Don't expect to find genetically modified salmon — or any other engineered fish or meat — on store shelves anytime soon.

The Obama administration has stalled for more than four years on deciding whether to approve a fast-growing salmon that would be the first genetically modified animal approved for human consumption.

During that time, opponents of the technology have taken advantage of increasing consumer concern about genetically modified foods and urged several major retailers not to sell it. So far, two of the nation's biggest grocers, Safeway and Kroger, have pledged to keep the salmon off their shelves if it is approved.

Proponents of genetically engineered fish and meat say they expect Food and Drug Administration approval of the salmon and still hope to find a market for it. However, the retailers' caution and lengthy regulatory delays have made investors skittish.

"The FDA delay has caused developers to take a pause," says Dr. David Edwards of the Biotechnology Industry Organization, the main industry group for genetically engineered agriculture. "They're not really sure where to go as far as the regulatory system."

By altering genetic materials of animals, scientists have proposed — and in some cases actually created — animals that would be bred to be free of diseases, be cleaner in their environment or grow more efficiently. Think chickens bred to resist avian flu, "Enviropigs" whose manure doesn't pollute as much or cattle bred without horns so they don't have to be taken off during slaughter.

But where the scientists see huge opportunity, critics see a food supply put at risk. They say modified organisms can escape into the wild or mingle with native species, with unknown effects.

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