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.

(See "Sex-Changing Chemicals Found in Potomac River.")

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