Up and down the beach at Ocean City, they washed ashore: dead fish, rotten and smelly, victims of what scientists believe was a sudden shift in the temperature of the ocean.

More than a million Atlantic croaker fish are believed to have died in recent days off the Maryland and Delaware coasts, and many of them have turned up on the beaches from Ocean City to Delaware's Indian River, Maryland officials said.

The fish kill, which could be the largest in Maryland's recent history, showed no signs of a man-made cause, such as a toxic chemical spill or pollution-fed algae bloom, said Rich McIntire of the Maryland Department of the Environment.

Instead, officials said the fish might have been killed by a natural phenomenon called an "upwelling," a sudden surge of cold water from the bottom of the ocean.

When the water reached the area where croakers live -- depths of 60 to 90 feet -- the temperature dropped from about 60 degrees to about 40 to 50, scientists said.

Many fish simply died of "thermal shock" -- stress caused by the change of temperature. McIntire said the fish were probably five to 14 miles offshore when they died.

Yesterday, people in Ocean City described a tide of croakers washing up on the beaches.

"It's a steady stream of them, floating out with the tide," said Bob Gowar, who owns a charter fishing boat. Gowar said he had also seen dead fish in nearby Chincoteague Bay, looking as if they had been dead for several days.

"They're just about rotten," he said. "The seagulls won't even pick on them."

From her condominium 13 floors above the city's famous beaches, Dottie Pinto said she could see workers walking slowly along the beach, throwing dead croakers into trucks.

"In the 10 years we've been there, we have never seen something like this before," Pinto said.

She said locals were joking about the problem -- "The croakers croaked!" -- but also dealing with a nasty smell.

"If you're in the Ocean City area, the air smells fishy," she said.

McIntire said it was unclear what might have caused the upwelling of cold water, but he said it might have been Hurricane Alex, approaching North Carolina. He said as many as 5 million croakers might have died.

An upwelling is typically caused by strong winds that blow surface water offshore, causing bottom water to rise to the surface and take its place, said Shenn-Yu Chao, a professor at the University of Maryland's Center for Environmental Science.

"If this upwelling is too intense, very cold water gets sucked up," Chao said. "And some fish may not be able to adapt to it, and they die."

Croakers are bottom fish that can grow to 18 inches long. They are related to black drum and red drum, and they all get their names because they use special muscles to vibrate their swim bladders and make a croaking or drumming sound.

Maryland officials said that croakers are particularly intolerant of cold water. They said other fish better adapted to cold, such as tautog and black sea bass, had been caught in healthy condition this week after the croakers died.

One of what could be as many as 5 million dead croakers lies on a beach in Ocean City.Experts say croakers are particularly intolerant of cold water. Other fish better adapted to cold have been caught in healthy condition this week.