As the Food and Drug Administration considers whether to approve genetically modified salmon, one thing seems certain: Shoppers staring at fillets in the seafood department will find it tough to pick out the conventional fish from the one created with genes from another species. Despite a growing public demand for more information about how food is produced, that won't happen with the salmon because of idiosyncracies embedded in federal regulations. The FDA says it cannot require a label on the genetically modified food once it determines that the altered fish is not materially" different from other salmon - something agency scientists have said is true. Perhaps more surprising, conventional food makers say the FDA has made it difficult for them to boast that their products do not contain genetically modified ingredients. The decision carries great weight because, while genetically modified agriculture has been permitted for years and engineered crops are widely used in processed foods, this would be the first modified animal allowed for human consumption in the United States. The AquAdvantage salmon has been given a gene from the ocean pout, an eel-like fish, and a growth hormone from a Chinook salmon. Consumer advocates say they worry about labeling for genetically engineered beef, pork and other fish, which are lining up behind the salmon for federal approval.