This story is part of a special National Geographic News series about global water issues.
The Gulf dead zone occurs when agricultural and waste runoff from the Mississippi River spark blooms of algae and microbes. These organisms gobble up oxygen, starving other marine life and creating huge swaths of "dead" ocean.
Between 2006 and 2007, nearly a quarter of female Atlantic croaker fish caught in the northern Gulf's dead zone had developed deformed, testes-like organs instead of ovaries.
It's unclear how long the fish were living in hypoxic—or low oxygen—waters before they began developing such sexual defects. But lab experiments showed that ten weeks of exposure is all that's needed.
The Gulf dead zone, which occurs annually, generally persists between May and September, and has more than doubled since the 1980s.
This zone, which often fluctuates in size, currently occupies a patch of ocean larger than the state of Connecticut. (Related: "Gulf of Mexico 'Dead Zone' Is Size of New Jersey .")
Low Oxygen Screws Up Fish Hormones
Lab analysis of the fish revealed that the masculinized female croakers had decreased levels of a key chemical found in the brain and ovaries called aromatase.
This enzyme regulates the production of the female sex hormone estrogen, which is critical for proper development of the ovaries.
The brain uses about 20 percent of the oxygen that the croakers breathe, said study co-author M.S. Rahman, a marine biologist at the University of Texas in Austin's Marine Science Institute.
"If the oxygen levels go down, it affects the brain and the neurohormones and neuropeptides that it produces."
In croakers and many other fish species, the sex organs are male by default—estrogen exposure is required to transform the testes into ovaries.
Rahman and colleague Peter Thomas, also at the University of Texas, think that when the croaker's estrogen levels were reduced as a result of hypoxia, some of the cells in the animals' ovaries reverted back to testicular tissue.
(Also see "Mercury Poisoning Makes Birds Act Homosexual.")
The sex organs of the masculinized female fish were smaller and less developed than normal male testes. While some of malformed organs even contained sperm, they were incapable of fertilizing normal female eggs, Rahman said.
The study also found that male croakers were affected by hypoxia, although to a lesser degree. Males caught in the Gulf dead zone, as well as those bred in hypoxic lab conditions, had smaller than average testes and lower sperm counts, according to the study, published online recently in the journal Proceedings of the Royal Society B.
Sexual impairments of both male and female croakers help explain the low hatching rates among fish exposed to dead zones, the scientists added.
While normal croaker hatching rates vary between about 40 to 80 percent, the hatching rate of the oxygen-starved fish was as low as 10 percent. What's more, affected females produced 1.5 times more male offspring than females.
Dead Zone Sex Changes to Impact Other Fish?
Scientists worry the reduced hatching rate and skewed sex ratio could lead to population changes that threaten croakers' long-term survival.
"The croaker is a very common fish, but it's not immune to crashing. So things like this skewed sex ratio can make a big difference," said Prosanta Chakrabarty, a fish biologist at Louisiana State University in Baton Rouge who was not involved in the project.
There is little reason to think that other fish would not be similarly affected, Chakrabarty said. Indeed, apart from a slightly higher tolerance to low-oxygen conditions than other species, croakers are not unique compared with other Gulf fish.
"It's a pretty typical Gulf fish," Chakrabarty said. "I wouldn't be surprised if these findings could be generalized to any advanced bony fish."
Croakers belong to a family of fish that includes dozens of other species, including ones that are more commonly targeted for commercial fishing, such as spot.
"People eat croaker, but there are more important commercial fisheries in the Gulf," said Charles Jagoe, an environmental toxicologist at Florida A&M University.
"And if these conditions are interfering with fish reproduction, it could cause population declines and have an impact on fisheries resources and the economies of fishing communities," said Jagoe, who was also not involved in the study.
(Related: "Sex-Changing Chemicals Can Wipe Out Fish, Study Shows.")
But study co-author Rahman said it's too soon to tell whether his team's findings can be extrapolated to other Gulf fish species. "We didn't study other species, so we're not sure," he said.
Gulf Fish Hit by "Double Whammy"
Jagoe said the new findings suggest Gulf fish suffer from a human-caused "double whammy."
"The Mississippi is a drainage for a third of the country, and there's all kinds of sewage-treatment plants and factories discharging chemicals into the river ... and some of these interfere with fish reproduction," he said.
"And now we've got this second factor, where low dissolved oxygen caused by nutrient loading from sewage-treatment plants and agriculture is apparently capable of causing reproductive effects in fish as well."
Scientists are also concerned that the 2010 Gulf oil spill created dead zone-like conditions as microbes expanded in number to feed on the plentiful oil and gas. (Read more about the Gulf oil spill a year later.)
Study co-author Rahman said his team is currently putting together a proposal to study this very question.
But Florida A&M's Jagoe suspects that any hypoxic conditions created by the Gulf oil spill would likely have been temporary, lasting no longer than the annual dead zone.