Smallmouth bass with eggs in their testes, a condition known as intersex, turned up at all 16 sites sampled in Pennsylvania from 2007–10, with the most severe cases coming from the Susquehanna River, according to a recent report by U.S. Geological Survey scientists.

The results highlight growing worries about the potential impacts that a class of chemicals known as endocrine disruptors may have on fish and wildlife in the Bay region, where smallmouth bass with intersex characteristics have also been widely found in the Potomac drainage.

The problem has caught the attention of Congress, which has funded a five-year study of the problem in the Bay watershed.

Intersex is the presence of characteristics of both sexes in a single organism. Several studies over the last decade found intersex fish in numerous parts of the Potomac basin, making it a focal point for attention.

But the new data show that Susquehanna sites “had as high, if not higher severity,” said Vicki Blazer, a USGS research fish biologist and lead author of the paper.

Male smallmouth bass from all 16 Pennsylvania sites sampled had immature eggs in their testes. The prevalence was lowest in the Ohio drainage, intermediate in the Delaware drainage and highest in the Susquehanna.

The study also found vitellogenin in the blood of white suckers sampled at several sites in the Susquehanna. Vitellogenin is a precursor of egg yolk and is normally found only in the blood of egg-laying female fish.

The new results, published in the journal, Environmental Monitoring and Assessment, also showed that intersex was most widespread in agricultural areas — with 100 percent of smallmouth bass sampled being affected in some locations. Some of the most severe cases, measured by the number of eggs in a fish, were found near wastewater treatment plant discharges. Those findings were consistent with results seen in earlier work in the Potomac.

The full impact of chemicals associated with inducing intersex in fish in the Bay watershed is unknown, but scientists believe that, among other things, they compromise the fishes’ immune system. Indeed, many smallmouth bass found in die-offs in both the Potomac and Susquehanna basins have shown signs of intersex. Blazer said intersex, or the chemicals that contribute to it, may have made those fish more susceptible to diseases.

Intersex is thought to be caused by exposure to endocrine-disrupting chemicals, which can mimic or block the naturally occurring hormones in an animal’s endocrine system, thus derailing normal processes that guide development, growth, reproduction, behavior and other bodily functions.

It’s not clear exactly what chemical, or mix of chemicals, is causing the problem.

But scientists think that will change soon. This year, Congress provided $1 million to the USGS to begin working with other state and federal agencies on a five-year study of the effects of endocrine-disrupting compounds on fish and wildlife in the Bay watershed.

“We think it is really going to help us finally better answer some of the key questions that people have been asking,” said Scott Phillips, USGS Chesapeake Bay coordinator. “What are the primary chemicals causing intersex? What are the sources of the chemicals? What is the risk to other species of fish and wildlife?”

One of the areas of focus, Phillips and Blazer said, will stem from the relationship between intersex in fish and land use, with the highest numbers of intersex fish consistently found in agricultural areas.

Scientists want to look at the suite of chemicals in the waterways, and determine whether those triggering intersex could come from herbicides used in row crops, or hormones used in animal operations.

If those chemicals and their sources could be identified, the next step would be to determine whether some of the nutrient control practices being promoted for the Bay cleanup could also help control those chemicals. If so, those practices might be targeted to problem areas.

“Understanding the sources is an important priority of the research so we can help managers know what they need to control,” Phillips said.

That also meshes with an outcome of the new Bay Watershed Agreement, which calls for more research on the impact of contaminants on fish and wildlife with an eye toward identifying management practices that could control both nutrients and chemical pollutants.

While concern about intersex in fish has been growing, there’s been little money to study the problem in the past. In some cases, fish caught in surveys have been frozen for years until there’s been time and money to examine them.

“So much of this has been done sort of piecemeal because of funding,” Blazer said. “This is giving us a real boost.”