GLOUCESTER POINT, Va. -- Bob Fisher plunged a net into a tank of water, scooping up a diamond-shaped creature with a face like a cow's and a jagged stinger under its tail.
"I don't totally trust these rays," the biologist said, glancing toward a puckered scar on his right arm. "You can still see the wound from the last time one stung me. The toxin made a severe burning sensation that went through my whole body."
Studying cow-nosed rays in the Chesapeake Bay can be a painful experience. But it's important, says Fisher, of the Virginia Institute of Marine Science, because some research has suggested that rising populations of the odd-looking animals are vacuuming up millions of native Chesapeake oysters - delivering a blow to recovery efforts.
Rays have been living in the bay at least since the days of Capt. John Smith, who was almost killed by a poisonous sting while fishing in 1608. But a recent article in the journal Science concluded that cow-nosed rays have become "hyperabundant" in recent years because of overfishing of sharks in the Atlantic Ocean.
More than 90 percent of the sharks are gone, slaughtered so their fins can be sold in Asia as a delicacy and aphrodisiac. This has thrown the ecosystem out of balance, allowing their prey, rays, to multiply perhaps 20-fold since the 1970s, to about 40 million today in the Chesapeake Bay, according to the article.
"If we take an ecosystem and knock off the equivalent of lions and tigers and wolves at the top, is there a consequence?" asked Charles H. Peterson, a University of North Carolina professor and one of the authors. "We discovered that the cow-nosed rays and other prey have increased dramatically."
Rays devour as much as 840,000 metric tons of oysters and shellfish a year in the bay - more than 2,000 times the amount caught by commercial watermen, according to the Science article. And the native oyster population is already depleted because of overfishing, parasites and pollution.
Some efforts to plant new oysters have been overwhelmed by rays. In May 2006, a swarm of cow-nosed rays gobbled 775,000 oysters planted by the Chesapeake Bay Foundation on an artificial reef. Two years earlier, rays sucked up oysters planted by the Army Corps of Engineers in the Great Wicomico River in Virginia's Nothern Neck.
"The rays can destroy hundreds of thousands of oysters and be gone in days," said Margaret Bevans Ransone, whose family owns an oyster company in Kinsale, Va. "We are trying to revive our industry, but one key question is what to do about the rays."
Fisher said he has some doubts about the numbers and conclusions in the Science article, to which he did not contribute. But he said he has heard many anecdotal reports from watermen about rising numbers of rays. He said he believes the population growth could be caused by a decline of the bay's fishing industry, which has meant fewer fishing nets to snag and kill rays.
"Not much is known about cow-nosed rays because nobody has really bothered to study them," Fisher said, watching the graceful swimmers cruise the perimeter of his lab's tank.
Cow-nosed rays, Rhinoptera bonasus, are an ancient species related to sharks with similar gray skin. But they are shaped more like stealth bombers than torpedoes. They lack sharp teeth, having bony plates in their sucker-like mouths to grind up shellfish and worms.
Although the rays have poisonous stingers at the base of their long tails, they seldom use them, Fisher said. People usually get stung only when they try to pick them up.
Chesapeake rays are of a smaller species than the Pacific stingray that pierced the heart of television naturalist Steve Irwin last year.
Cow-nosed rays are about three feet across when fully grown, weighing about 35 pounds. Their nickname comes from their slightly bovine appearance, with eyes on the sides of their heads.
The animals spend their winters in the warm waters near Florida, flocking into the Chesapeake Bay every May in schools of sometimes millions. They mate in June, traveling as far north as Baltimore's industrial harbor during the summer. Then they swarm southward again in September.
Like sharks, rays have specialized pores in their skin, called Ampullae of Lorenzini, that can detect the faint electronic pulses generated by the beating of hearts. With these tiny sensory organs, they can find food hidden under the sand. Fisher is experimenting with these pores to try to find a way to repel rays.
He keeps four rays - one he nicknamed Nacho, another Scar - in a 21-foot-long tank in his lab. He records their movements with an underwater video camera. He discovered last year that when he put magnets into the tank with crabmeat as bait, the rays avoided the meat with the magnets nearby and ate the bait farther away.
His hypothesis is that magnets irritate rays the same way they repel some sharks. He is expanding the experiment by putting 400 magnets on the bay's floor to see if they can protect a small oyster bed he's planting. It may be impractical to guard all of the bay's shellfish with magnets, he said, but perhaps oyster farms could be guarded with machines that generate low electronic fields.
Another idea is to encourage people to eat rays before the rays eat all the oysters. Fisher has worked with a seafood trade organization to distribute ray meat to chefs in Virginia restaurants. They've tried ray fajitas, cow-nosed ravioli and stingray soup.
But the idea hasn't taken off with the public, in part because the flesh has an unusual texture. "It's a very bloody meat, and that's a drawback in a seafood product."
He sees ray skins as a possible leather substitute in luxury goods. "The skin of rays tans up very nicely, it's very durable and has a nice texture," he said. "Ray purses are very popular in Asia."
Dean Grubbs, a biologist at Florida State University, said he worries rays are getting a bum rap, being scapegoated for the disappearance of oysters. Creating a commercial fishery for rays could be disastrous because they reproduce very slowly. He noted that South American dealers sold Brazilian cow-nosed rays to Korea as a food in the 1980s and 1990s, only to see that species nearly become extinct.
"It takes them eight years to reach maturity, and they have only one pup a year," Grubbs said. "If you started a fishery, you could fish them out very quickly."