Scientists call the growing oxygen-starved patches of world waterways "dead zones." That also could describe the not-so-swinging mating scene for some of the fish that live there.

For zebrafish, low oxygen levels in the water turn their habitat into the equivalent of a freshwater locker room.

When oxygen is reduced, newly born male zebrafish outnumber females 3-to-1, and the precious few females have testosterone levels about twice as high as normal, according to a scientific study released Wednesday.

Earlier studies also have found reproductive problems for males in other species in oxygen-starved waters. And though all the research is conducted in controlled laboratories, scientists say the gender bending is something that could explain what they are seeing in the nearly 150 dead zones worldwide.

This could be a serious problem because with the expansion of dead zones — such as the massive area in Gulf of Mexico that's reached the size of New Jersey — many fish die, and those that don't die may not be able to keep the species alive, scientists say.

Having too many males "is not a good strategy for survival," said Alan Lewitus, who manages the dead zone program for the National Oceanic and Atmospheric Administration.

The world's dead zones add up to about 100,000 square miles, and most of those zones are man-made because of fertilizer and other farm run-off, said Robert Diaz, a professor of marine sciences at the College of William and Mary. More than 30 dead zones are in U.S. waters and are part of key fisheries.

The stress of hypoxia — the lack of oxygen in water — tinkers with the genes that help make male and female sex hormones, said study lead author Rudolf Wu, director of the Centre for Coastal Pollution and Conservation at the City University of Hong Kong. Wu's peer-reviewed study will appear in the May issue of the journal Environmental Science and Technology.

Wu restricted the oxygen of zebrafish, which are freshwater aquarium fish, but said similar changes are possible in other species of fresh and saltwater fish.

Fish often change genders during their lives, but this is different, he said.

"Since development of sex organs is modulated by sex hormones, hypoxia may therefore affect sex determination and development," Wu wrote in an e-mail interview. "Hypoxia covers a very large area worldwide, many areas and species may be affected in a similar way."

Wu and others said oxygen starvation may be a more powerful sex hormone-altering problem than the chemical pollution that has gotten widespread attention.

In the Gulf of Mexico, sexual development problems have been found with shrimp and croakers, said Nancy Rabalais, executive director of the Louisiana Universities Marine Consortium.

The trend is worrisome, said Peter Thomas, professor of marine sciences at the University of Texas.

"Hypoxia is emerging as a really important stressor, possibly of even greater significance than chemicals," Thomas said. "When it does act, it shuts things down completely."