Study: Male Fish Growing Female Eggs in Chicago-Area River

Male largemouth bass are the focus of a newly published study from the Illinois Natural History Survey. (Jonathunder / Wikimedia Commons)Male largemouth bass are the focus of a newly published study from the Illinois Natural History Survey. (Jonathunder / Wikimedia Commons)

Female eggs are appearing in the testes of some male fish in Illinois, a new study shows.

Scientists at the Illinois Natural History Survey dissected 51 male largemouth bass, a common game fish, in the spring of 2014 and found that 21 – some 41 percent – had grown oocytes, or female eggs, in their testicular tissue.

The fish were collected from the lower region of the Des Plaines River near Joliet, about 25 miles downstream from Chicago.

This part of the river isn’t exactly pristine: the Chicago Sanitary and Ship Canal, where treated sewage is carried out of the city, joins with the Des Plaines River north of this point. In its 2016 water quality report, the Illinois Environmental Protection Agency found 12 out of the 14 segments of the river it tested were impaired by contaminants such as fecal bacteria and toxic industrial chemicals.

Yet the region is still a popular one for recreational fishing of largemouth bass and other fish for anglers to reel in, according to the study’s co-author and former INHS scientist, Mark Fritts.

He also said the wild population of largemouth bass they examined don’t appear to be negatively affected by such feminization.

“This is an emerging field of research. We're kind of on the tip of the iceberg. There are still a lot more questions than answers.”

–Fish Biologist Mark Fritts


Scientists describe these fish as intersex, and say their disrupted hormonal systems could be caused by hormone-based pollutants in the water called endocrine disrupting chemicals (EDCs). These can be found in birth control pills, pesticides and other consumer, industrial and agricultural products.

Similar findings have been made elsewhere in the country, but this study is believed to be the first to measure the intersex condition of wild fish within Illinois’ waterways.

“Long-term surveys conducted by the INHS in this region have shown big increases in largemouth bass over the past 40 years since the implementation of the Clean Water Act,” Fritts said. “It's a dichotomy here because we're seeing a population that has increased dramatically, but we're also seeing this potential problem rising.”

While it's not one of the five longest rivers in Illinois, the Des Plaines River is the longest stream in the Chicago area, flowing 133 miles from south Wisconsin to form the Illinois River, which feeds into the Mississippi River.

Fritts, who's now a U.S. Fish and Wildlife Service fish biologist in La Crosse, Wisconsin, spoke to “Chicago Tonight” about the study’s results, which were published in the July 2016 edition of the scientific journal The American Midland Naturalist.

Chicago Tonight: Were you surprised that almost half of the male largemouth bass you collected in this river had grown female eggs?

MF: Not really. Given the environmental history of the Chicago area and the results of recent studies that have documented the presence of high concentrations of EDCs in the Chicago region’s water bodies, we assumed that fish in this downstream location would probably be affected in some way or another.

A microscopic view of eggs in the testicular tissue of an intersex male largemouth bass. (Mark Fritts)

CT: What factors could be causing this?

MF: We can’t say anything directly from our study because we didn’t measure the concentrations of any chemicals within the water. But previous researchers have found really high concentrations of estrogenic compounds coming out of the wastewater treatment plants in Chicago. And, I mean, they were really significant concentrations coming out into the streams that eventually find their way into the Des Plaines River. Those are older studies, but they illustrate that these EDCs are present in Chicago area water bodies and they naturally flow downstream.

CT: What exactly are EDCs, and can you give us some examples?

MF: EDCs are any natural or synthetic chemical compounds that can disrupt the endocrine development pathways of an organism. Our growth and our reproductive development are all tied to our endocrine system. These chemicals can affect the way these systems develop. There are literally thousands of compounds classified as EDCs around the world.

Environmental estrogen is associated with birth control pills, things like that. Whenever a person takes medicine and goes to the bathroom, a very large amount of that medicine goes through their body and goes into the wastewater treatment where it’s not always picked up and finds its way back in the river. Another example is the very common drug Metformin, which is used to treat diabetes. It’s very, very widespread and pervasive especially around the American Midwest and laboratory studies have shown it causes feminization in male fishes.

BPA (Bisphenol A.) is another one. It’s in children’s bottles and toys and known to cause intersex in fishes as well. And a lot of flame retardants are known to cause these problems. But this is not an exhaustive list – there’s more out there.

"Largemouth bass appear to be susceptible to intersex condition and it’s a very important recreational species," Fritts said. "There are a lot of people that go out and pursue that fish." (Thephpdude / Wikimedia Commons)

CT: Do you know if people actively fish in the area where you found intersex fish?

MF: Yes. When I was with the INHS, we were involved in research-based fishing in that region for a very long time. I do know it was very common to see recreational fishermen pursuing bass in that area. A couple years ago there was a paper about how the Des Plaines River was the hidden bass fisherman’s haven in suburban Chicago. People are aware that this is a good sport fishing area.

CT: Would an intersex largemouth bass be dangerous for humans to eat?

MF: That’s another question that we have, too. That’s a future area of research. We don’t really understand the models or have one in place for understanding how a lot of these contaminants get transferred to humans. These are highly complex molecules. Understanding how they get transferred between pieces of the food chain is very difficult. We just don’t know that. We don’t have really good research studies to answer those questions. This is an emerging field of research. We’re kind of on the tip of the iceberg. There are still a lot more questions than answers.

CT: Are you worried about potential negative effects on humans?

MF: That’s my biggest concern. The fish are a biomarker, they’re an indicator that there’s a problem, but my primary concern is making sure that people who use the resources don’t get sick either.

Interview was edited and condensed.

Follow Evan Garcia on Twitter: @EvanRGarcia


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