Fish exposed to even small amounts of estrogen produce fewer males

Water tainted with even a small concentration of human hormones can have profound effects on fish, according to a University of Cincinnati biologist.

UC assistant professor Latonya Jackson conducted experiments with North American freshwater fish called least killifish. She found that fish exposed to estrogen in concentrations of 5 nanograms per liter in controlled lab conditions had fewer males and produced fewer offspring.

Scientists have found estrogen at as much as 16 times that concentration in streams adjacent to sewage treatment plants.

The study suggests that even this small dose of estrogen could have significant consequences for wild fish populations living downstream from sewage treatment plants.

The study was published this week in the journal Aquatic Toxicology.

What’s special about least killifish is they have a placenta and give birth to live young, Jackson said. It’s uncommon among fish, who more typically lay eggs.

Jackson studied a synthetic estrogen called 17α-ethinylestradiol, an active ingredient in oral contraceptives also used in hormone replacement therapy. Estrogen been found in streams adjacent to sewage treatment plants in concentrations of as high as 60 nanograms or more per liter.

“Anything you flush down the toilet or put in the sink will get in the water supply,” she said. 

This includes not only medicine people flush (never do that) but also unmetabolized chemicals that get flushed when people use the bathroom.

“Our wastewater treatment systems are good at removing a lot of things, but they weren’t designed to remove pharmaceuticals,” Jackson said. “So when women on birth control or hormone therapy go to the bathroom, it gets flushed into wastewater treatment plants.”

Chronic exposure of fish to estrogen led to smaller populations and a gender ratio imbalance with more females than males.

Now Jackson wants to know how the exposure to hormones such as estrogen and androgen in a female fish affects her offspring. She is collaborating with the U.S. Environmental Protection Agency to examine local waters in southwestern Ohio.

Jackson said the impacts on streams are not limited to fish. Hormones and other chemicals that are not removed during treatment can bioaccumulate in the food chain or end up in our drinking water.

“Our drinking water is not a renewable resource. When we run out of clean drinking water, it’s gone,” Jackson said. “It’s very important that we keep this resource clean.”

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