Extensive testing of Gulf of Mexico seafood by federal scientists has found just minute traces of the dispersant Corexit, which was used to break up oil from the BP spill, officials say. About 1.8 million gallons of dispersant were applied to the waters’ surface and at the wellhead, almost a mile undersea.

Of 1,735 tissue samples analyzed, only 13 showed trace amounts of dispersant residue, in concentrations fine below safety thresholds established by federal agencies. Other current tests from federal waters reopened to commercial fishing have shown little or no detectable oil, and no samples that exceed federal safety guidelines.

“The overwhelming majority of the seafood tested shows no detectable residue, and not one of the samples shows a residue stage that would be harmful for humans,” Margaret A. Hamburg, commissioner of the Food and Drug Administration, said on Friday. “There is no question gulf seafood coming to market is safe from oil or dispersant residue.”

Roughly 96 percent of federal waters in the gulf are now open to commercial and recreational fishing. At the height of the spill, 37 percent of federal waters, an area of nearly 90,000 square miles, was closed to fishing.

Despite what the federal government insists are rigorous and transparent testing procedures, some environmentalists and gulf residents continue to express skepticism that the fish is safe to eat.