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(CNN) -- U.S. health officials said Tuesday that fish intended for human consumption were fed meal that was contaminated with the chemical melamine.
But the officials said the fish doesn't pose any significant risk to human health.
Federal authorities made the discovery as part of an ongoing investigation into the addition of the industrial chemical melamine and related compounds into pet food, said Dr. David Acheson, the Food and Drug Administration's assistant commissioner for food protection.
One of two Chinese companies already under investigation for having shipped adulterated and mislabeled wheat flour to the U.S. also shipped some of the product to Canada.
In Canada, it was used to make fish meal, Acheson told reporters in a conference call.
That meal was then shipped from Canada to companies in the United States for use in fish farms and to feed fish used to stock bodies of water, he said.
Chinese firm manager detained
The manager of one of the Chinese companies that sold the suspected tainted wheat flour has been detained for nearly two weeks outside Beijing, CNN has learned.
Tian Feng is the manager of Binzhou Futian Biology Technology Co. Ltd., which U.S. pet food distributors have identified as the company that sold them wheat flour, which was mislabeled as wheat gluten or rice protein concentrate and contained melamine and related products. (Watch where Tian's company fits into the chain of events )
Tian's company was shut by local police on April 25, the day he was detained.
"I didn't do anything wrong," Tian said in an interview with CNN from the detention center in Binzhou in China's eastern Shandong Province.
Dressed in a white T-shirt and orange prison vest, Tian said, "I don't know about melamine. I don't even know what this melamine is. I have never heard of anyone using it."
Under Chinese law, police can hold Tian for 30 days while the investigation continues. After that, he must be tried or released.
Officials: Fish pose no 'significant' danger
Acheson, who would not divulge what states the meal was sent to, said there is no reason for alarm.
"We do not believe that there is any significant risk associated with consuming these fish," he said.
Acheson added that investigators are traveling to the companies "to determine exactly what they are doing with the fish."
The Oregon Department of Fish and Wildlife said Tuesday that the FDA confirmed the finding of melamine in fish feed at its Marion Forks Hatchery, and that the Canadian fish feed producer, Skretting, told the department the same lot of fish feed was sent to six other hatcheries in the state.
"Hatchery managers immediately discontinued using any remaining fish feed from the lot in question and notified the FDA," the statement said.
Skretting, based in Vancouver, British Columbia, said it was voluntarily recalling all feed related to the batch of Bio-Oregon feed in question, although it had received no complaints related to "unusual fish health issues."
Customers would be offered replacement feed, Skretting said, and it was working closely with the FDA and the Canadian Food Inspection Agency.
"Please be assured that Skretting takes any food safety issue very seriously, and that our tracking and tracing ability ensures that any emerging problems can be evaluated and dealt with quickly and efficiently," the company said.
The discovery of the contaminated imports from China was announced Tuesday after analysis of the products showed they had been mislabeled, Acheson said.
The FDA is "considering possible enforcement actions," he said.
Agency investigators are expected to return to the U.S. next week from China, he said. He would not say what they have found.
Acheson said officials suspect that the Chinese substituted wheat flour for wheat gluten and rice-protein concentrate, then attempted to make them appear to be the protein-rich substances by adulterating them.
Melamine is used to make plastics and fertilizer. It has no food value but is rich in nitrogen. In tests, that nitrogen level makes products seem higher in protein, which can lead to higher prices.
"If your measurement of protein is actually measuring nitrogen, what you've got is a wheat-based pellet that appears to have high protein because the nitrogen is high, due to the addition of melamine," Acheson said.
Cats dead from kidney failure
The FDA began its investigation in March, when Ontario-based Menu Foods discovered that nine of 25 cats involved in a taste test died of kidney failure.
The company has voluntarily recalled 60 million cans of its pet food.
Since then, the FDA has received thousands of complaints from pet owners alleging pet food killed their pets.
The investigation recently widened to include human food after it was discovered that swine and poultry were fed contaminated scraps from the pet food makers.
But authorities said this week that a risk assessment that took into account the "dilution factor" determined that the pork and poultry products present an extremely low risk to people.
The "dilution factor" notes that the amount of tainted food eaten by people would be far less than the amount of product that proved fatal to pets.
Dr. Kenneth Petersen, assistant administrator for field operations of the Food Safety and Inspection Service, a part of the U.S. Department of Agriculture, said 20 million chickens that were fed contaminated feed tested negative for melamine.
The feed was ordered held last week, but the chickens have been approved for slaughter and subsequent sale.
Tests on another 100,000 breeder birds in Indiana have not yet come back negative, and they remain on hold, he said.
Some 2.7 million birds from roughly 30 farms in Indiana have already made their way into commerce, he said.
Negative tests have not yet been received for 50,000 swine that ate the contaminated feed, and they also are being held, he said.
Copyright 2007 CNN. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed. Associated Press contributed to this report.
U.S. officials say Melamine has been found in feed given to about 20 million poultry and thousands of hogs.
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Sources: OSHA, CDC