A strange thing is happening to male smallmouth and largemouth bass swimming in waterways in parts of the Northeast: They are increasingly showing up with characteristics of the opposite sex.
In a survey of 19 national wildlife preserves, researchers from the US Geological Survey (USGS) and the US Fish and Wildlife Service found this month that a whopping 85 percent of smallmouth bass and 27 percent of male largemouth bass were intersex.
"What we're finding is the presence of immature eggs in the gonads of the male fish," said Luke Iwanowicz, a USGS research biologist. "This has basically been an indication of exposure of estrogens such as compounds that enforce the female physiology."
Iwanowicz is the lead author of a paper on the phenomenon, which has been published in Ecotoxicology and Environmental Safety.
Fish are often used as indicators of aquatic health, and in this case, are being used to showcase the increasing threat of endocrine disrupting chemicals (EDCs), which are being dumped into our rivers, streams, swamps, and lakes. Among the main culprits are hormones in livestock manure and herbicides and pesticides found in runoff. Trace amounts of pharmaceuticals, like birth control pills, are also showing up in waterways after being discharged from wastewater treatment plants.
"Estrogens are hormone signaling molecules that work in low concentrations," Iwanowicz said of the chemicals, which are found mostly in the organs of the fish, rather than the flesh that people eat.
"The low concentrations of these hormones can have potentially profound effects," he said. "We are looking at fish but, of course, there is that concern that, if this stuff is in the water, it can be affecting other wildlife."
He added, "We're not sure how it works into the human dimension but it's kind of worth considering."
In conducting the study, Iwanowicz and his colleagues did not conduct a chemical analysis on the water so they could not say for sure what was to blame for the intersex fish. But the findings suggest that EDCs were the most likely the culprit.
The problem of intersex fish has been documented over the past two decades in nearly 40 fish species worldwide. Scientists believe their prevalence is more widespread than previously thought and could be starting to take a toll on some populations, either by reducing a fish's ability to reproduce or weakening its immune system and making them more vulnerable to disease.
"Estrogenic exposure is a problem throughout the developed world and there are many instances in which fish population health was degraded through the presence of estrogenic compounds," said Heiko L. Schoenfuss, director of the Aquatic Toxicology Laboratory at St. Cloud State University in Minnesota.
"Although estrogens by themselves seldom reach environmental concentrations that would result in the collapse of a fish population, they are an important and potent environmental stressor that may impeded the recovery of aquatic ecosystems that have experienced other sources of disturbances," he said.
A 2009 study by the USGS examined 16 fish species from 1995 to 2004 and found a third of all male smallmouth bass and a fifth of all male largemouth bass were intersex. Intersex fish were found in a third of all sites examined from the Apalachicola, Colorado, Columbia, Mobile, Mississippi, Pee Dee, Rio Grande, Savannah, and Yukon River basins.
John Arway, executive director of the Pennsylvania Fish and Boat Commission, said the most recent findings reflect what has been found in the state's waters. A USGS study last year found intersex fish in Pennsylvania's Susquehanna, Delaware, and Ohio River basins. Four of the wildlife refuges in the latest study are at least partly located in the state.
"Obviously we're concerned not only about the Susquehanna, but the other waters that either drain into or drain out of those national wildlife refuges," Arway said.
Arway said his agency, along with the US Environmental Protection Agency, found that until 2005 there were no reports of disease among smallmouth bass on the Susquehanna River. Since then, sampling in some sites has revealed disease rates of up to 70 percent.
"One of the things found was that EDCs and herbicides were the likely and probable causes for the effects we are seeing with our bass on the Susquehanna," he said, adding that there has been increased incidences of sores and lesions on the fish, as well as cancerous tumors.
"[W]e are seeing the population of fishable bass declining on the Susquehanna," he said. "That is not to say EDCs are the only stressor that are effecting these young fish but we are starting to narrow down the causes and EDCs are the likely cause that is infecting and killing these young fish."
Combating the problem won't be easy, since estrogen is so widespread — all living things produce it, after all — and its effects are often felt at very low concentrations. Fixes like upgrades to treatment plants, would be costly and limiting the use of herbicides and pesticides or controlling runoff from farms faces opposition from the agricultural industry.
"We need to start acting on some of the chemicals we know cause these effects and not worry about those we can't control like a lot of the antibiotics," Arway said. "We need to trace those concentrations back up the tributaries, find out the major sources of where these chemicals are being applied and then work with the applicators — whether it's the farmers or the lawn care companies — to try to get them to adhere to the label restrictions."
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