Scientists are still trying to sort out exactly what’s causing sexual abnormalities among smallmouth bass in Chesapeake Bay rivers, but they may be getting closer to figuring out how to reduce them.

Prompted initially by disturbing fish kills in the Potomac and Susquehanna rivers, researchers have been on a quest for nearly 20 years to understand what’s impacting the health of smallmouth bass, a popular freshwater recreational catch throughout the Bay watershed.

While studying die-offs, skin lesions and infections seen in both adult and juvenile bass, scientists began noting “intersex” conditions in Potomac and Susquehanna fish. They’ve found cells in the sex organs of males that are usually found only in females, as well as a protein that’s produced by females to form the yolk around an egg.

Vicki Blazer, fish biologist with the U.S. Geological Survey, removes kidney of euthanized fish collected from the South Branch of the Potomac River. Credit: Heather Walsh / U.S. Geological Survey

Extensive water sampling in Bay rivers has also documented the presence of hormone-disrupting chemicals, which have been linked to the development of intersex traits in bass.

There appears to be no one source of those chemicals in Bay tributaries, but a pair of new research papers suggest that efforts to reduce nutrient and sediment pollution also could help reduce chemical contaminants — and possibly the intersex abnormalities.

After conducting a statistical analysis of water and fish samples collected at multiple sites over several years, a team of U.S. Geological Survey scientists found that land use nearby or upstream was linked to the levels of hormone-disrupting contaminants measured in the water.

Higher levels generally were found in waters that drained farmland, the USGS team reported in the March issue of the journal Chemosphere. But the levels could also be affected by the extent of crop cultivation, and the type of crop being grown. Soybeans, for instance, generate natural estrogens. Levels were even higher near fields where hormone-disrupting herbicides such as atrazine and metolachlor had been applied, the study found.

The USGS team also found elevated estrogenic or hormone-disrupting substances in waters draining some more urban settings, especially those with a lot of pavement or high numbers of septic systems treating wastewater.

“We did find … that scale matters,” said Vicki Blazer, the study’s lead author. In the immediate catchment basin being studied, they saw impacts associated with pesticide applications, percent of land cultivated, and the presence of septic tanks. But some impacts could also be connected to factors farther upstream such as runoff and the extent of pavement and buildings. They also sampled early and late in the year to see if the effects varied by season.

Previous studies have linked feminization of male bass with exposure to effluent from municipal wastewater treatment plants, but sampling in the Potomac River near sewage plants found no such pattern. Still, Blazer indicated she isn’t ready to rule out wastewater plants as sources, saying there hasn’t been enough sampling to do that.

Another clear message from the data: Levels of hormone-disrupting chemicals tend to be lower in stretches of river lined with trees, or even bushes.

“I think that basically what that’s telling us is that riparian zones are important,” said Blazer, a biologist in the USGS Leetown Science Center in Kearneysville, WV. “We’ve known that, but it gives us more evidence.”

Streamside forests have long been recognized as one of the most effective ways to reduce nutrient and sediment pollution from land. Trees and shrubs slow down stormwater runoff, trapping sediment and allowing nutrient-laden water to soak into the spongelike soil in forests.

As part of their commitment to reducing nutrient and sediment pollution, the Bay states have pledged to plant 900 miles of forest buffers annually, but they have fallen far short of their buffer-planting goal to date.

Kelly Smalling, a research hydrologist with the U.S. Geological Survey, prepares to filet a fish for chemical analysis.

Other Bay restoration practices may also offer hope for limiting estrogenic chemicals in rivers and streams. In a separate study, USGS scientists found indications that the number or density of best management practices put on farmland to control nutrient and sediment pollution influenced the levels of hormone-disrupting compounds detected in nearby streams.

Researchers tested water samples for 301 organic chemicals to assess the benefits of agricultural best management practices to contaminants levels in streams and rivers. They focused their analysis on a handful of contaminants that had shown up most often: the herbicides atrazine and metolachlor; natural estrogens produced by crops and other vegetation; and cholesterol, which might be coming from either human or animal waste.

They compared those contaminant levels to U.S. Department of Agriculture data on the number of best management practices that had been installed near the five study sites — three in the Susquehanna and two in the Potomac watersheds.

Writing in the journal Science of the Total Environment, the USGS scientists reported that contaminant levels were lower in areas that had the most farm conservation practices.

The effect of farm runoff controls was most apparent when looking at crop fields where the herbicides atrazine or metolachlor had been used, said Kelly Smalling, lead author and research hydrologist in the USGS New Jersey Water Science Center.

“As BMP density increased, the concentrations of atrazine and metolachlor decreased” in the water, Smalling said. Atrazine is of concern because it’s one of the most widely detected chemicals in surface water in the Bay watershed. Metolachlor, another herbicide, is the second most heavily used agricultural weed killer after glyphosate.

But the researchers also found that BMP intensity had a positive effect on levels of other contaminants in streams, including estrogenic compounds produced by plants and cholesterol. “There’s less runoff making it into the streams” Smalling said, and that’s reducing contamination.

She cautioned that the findings were based on sampling at only five sites and therefore too limited to be conclusive. Nor could this study determine whether some conservation practices were more effective than others, she noted.

But she added that “we think this is the beginning of saying there could be co-benefits” to reducing hormone-like contaminants from efforts to control nutrient and sediment runoff. Smalling called that “pretty exciting.”

This article was originally published on on February 25, 2021.

Timothy Wheeler, Bay Journal Media

Tim Wheeler is the Bay Journal's associate editor and senior writer, based in Maryland. You can reach him at 410-409-3469 or

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