U.S. health regulators on Thursday cleared the way for a type of genetically engineered Atlantic salmon to be farmed for human consumption - the first such approval for an animal whose DNA has been scientifically modified.

Five years ago, the U.S. Food and Drug Administration first declared the product, made by Massachusetts-based AquaBounty Technologies, to be as safe as conventional farm-raised Atlantic salmon.

AquaBounty's product will not require special labeling because it is nutritionally equivalent to conventional farm-raised Atlantic salmon, the FDA said on Thursday.

AquaBounty developed the salmon by altering its genes so that it would grow faster than farmed salmon, and expects it will take about two more years to reach consumers' plates as it works out distribution. AquaBounty is majority owned by Intrexon Corp, whose shares were up 7.3 percent at $37.55 in afternoon trading.

AquaBounty says its salmon can grow to market size in half the time of conventional salmon, saving time and resources. The fish is essentially Atlantic salmon with a Pacific salmon gene for faster growth and a gene from the eel-like ocean pout that promotes year-round growth.

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Activist groups have expressed concerns that genetically modified foods may pose risks to the environment or public health. Several on Thursday said they would oppose the sale of engineered salmon to the public, while some retailers said they would not carry the fish on store shelves.

Kroger Co, the nation’s largest traditional grocery chain, has “no intention of sourcing or selling genetically engineered salmon,” spokesman Keith Dailey said. Trader Joe's and Whole Foods Market Inc also confirmed that they do not intend to carry the product.

Target Corp eliminated farm-raised salmon in favor of wild-caught salmon in 2010, which spokeswoman Molly Snyder said was the first step in a long-term commitment to improving the sustainability of our seafood assortment. "We are not currently planning to offer genetically engineered salmon," Snyder said.

AquaBounty Chief Executive Ronald Stotish said the approval is "a game-changer that brings healthy and nutritious food to consumers in an environmentally responsible manner without damaging the ocean and other marine habitats."

The approval for the fish, to be sold under the AquAdvantage brand, requires that the salmon be raised only in two designated land-based and contained hatcheries in Canada and Panama, and not in the United States. All of the fish will be female, and reproductively sterile, to prevent inadvertent breeding of the genetically modified fish with wild salmon, FDA officials said.

The agency on Thursday also issued draft guidelines on how food manufacturers could identify whether the salmon in their products are genetically modified. The guidelines state that such labeling would be voluntary.

Stotish said in an interview that AquaBounty will follow the FDA’s rule for labeling and currently “there would be no requirement for labeling."

"Frankly, it’s an area that we don't have to address today,” he said.

CREDIBLE EVIDENCE

Dr. William Muir, a professor of genetics at Purdue University who had urged the government to approve the salmon, said there is "no credible evidence" that these genetically modified fish are a risk to either human health or the environment.

"The current practice of using wild-caught salmon as a food source is not sustainable, our oceans are overfished," Muir said.

Muir was among 80 scientists and biotech industry executives who in 2014 sent a letter to President Barack Obama asking for administration support for this approval.

AquaBounty's salmon was originally developed as a fast-growing variety by a group of Canadian public university scientists over a quarter of a century ago, and the company has been trying to get regulatory approval for almost two decades, said Dr. Alison Van Eenennaam of the University of California, Davis, who served on the 2010 FDA Veterinary Medicine Advisory Committee which looked at the AquaBounty salmon.

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She called the FDA's five-year decision-making process on the fish "unprecedented" and said the approval was "long overdue." FDA policy analyst Laura Epstein told reporters that because the approval was the first of its kind, the agency wanted "to get everything right" and offer many opportunities for public comment.

FDA spokeswoman Juli Putman said AquaBounty first began a conversation with FDA in the mid-1990s. She said the agency reviewed data and information from the company as it was submitted, up until July of this year.

Joe Perry, former chair of the European Food Safety Authority, said European regulators would require a lot more data than the FDA before giving a similar green light to engineered salmon.

U.S. consumer and environmental groups also renewed their opposition to the product.

Patty Lovera, assistant director for Food & Water Watch, said the group is talking to members of Congress about rolling back the FDA approval. The group is also considering a lawsuit to block genetically modified salmon from reaching the market.

Activist group Friends of the Earth estimates that at least 35 other species of genetically engineered fish, along with chickens, pigs and cows, are under development. The FDA's decision on salmon may set a precedent that could make approval for other genetically modified animal species easier.

FDA officials would not comment on whether it has received any other applications for genetically modified animals.