WASHINGTON - Consumer advocates urged the U.S. Food and Drug Administration on Tuesday to ensure that salmon engineered to grow twice as fast as the conventional variety are labelled in the grocery store as genetically modified.

The FDA is holding a hearing to hear opinions about whether, if the FDA should approve sale of the fish, the salmon should be labelled. According to FDA rules, the fish would not be labelled as genetically modified if the agency should decide it has the same material makeup as conventional salmon.

The agency already has said it believes the two fish are the same, but is holding the hearing to hear thoughts from both critics and supporters before the agency makes a decision whether to allow the fish to be sold.

Consumer advocates say it is the public's right to know. Dr. Michael Hansen, senior scientist at Consumers Union, the publisher of Consumer Reports, testified that his organization disagrees with the FDA that genetic engineering itself does not constitute a material difference in the two fish. He added that the agency has the authority to demand the labels, citing previous rules that allowed foods to be labelled so people with religious or cultural sensitivities could avoid them.

Labelling also is a safeguard for the safety of the fish, he said.

"You need this labelling so if there's a problem down the road, you can trace it back," Hansen said.

Ron Stotish, CEO of AquaBounty, the company that has developed the fish and is applying to the FDA to market it, argued that genetically modified salmon have the same flavour, texture, colour and odour as the conventional fish.

"This fish is an Atlantic salmon," he said.

The Atlantic salmon engineered by AquaBounty has an added a growth hormone from a Chinook salmon that allows the fish to produce growth hormone all year long. The engineers were able to keep the hormone active by using another gene from an eel-like fish called an ocean pout that acts like an on switch for the hormone, which conventional salmon produces only some of the time.

In documents released ahead of the hearing, the FDA agreed with the company, saying there were no biologically relevant differences between the engineered salmon and conventional salmon, and there is a reasonable certainty that no harm will come from its consumption.

At a hearing on the science of the fish held Monday, FDA scientists said there are very few differences between the modified and conventional fish. An advisory panel that heard the evidence was more cautious, saying more study is needed to be sure.

Two experts speaking at Tuesday's labeling meeting agreed with the FDA that there are no obvious material differences between the two fish. Alison L. Van Eenennaam of the University of California, Davis, and Gregory Jaffe of the Center for Science in the Public Interest said there was no evidence from the data provided that the two were different enough the be labelled as such, though Jaffe urged the FDA to ensure the fish is somehow branded so consumers know what they are eating.

"The reality is there are consumers out there who want to know if their salmon has been genetically engineered," he said.

If the FDA should approve the sale of the salmon, it would be the first time the government had allowed such modified animals to be marketed for human consumption. AquaBounty submitted its first application for FDA approval in 1995, but the agency did not decide until two years ago to consider applications for genetically engineered animals.

Genetically engineered animals are not clones, which the FDA has already said are safe to eat. Clones are copies of an animal. In GE animals, the DNA has been altered to produce a desirable characteristic.

Critics have two main concerns about the modified fish: The safety of the food to humans and the salmon's effect on the environment.

Because the altered fish has never been eaten before, they say, it could include dangerous allergens, especially because seafood is highly allergenic. They also worry that the fish might escape and intermingle with the wild salmon population, which is already endangered. They would grow fast and consume more food to the detriment of the conventional wild salmon, the critics fear.

The company has several safeguards in place to quell concerns. The fish would be bred female and sterile, though a small percentage might be able to breed. They would be bred in confined pools where the potential for escape would be low.

The FDA has said the fish should not cause any allergies not already found in conventional salmon, and there is little chance they could escape. But Monday's advisory panel, which was formed to give input to the agency and did not hold a final vote, cast some doubts on whether enough evidence had been presented to back up those assertions.

It remained unclear whether the public will have an appetite for the fish even if it were approved. Genetic engineering is already widely used for crops, but the government until now has not considered allowing the consumption of modified animals. Although the potential benefits, and profits, are huge, many people have qualms about manipulating the genetic code of other living creatures.

If approved, the fish could be in grocery stores in two years, the company estimates.