Male fish are turning female and growing eggs in their testicles due to toxic runoff largely from prescription drugs, ecologists are warning.

Professor Charles Tyler is an ecotoxicologist who for more than a decade has studied the effects that prescription drug runoff have on ecosystems. Nearly a decade ago, he conducted a study in England that found male roach fish were showing signs of becoming female including growing eggs.

Now, Tyler is once again sounding the alarm about the pollution crisis and the devastating effect it could be having across the world.

Speaking to the Independent, Tyler said the fish he studied were largely impacted by the chemicals in prescription birth control. These drugs contained high levels of estrogen, which is used along with progestin to prevent ovulation in women who take the medication.

Some of this chemical makes it into waterways through the chemical runoff or drugs that have been flushed, but other comes through humans who take the drug and pass it through their bodies and into the sewer system, the Independent reported. The problem was so acute that in some rivers, all of the male fish he studied had feminized to some degree, Tyler found.

Tyler will be speaking about the pollution crisis at the upcoming 50th Anniversary Symposium of the Fisheries Society in the British Isles, the Independent noted. He said the effects of toxic runoff caused by prescription drugs can be difficult to study and the effects not as obvious as more deadly forms of pollution.

The contraceptive pill is making fish change from male to femalehttps://t.co/Tbkc3iMPWH — PinkNews (@PinkNews) July 3, 2017

“It’s blindingly obvious when you stick a poison out and it kills something. It’s an incredibly difficult challenge to understand sub-lethal effects and how these things affect behavior,” he told the Independent.

“If we get sufficient evidence indicating there’s a high likelihood of a population effect, perhaps we need to be more proactive about restricting … or banning these chemicals.”

Contraceptive pills that are flushed down the toilet are turning one in five male river fish transgender https://t.co/8IOiHQGdQU — Daily Mail U.K. (@DailyMailUK) July 3, 2017

This is not the first time that scientists have warned about the effects prescription drug runoff is having on fish. In 2013, a study published in the journal Science found concentrations of the anti-anxiety medicine Oxazepam in wild European perch. This drug concentration caused them to become less social, more active, and eat faster, the New York Times reported.

The United States Geological Survey also found what it termed “intersex fish” in the Potomac River and its tributaries, the result of hormone runoff. The New York Times also noted a study from the journal Environmental Science and Technology that found antidepressants in the brains of fish collected downstream from wastewater treatment plants in Colorado and Iowa.

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