Study links agriculture to increase of intersex fish in Potomac basin
Study cautions that condition is likely caused by multiple contaminants and not just one source
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Scientists have been perplexed for years as to why large numbers of male smallmouth bass in the Potomac River basin contain immature egg cells, but they offer some clues in a recent journal article.
Results published in the Journal of Aquatic Animal Health suggest that the high rate of “intersex” characteristics in smallmouth bass from the Shenandoah River and the South Branch of the Potomac appears to be linked to areas with large human populations or intense agricultural operations.
The results were not a complete surprise. The presence of intersex is thought to be caused by exposure to estrogenic compounds in the water, many of which are byproducts from human activities or chemicals used in agriculture.
But unexpectedly, the new study reported that some of the highest intersex rates—reaching 100 percent in some samples—came from agricultural areas.
Past studies by scientists in other areas tended to find the strongest links between intersex in fish with wastewater discharges or urban areas.
“We were surprised when we saw it in the South Branch, and even in the Shenandoah, at a higher level because we are not talking about huge cities in those areas,” said Vicki Blazer, a scientist with the U.S. Geological Survey who led the study.
Intersex is thought to be caused by any of a number of chemicals—from pesticides and other compounds used in agriculture to hormones from birth control pills and other pharmaceuticals—which can affect the reproductive system.
The paper said a number of chemicals known to have estrogenic properties were also found in the water where smallmouth bass were collected, including several found in herbicides and pesticides used in agriculture.
But the paper cautioned that it's “unlikely that only one contaminant or source is responsible” for intersex. Rather, it suggested, the condition is likely caused by exposures to multiple contaminants.
Making links to specific chemicals, or combinations of chemicals, is difficult and is further complicated by the fact that most smallmouth bass studied by scientists were 2 to 7 years old. Intersex, Blazer said, is likely caused by chemical exposure in the first few weeks of fishes' life. “You need to think about what was going on back when they were really young fish,” she said. “It is not a really easy thing to tease out. We are working on it.”
The notion that some human activity is causing the problem was bolstered when the scientists examined smallmouth bass from less developed areas outside the Potomac watershed, including the Greenbrier, Elk and Gauley rivers in West Virginia. Intersex rates in those rivers were much lower, ranging from 17 percent to 36 percent.
Blazer said even those sites were not truly unimpacted. Evidence of chemicals suspected of causing intersex were still present, albeit at lower levels.
Scientists discovered the intersex in smallmouth bass in 2003 while investigating fish kills on the rivers. Under the microscope, they unexpectedly spotted egg cells in the testes of male fish.
Intersex was not the cause of the fish kills, Blazer said—many of the fish killed were other species that did not show intersex characteristics.
But the fact that fish kills and high levels of intersex were found in the same areas suggest that the same pollution issues may play a role in both problems, she said. “To me, intersex is important as an indication of environmental contaminants that might be affecting the fish health in other ways, like the fish kill,” Blazer said.
The exact causes of recurring fish kills on the Potomac and Shenandoah remains under investigation.
Research is continuing to determine whether intersex is affecting fish health or reproduction. Scientists are also continuing to examine fish from other locations both inside and outside the Potomac basin to better correlate intersex with land-based activities.
One surprise from that work is that—unlike other areas—intersex in Potomac smallmouth bass does not seem to be linked to wastewater discharges. When scientists examined fish above and below outfalls of two wastewater plants in Maryland, Blazer said they found similar levels of intersex. That suggests the chemicals causing the change may be coming from more diffuse sources, such as stormwater runoff, she said.
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