Potomac Fish Study Turns Up No Exact Cause of Abnormalities

By David A. Fahrenthold
Washington Post Staff Writer
Wednesday, April 22, 2009

More than 80 percent of the male smallmouth bass in the Potomac River are growing eggs, but after six years of searching, scientists still have not pinned down the pollutants that are causing the abnormality, according to research unveiled yesterday.

Federal officials released the results of the largest-ever investigation into "intersex" fish in the Potomac watershed. The bass, first identified in a West Virginia tributary in 2003, have made the Potomac a focus of research into "endocrine disruptors," pollutants that interfere with an animal's natural chemical signals.

The study found a substantial proportion of abnormal fish. In some places, between 82 percent and 100 percent of the male fish had some female characteristics.

But it did not produce the smoking gun that scientists were looking for.

They tested fish upstream and downstream from sewage treatment plants, hoping to find evidence that the fish were being altered by substances such as human hormones, soaps and personal-care products in processed sewage. They didn't. The male fish in both locations were growing eggs.

"Right now, we're shooting in the dark," looking for other possible pollution sources, said Leopoldo Miranda-Castro, supervisor of the U.S. Fish and Wildlife Service Chesapeake Bay Field Office. "Because it could be pharmaceutical, it could be agricultural," or something else, he said.

Scientists said they believe that the problem is caused by a mixture of pollutants, including some in sewage, animal hormones from farm manure and pesticide runoff.

The survey examined fish in the Potomac in the District, and in two Maryland tributaries, the Monocacy River and Conococheague Creek. Instead of illuminating a single cause, it made a tangled problem seem even more complex by revealing new changes in female fish.

In the District, U.S. Geological Survey scientist Vicki S. Blazer said, female largemouth bass showed low levels of a protein called vitellogenin, which is used to produce the yolk in their eggs. In fact, Blazer said, in some cases the levels of vitellogenin in females were actually lower than in the Potomac's male fish -- which should not produce the protein.

"That indicates that it's not just estrogenic compounds" in the river, but also some that mimic male hormones in female fish, Blazer said.

The Fish and Wildlife Service is beginning to look for intersex fish and amphibians in wildlife refuges along the East Coast.

For now, Maryland authorities say that the problem does not seem to affect the bass's ability to reproduce. The Potomac's smallmouth population is at a 20-year high, one biologist said.

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