Ocean "dead zones" -- oxygen-starved patches -- may be turning normal breeding grounds into the equivalent of male-dominated locker rooms for fish.
In lab experiments, newborn male zebra fish outnumber females 3 to 1 when oxygen is reduced. And the few females have testosterone levels about twice as high as normal, according to a study released Wednesday by the journal Environmental Science and Technology.
The stress of hypoxia, or lack of oxygen, alters the genes that help make male and female sex hormones, said lead author Rudolf Wu, director of the Centre for Coastal Pollution and Conservation at the City University of Hong Kong.
Wu restricted the oxygen of zebra fish, which are freshwater aquarium fish, but he said that similar changes were possible in other species of fresh- and saltwater fish.
"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."
Earlier studies also have found reproductive problems in other species in oxygen-starved waters. Scientists say the gender changes might explain what they are seeing in the nearly 150 dead zones worldwide.
This could be a serious problem: With the expansion of dead zones -- an area in the Gulf of Mexico, for example, is now the size of New Jersey -- fish die, and those that don't die may not be able to keep the species alive, scientists say.
The world's dead zones, most of them caused by fertilizer and other farm runoff, add up to about 100,000 square miles.