WASHINGTON, May 17 (Reuters) - Two U.S. commercial fish farms have been cleared to resume selling fish after tests showed no contamination with the industrial chemical, melamine, the Food and Drug Administration said on Thursday.

Investigators with the FDA and the U.S. Agriculture Department are tracing shipments of melamine-tainted vegetable protein concentrates from China that were used to make U.S. feed for pets, pigs, chickens and fish.

So far, all the melamine contamination has been linked to shipments from two Chinese companies.

Dr. David Acheson, the FDA assistant commissioner for food protection, said tests showed no melamine present in fish grown at the Kona Blue farm in Hawaii and the American Gold farm in Washington state. Both bought feed made with the Chinese- sourced protein concentrates.

In addition to the two commercial fish farms, tainted feed may also have been sold to 196 fish hatcheries, the FDA said.

"There is a very low risk of harm from eating fish that consumed this tainted feed," Acheson told reporters.

As a precaution, the FDA has detained a total of 46 shipments of vegetable protein concentrates imported from China since April 27.

"They remain in detention until we are satisfied" that the shippers have documentation proving their cargo is free of melamine, Acheson said.

The two Chinese companies are believed to have substituted melamine -- a chemical used in plastics and fertilizers -- for more costly ingredients in the vegetable protein concentrates.

Acheson acknowledged that similar substitutions "certainly could be going on" by other Chinese manufacturers. The FDA is in discussions with the Chinese government to tighten food safety standards, he said.

In a related step, the U.S. Customs and Border Protection is sampling imports of wheat and corn gluten from all countries. No melamine and related compounds have been found in any of the samples, said Vera Adams, director of enforcement for the customs agency.

Within the domestic market, FDA inspectors visited feed manufacturing plants in six states to collect samples of finished products made with vegetable protein concentrates from China. Of 63 samples, 37 were negative for melamine and 23 are still being tested in laboratories, Acheson said. Three samples could not be analyzed because they were a mixture of rawhide dog chews and other materials.

The federal government's probe of the tainted feed began in mid-March following reports of kidney failure in some dogs and cats. Pet food makers that used ingredients from the two Chinese companies recalled more than 100 brands of melamine- laced feed.

The FDA and USDA investigation has widened to other animal feed and officials placed a temporary hold on 56,000 hogs in seven U.S. states and 80,000 breeder chickens in Indiana.

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