in which female fish have malformed ovaries or produce sperm.
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