Every year, roughly between May and September, a "dead zone," bigger than the state of Connecticut forms in the Gulf of Mexico.
One of the most interesting and troubling effects is the changing of female fish into males. So far, this has only been observed in croakers. In a study by the biologists at the University of Texas, a quarter of the croakers taken from this dead zone that were females had begun to develop some degree of male reproductive organs.
Croakers are by default male, and in the developmental stages they need exposure to estrogen to develop female organs. When there's not enough oxygen, their brains can't produce estrogen, which means they'll start reverting back to males.
This is bad for a number of reasons, but mostly because it highly interferes with the success rate of reproduction, or hatching rates, as they're called. It's also troubling because the size of the dead zone has doubled since the 1980s, when it was first discovered.
Another thing that's got scientists worried is that croakers are a pretty typical fish, meaning that if this can happen to them, it's likely to impact other more commercially important species, although the effects haven't yet been documented in other species.