Despite the approval, it is likely to be at least two years before any of the salmon reaches supermarkets, and at first it will be in tiny amounts.

Ronald Stotish, the chief executive of AquaBounty, which is majority-owned by Intrexon Corporation, said he was delighted and somewhat surprised by the approval after all this time. “We had no indication that approval was imminent,” he said in an interview.

Mr. Stotish declined to say what the plans were for bringing the fish to market, other than that the salmon would not be in stores immediately because it would take about two years for even these fast-growing salmon to reach market size. It is also not likely there will be much of the salmon on the market because the approved production facility, which is in Panama, has the capacity to produce only about 100 tons of fish a year — a tiny amount compared with the more than 200,000 tons of Atlantic salmon the United States imports each year.

Mr. Stotish said he did not know if approval was still needed from Panama to export the fish.

It is not clear how well the salmon will sell. Some leading supermarkets have already said, in response to the vocal opposition, that they have no plans to sell it.

The fish are supposed to be raised inland in contained tanks to lessen the chances that they will escape into the wild. AquaBounty and its supporters say this will also be less stressful on the environment than using pens in the ocean. And it could eventually allow the fish to be raised in the United States, rather than being imported, as most farmed Atlantic salmon is.

For now, however, the fish are being raised in Panama, from eggs produced in Prince Edward Island, Canada. If the salmon were bred or raised elsewhere, for marketing to Americans, that would require separate approvals.