At the Xulong eel factory here, a team of workers slice eels, lop off their heads and push them through a huge assembly line that will cook and package them for millions of customers around the world.

The precision round-the-clock operation, aided by a roasting oven that spans the length of a football field, is one reason China now dominates the world’s seafood trade, and supplies 80 percent of America’s imported eel and 70 percent of its tilapia.

But the Food and Drug Administration says Xulong and other Chinese companies will be restricted from selling certain types of seafood in the United States because regulators keep finding Chinese imports contaminated with carcinogens and excessive antibiotic residues.

Here in the Pearl River Delta area, near Hong Kong, it is not hard to see why. Rivers, lakes and coastal waterways are so fouled with industrial chemicals or farm effluents that many seafood exporters are forced to rely on antibiotic drugs to keep their fish alive.

China’s coastal regions, after all, are also home to its biggest factories, which are famous for churning out electronics, processing chemicals and dumping mountains of toxic waste.

At the Xulong factory here, officials offered a tour of what they said was an up-to-date plant that forces workers to disinfect themselves by going through multiple washing stations. The officials showed off on-site testing labs and boasted that pure water from a local reservoir made their eel the best in China.

Even so, the company’s eel has been refused entry into the United States on multiple occasions. Last April, the F.D.A. refused four shipments of roasted eel from a nearby Xulong factory because they contained residues of banned antibiotics that could prove harmful to consumers.

In an interview here on Saturday, Xu Liming, vice chairman of the Xulong Group, defended the quality and safety of his products.

“There are a lot of poor places in China that don’t care about food safety,” said Mr. Xu, who help found the company with two brothers in 1983. “But we’re a big company and we’ve invested a lot in food safety. We’re the only eel producer certified to ship to Europe.”

But if Xulong — which is the world’s biggest eel producer and claims to have some of the cleanest operations in China — at times cannot pass muster with American regulators, how many Chinese seafood companies can?

The question has huge implications for the global seafood trade, and for the United States, which imports 80 percent of the seafood Americans consume.

The heightened concern has also set the stage for a nasty trade dispute. After a series of high-profile recalls of Chinese-made goods — from tainted toothpaste and pet food to toxic toys and defective tires — some members of Congress are pushing for stronger measures against Chinese imports. And European Union officials say they are considering their own restrictions.

Experts say a broader crackdown could be a severe blow to China’s $35 billion fish- and seafood-farming, or aquaculture, industry, which is helping meet soaring demand for seafood at a time when supplies of wild fish stocks are being depleted.

“This is certainly bad for Chinese aquaculture,” said Rohana P. Subasinghe, a fish-farming expert at the United Nations Food and Agriculture Organization. “A ban on any product to any major region or country has tremendous repercussions for the country and the industry.”

The new F.D.A. restrictions, announced Thursday, effectively ban some of China’s biggest seafood imports, including shrimp, catfish, eel and a type of carp. The move drew a quick rebuke from China, which on Friday warned the United States about acting “indiscriminately.”

China is already the leading supplier of seafood, garlic and apple juice concentrate to the United States, and it is gaining market share in processed vegetables, frozen foods and food ingredients. That is worrying food-safety experts, who say American regulators are ill equipped to deal with China’s rise as a major food supplier.

“China has gone from literally nowhere to No. 3 in food imports behind Canada and Mexico,” said Michael Doyle, director of the Center for Food Safety at the University of Georgia. “And if we’re going to continue to import more and more of our food, we’re going to have to have a better inspection program.”

In the United States alone, Chinese seafood imports jumped from about $550 million in 2001 to about $1.9 billion last year, about 22 percent of total seafood imports. But 60 percent of the seafood shipments that were refused entry by American regulators came from China.

And those figures may not tell the full story. Robert Schubert, director of research at Food and Water Watch, a nonprofit group, says the F.D.A. is sampling only a tiny fraction of the food shipments entering American ports, which means much of the tainted seafood may be making it to stores.

“The F.D.A. needs its budget massively increased, and it needs to respond with more testing,” said Mr. Schubert, co-author of a study on the growth of American seafood imports.

What has been stopped by inspectors is alarming. In May alone, regulators tagged “filthy frozen scallops”; catfish, eel and shrimp laced with banned chemicals; unsafe additives; pesticides; and cancer-causing agents.

European Union officials say they have also noticed a rise this year in the number of Chinese seafood shipments turning up with banned chemicals, despite strict procedures, including food-safety test certificates presented by the Chinese government.

[“We are reviewing our measures in light of a number of factors,” Philip Tod, a spokesman for the European Commission said Monday, noting that European Union member countries have issued nine Chinese seafood alerts so far this year, up from three in all of 2006. “That is a cause of concern. We are aware there appears to be a problem with veterinary medicine residues.”]

This is not the first time Chinese seafood has run into problems. In recent years, the European Union and Japan have both placed restrictions on imports of Chinese seafood after detecting banned antibiotics, like malachite green. And this year, several Southern states in the United States banned or blocked imports of Chinese catfish after detecting illegal antibiotics.

Part of the problem, experts say, is that breeding ponds in China are overcrowded to bolster production in the gigantic factory-style fish farms. And fish excrement and bacteria in the water can devastate large schools of fish.

“When you’re raising thousands and thousands of fish together, you have disease spreading,” Mr. Schubert said. “And the operators try to control that by using drugs and antibiotics.”

In addition, a recent study by scientists from the Chinese Academy of Sciences found that seafood products in 11 coastal cities in the Pearl River Delta area were heavily contaminated with pesticides, including DDT, which was banned in China in 1983.

“The only region that reports higher levels of DDTs is Egypt,” the report said. “This indicates that the coastal region of southern China is probably one of the most DDT-polluted areas in the world.”

Another study released in May by local scientists was just as damning, finding that the coastal waters around Guangdong are being devastated by large deposits of oil, lead, arsenic, mercury and copper.

So when heavy rains hit the area earlier in June, government scientists issued a seafood alert because of a huge toxic “red tide,” an algal bloom that was carrying industrial waste to some of the region’s biggest seafood-producing areas.

Consumers were warned not to go swimming and not to eat local seafood.

Given the problems found with Chinese seafood, American regulators say they had no choice but to impose new restrictions. “There’s been a continued pattern of violation with no sign of abatement,” said Dr. David Acheson, the F.D.A.’s assistant commissioner for food protection.

Many Chinese seafood exporters say they get their supplies from local fish farmers, who sometimes overuse antibiotics. But the exporters also say the F.D.A. restrictions are overly harsh and smack of politics.

“This is all about trade protectionism,” said Gao Hua, director of quality at the Meihua Aquatic Processing Factory in Fujian Province. “Some U.S. states suddenly raised their standards on the content of antibiotics in seafood in April. Maybe they saw too many imports from China.”