That's what USGS researcher Jo Ellen Hinck, a biologist with the U.S. Geological Survey, could have been singing when she dissected fish taken from rivers all over the U.S. 

At one third of the sites sampled, the researchers found male fish with eggs in their sperm sacks. 

"We didn't see the eggs until we took thin slices and looked under the microscope and then we'd see a normal bass testes with sperm in different stages of development and we'd also see these immature eggs." Hinck describes.

The study sampled 16 different species of fish in U.S. river basins including the Apalachicola, Colorado, Columbia, Mobile, Mississippi, Pee Dee, Rio Grande, Savannah, and Yukon. One third of the male smallmouth bass and one sixth of the male largemouth bass sampled exhibited intersex characteristics and, for the first time, intersex characteristics were documented in channel catfish.  

This is the first nation-wide study of intersex fish in the U.S., but Hinck, who led the study for the USGS, wasn't out looking for weird fish.  The samples were not purposefully taken in polluted areas or heavily populated areas but instead represent a broad range of environments, Hinck says. "We weren't looking for intersex. It was one component of the study that was included in our monitoring program. We weren't trying to target endocrine disrupting compounds in the environment."

Endocrine disrupting compounds are compounds that mimic the hormonal messengers in our body, such as estrogens and androgens, thereby disrupting natural development. Pesticides, PCB's, bisphenol A, pharmaceuticals and some household cleaning products are known to have disruptive effects in animals.  

It's a sort of who-done-it mystery at this point, and there are many potential compounds to which these fish could have been exposed. Hinck and her team believe the mutations are related to both environmental and genetic factors. "We're still trying to figure out that mechanism and what it is that causes an egg cell to form in the testes but certainly hormones are a very key part of this situation in the fish."

It's not yet known how well these intersex fish may be able to reproduce. One study on wild roach (Rutilus rutilus) conducted by a team of European researchers showed that intersex fish of this species had lower reproductive rates.  

 

Hinck says the findings of the USGS study could be a precautionary tale for people. "With the endocrine system of fish, if you think about humans and our hormone cycle, similar things happen with fish. So we need to understand the possible environmental factors here." 

Hinck and her team hope to study intersex fish in the lab in order to better understand how environmental pollutants, genetics and even temperature may play a role in the reproductive health of certain types of fish.