By Richard Black
BBC science correspondent
A Canadian firm developing genetically modified fish says it aims to apply for regulatory approval by the end of 2004.
A modified fish alongside a normal fish of the same age
Aqua Bounty has developed salmon which grow several times faster than normal.
Environmentalists oppose the technology because, they say, studies have shown how escaped GM fish could breed with and damage populations of wild salmon.
But Aqua Bounty, based in Newfoundland, says such impacts can be avoided by making the novel fish sterile and growing them in isolated tanks.
If regulatory approval is granted, it could open the door to other varieties of GM fish; about 30 different types have so far been created in various laboratories around the globe.
The BBC World Service programme Earth Files travelled to Canada to see Aqua Bounty's transgenic salmon.
In a giant tank in the research laboratory of the Ocean Sciences Centre, the company is rearing modified fish alongside natural counterparts of the same age. The difference in size is startling.
"In the early stages, the transgenic fish will grow four or five times faster," says Aqua Bounty's president and CEO Dr Garth Fletcher.
"Right now, these growth hormone transgenic fish are about three-and-a-half kilogrammes, and the control salmon would be about a kilogramme."
The aim is not to produce bigger fish, but to bring them to market size faster, if possible within a year.
Aqua Bounty has been discussing its salmon with the United States Food and Drug Administration for nearly a decade.
The company has submitted a substantial amount of data in response to questions from the FDA and other US government agencies, such as the National Marine Fisheries Service.
It believes this data shows that the enhanced fish can be grown and eaten safely, and Dr Fletcher hopes to make a formal application for FDA approval before the end of the year; processing that application would take a mandatory six months.
Environmental groups are concerned that GM salmon would escape from farms, and could devastate populations of wild salmon.
Escapes of farmed fish are routine - it is estimated about two million escape from farms in Europe each year.
Last year, a team led by Dr Phil McGinnity, from Ireland's Marine Institute, published research showing that interbreeding between farmed and wild salmon is leading to a decline in fitness.
Would the market for salmon accept GM fish?
"Principally we observed hybrids - the results of interbreeding between farmed salmon and the wild population - and this showed there was reduced survival compared with the wild salmon."
The same 10-year study also showed that farmed salmon are forcing their wild relatives out of their native rivers.
"The farmed salmon grew faster, and we found that they displaced wild fish; and if you're being out-competed and pushed out, you're not going to survive, you're going to die," said Dr McGinnity.
"With GM, the situation is likely to be comparable and is likely to be more extreme."
Sue Scott, from the conservation group the Atlantic Salmon Federation, told the Earth Files that wild populations had already declined markedly over the last 10 years; and interactions between wild and farmed salmon were a major cause.
"It can only be assumed that the transgenic salmon will cause the same problems as the farmed salmon; only because they're bigger, they're really going to compete for food and habitat," she said.
"We want to see a moratorium on transgenic salmon until we can safeguard against any problems which may result from this industry."
However, Aqua Bounty says there are ways to ensure that transgenic salmon do not escape; and that if they do, they cannot breed with wild relatives.
It plans only to sell sterile females which do not lay eggs, so there will be no cross-breeding. It says the sterilisation process, known as triploidisation, is around 99.8% effective.
Triploid fish have three sets of chromosomes in each cell rather than the usual two sets; triploid females do not lay eggs.
Aqua Bounty also says that escapes could be prevented altogether by isolating the fish in inland pools rather than in underwater pens in rivers and coastal waters.
Is there a demand?
With the benefits that genetic manipulation could potentially bring - faster growth, improved flesh quality, disease resistance, cold tolerance - it would be easy to assume that the aquaculture industry would be keen on transgenic fish.
But Nell Hulse, president of the Canadian Aquaculture Industry Alliance, said the farmers she represented were against it.
"We do not support the commercial growth of genetically modified salmon on our farms, and that policy won't change until a number of things have taken place," she told the Earth Files.
There are concerns about the impact escapees would have on wild salmon
"It would have to be declared very clearly to be a safe product, from the point of view of human health; and more importantly, there has to be a market demand."
Genetically modified crops are widely used in North America, though they are regarded with intense suspicion in many other parts of the world. But there are signs that transgenic fish might not be as well received.
The pressure group the Centre for Food Safety lists 468 businesses in the US which it says have pledged not to buy or sell GM fish.
One is the four-star New York seafood restaurant Le Bernardin, once named top restaurant in the US.
"It's totally unnatural, and it could actually be dangerous," said executive chef Eric Ripert.
"The fact that they are not going to let us know on the markets where we buy the fish whether they are genetically modified or not is to me a big issue, and I refuse to buy that fish.
"We find wild salmon in North America and in Scotland and in Norway; and I think if we are reasonable in fishing and we don't over-fish, we can keep those stocks forever and enjoy these natural fish."
The Food and Drug Administration keeps negotiations secret and will not say how it views Aqua Bounty's salmon; neither will it say whether it is in discussion with other companies on other transgenic fish and shellfish.
Garth Fletcher is convinced that GM fish will inevitably come to market; and if his negotiations with the FDA go well, he says that could be as early as next year.
"Test marketing could be done just after FDA approval, and that could be 2005; commercial sales, you're looking at two to three years beyond that."