WASHINGTON - Farmed fish have been fed meal spiked with the chemical linked to the pet-food recall, but the contamination was probably too low to harm anyone who ate the fish, federal officials said yesterday.

The Canadian-made meal included what was purported to be wheat gluten, a protein source, imported from China. The material was actually wheat flour spiked by the chemical melamine and related nitrogen-rich compounds to make it appear more protein-rich than it was, officials said.

After pigs and chickens, the farmed fish mark the third food animal given contaminated feed. The level of contamination is expected to be too low to pose any danger to human health, said David Acheson, an assistant commissioner at the Food and Drug Administration.

It was not immediately clear if any of the farmed fish entered the food supply. But Acheson said that at least one firm's fish were still too young and small to be sold. Investigators were visiting other American aquaculture farms that used the contaminated feed.

Melamine, found in plastics and pesticides and not approved for use in pet or human food in this country, contaminated pet food that sickened or killed an unknown number of dogs and cats.

Federal health and food officials have said about 20 million chickens and thousands of hogs also were fed feed contaminated by melamine. As with the fish, they said the risk to human health was very low.

U.S. investigators also have learned that the purported Chinese wheat gluten and a second ingredient, rice-protein concentrate, were actually simple wheat flour. The flour was spiked with melamine and related nitrogen-rich compounds to make it appear to be richer with protein than it was.