What’s causing female eggs to grow in the testes of these male fish?

The Problem

  • Estrogen-like chemicals mimic natural hormones and can affect the endocrine (hormonal) system of animals. They come from many different sources, including some types of birth control pills, natural hormones in livestock manure, herbicides and pesticides.
  • These chemicals are showing up in our waterways.
  • Studies of waterways around the country have found evidence that something is causing male fish to show signs of having female characteristics (aka intersex fish).
  • These fish could be telling us about dangerous the chemicals in the water that we drink, swim in, and yes, fish in (sorry fish).
Billy Bass? Let's take a closer look...
Billy Bass? Let’s take a closer look…
  • Scientists have never done a study on fish in National Wildlife Refuges in the Northeast to look for signs of intersex fish.
  • They wanted to see if they would find intersex fish, and if so, how many there are.
  • They also wanted to see if the water that the fish live in contains the female hormone estrogen or estrogen-like chemicals.

The Study

  • Researchers from the U.S. Geological Survey collected fish from 19 wildlife refuges in the Northeast U.S.
19 sampling sites across the Northeast U.S.
19 sampling sites across the Northeast U.S. Credit: L.R. Iwanowicz


  • They collected most of the fish by electroshocking them or just hooking them on an old-fashioned fishing line.
  • They caught 118 male smallmouth bass and 173 male largemouth bass.
  • They removed the reproductive organs from the adult male fish to preserve and study them.
  • Researchers looked for the presence of oocytes (oo = egg; cytes = cells) in the male reproductive organs.
  • They tested blood samples for a protein (Vitellogenin or Vtg) that can tell us if there is a lot of estrogen in the fish.
  • Vtg is a a protein that makes up the proteins that then make up egg yolk sacs in egg-laying female animals.
  • Water samples were also taken at each site where fish were caught.
  • The water samples were tested for the presence of chemicals that act like estrogen.

The Results

  • 85% of male smallmouth bass and 27% of male largemouth bass sampled were intersex (i.e. they had eggs in their testes).
  • Estrogen-like chemicals were found in 79% of the water tested from the sampling sites where the fish were caught (but the scientists did not study what exact chemicals were there).
  • Elevated levels of Vtg (indicator of female hormones in fish) were found in smallmouth bass from 4 sampling sites and largemouth bass at 1 sampling site.

The Takeaway

Don't worry, fish. We've got you covered.
Don’t worry, fish. We’ve got you covered.
  • More and more often, we are finding high percentages of intersex fish in these waterways.
  • More intersex fish were found in the Northeast than in any other previous study (and they’ve done lots of these studies).
  • This was the largest study ever done on intersex fish at National Wildlife Refuges.
  • More studies need to be done on where these chemicals are coming from and how they might be affecting other animals and humans.

Think this smells fishy? Allow me to waft some more good science your way:




Author: Emily

I started Rise and Sci because I love science, I love teaching people new things and I want to help build a greater public understanding of all things science. My goal is to take hard to understand concepts and make them accessible to everyone- all in a fun an interesting format!

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