GULF OF MEXICO (CNN) -- Fisherman Terry Pizani turns his captain's wheel with a mournful expression on his face. Far below, the fishing grounds off the Louisiana coast where the 63-year-old has made a living for five decades have become an aquatic graveyard known as a "dead zone."
Fisherman Terry Pizani's shrimp catch is not as plentiful because of the Gulf of Mexico's "dead zone."
"You don't see nothing," he said. "Usually you see bait fish on the water. You don't see no bait fish, nothing. Nothing's there.
"I don't have no kind of testing material to test the water, but I know something's wrong."
Oceanographers who test the Gulf of Mexico waters every month confirm that the veteran fisherman is right.
"We're not finding enough oxygen to support life, aquatic life," scientist Lora Pride said aboard the Pelican, the Louisiana Universities Marine Consortium research vessel that studies the Gulf.
CNN traveled aboard the ship August 14-15 as consortium researchers sent sensors to the bottom of the sea, scooped up sediment and collected water samples for analysis at nine testing stations in the Gulf.
As an oxygen meter sank far below the Pelican, Pride pointed to an onboard computer screen displaying the meter's findings in real time.
"This green line is the oxygen right here, and at the bottom, it's reading less than 2 milligrams per liter," Pride said.
Six of the nine stations revealed such oxygen-deprived, hypoxic water, compared with a normal reading of 6 milligrams per liter.
As Pride and her crew aboard the Pelican monitored the Gulf waters, the journal Science published a study last week revealing that there are more than 400 dead zones around the globe, double the number found by the United Nations two years ago.
One of the major dead zones is in the Gulf of Mexico. It is 8,000 square miles, nearly the size of New Jersey, according to the marine consortium's annual measurement completed in July.
"There's no oxygen in the water for shrimp, crabs, fish to live," said Nancy Rabalais, executive director of the consortium.
Fish and shrimp "can sense that and they start to move out of the area. Otherwise they would die. The animals that still remain in the sediments have to keep breathing. There is not enough oxygen, and eventually they will die off," Rabalais said.
Scientists have been studying the Gulf's dead zone for about 20 years, although its existence has been known for decades. So why is oxygen disappearing from fishing waters in the Gulf of Mexico? The answer, scientists say, is found hundreds of miles to the north, up the Mississippi River in corn country.
Farmers in Iowa and across the Midwest use tons of nitrogen and phosphorous to make their cornfields more productive, which allows the farmers to take advantage of high corn prices resulting from growing demand from ethanol factories and developing countries.
Rain always causes some fertilizer to run off farmland, but this summer's historic flooding caused even more runoff into rivers that flow into the Mississippi.
"That's the primary source of the nutrients that go to the Gulf of Mexico," Rabalais said. "And so the size of the low-oxygen zone has increased in proportion to these nutrients reaching the Gulf."
Fertilizer flowing into the Gulf of Mexico triggers an overgrowth of microscopic algae, which eventually die and fall to the bottom.
"When they die, they decompose, and decomposition requires oxygen," Pride said. "So these things will fall to the bottom, and as they decompose, they consume oxygen."
So much oxygen is taken from the water that slow-moving sea life like clams, small crabs, starfish and snails suffocate.
"We go diving down there quite frequently," said Melissa Baustain, a doctoral candidate at Louisiana State University. "The deeper we go down in the water, it gets kind of scary, because there's nothing there. There's no fish, there's no organisms alive, so it's just us.
"It's dark, and it's turbid because all that algae that is dying, that's sinking through the water column."
To find lots of shrimp, fishermen like Pizani have to travel to the edge of the dead zone. He calculated that it costs him $450 a day in diesel fuel to fish.
"You just gotta keep going miles and miles and miles, and hopefully you'll run into something," he said. "The fuel costs are so high, it's just not feasible to get out there unless you can catch a boatload, really make any money out of it."
So, many boats are idle. Others are staying away from their home port in Grand Isle, Louisiana, a disaster for seafood processor Dean Blanchard, who buys shrimp from fishermen.
"All my boats have to go somewhere else to make a living. It's a shame," Blanchard said.
"This is the prime shrimping ground in the country right here, and it shut us down. It just shut us down. It's unreal."
With demand for corn growing, scientists say, the dead zone could expand in coming years.