National Geographic News | Scientists have created hundreds of mutant fish with "six-pack abs" and bulging "shoulders" by beefing them up with new genes.

While the fish aren't going to win any beauty contests, the genetically engineered rainbow trout could hold some appeal at market, because they each provide 15 to 20 percent more flesh than standard tout, researchers say.

(See pictures of the world's largest trout in the wild.)

Developed with fish farming in mind, the genetically modified trout is the result of ten years of experimentation by a team led by Terry Bradley of the University of Rhode Island's Department of Fisheries, Animal, and Veterinary Sciences.

The team injected 20,000 rainbow trout eggs with different types of DNA from other species, making them transgenic. The added DNA was intended to suppress a protein called myostatin, and it apparently worked in about 300 of the eggs, turning them into the muscle-bound superfish.

The transgenic trout incorporate genes modeled on myostatin-inhibiting proteins found in powerfully built Belgian blue cattle, a beef breed noted for its "double muscled" appearance.

In mammals, including humans, mysostatin is known to keep muscle growth in check—controlling myostatin is touted as a potential way to reverse muscle-wasting diseases in humans.

(Related: "Forty Percent of North America's Freshwater Fish at Risk.")

New Genes Showcase Six-Pack Abs

The muscle-bound trout is the first real proof that mysostatin inhibition has a similar effect in both fish and mammals.

Although fish lack abdominal muscles, the modified trout exhibited a "six pack" effect along the sides of the their midsections and developed prominent humps on their backs, Bradley recently reported.

"Our findings are quite stunning," Bradley said in a statement. "The results have significant implications for commercial aquaculture."

If met with regulatory approval, the fish-gene modifications could mean cheaper trout for consumers, as farmers would be able to grow larger fish without having to feed them more, he said.

(Video: Biggest Trout Fished in Asia.)

Mutant-Trout Takeover?

Though some trout with altered genes have been approved for release, trout with added DNA from other species have yet to be approved for commercial use, according to zoologist Fredrik Sundström of the University of Gothenburg in Sweden.

Other genetically modified trout in the works have been engineered for faster growth, disease resistance, or frigid-water survival (via "antifreeze genes").

Sundström, who has investigated the potential risks of transgenic trout escaping into the wild, said studies suggest the fish can not only breed in rivers but also pass on their lab-altered genes to natural populations. (Read about threats to freshwater fish.)

"Under certain conditions the transgenic fish do better than the wild types, but under other conditions we see the opposite," he added.

"If they have a lot of food, transgenic fish can use that food to a greater extent, but if you have predators nearby they also seem to be more susceptible to predation," Sundström said.

He doubts, however, whether this latest transgenic trout would find enough food in the wild to support its body builder physique—or that the bulky fish would be able to maneuver swiftly enough to avoid being eaten.

But if the fish did survive in the wild—for instance, if juveniles are able to "grow too big for birds to feed on them"—they could overturn their ecosystems, Sundström said. For one thing, he said, the six-pack trout's greater size could allow them to outcompete their unmodified cousins, leaving them with little food and an imperiled future