The fish grows twice as fast as normal salmon, so it reaches market size more quickly. It has an added growth hormone from the Pacific Chinook salmon that allows the fish to produce growth hormone all year long. The engineers were able to keep the hormone active by using another gene from an eel-like fish called an ocean pout that acts like an "on" switch for the hormone. Typical Atlantic salmon produce the growth hormone for only part of the year.
The FDA has also said the fish is unlikely to harm the environment. The fish would be bred female and sterile, though a very small percentage might still be able to breed. The company has argued the potential for escape is low.
There is no evidence that the foods would be unsafe, but for some people, it's an ethical issue.
Some retailers have pledged not to sell the salmon, and it's still unclear whether the public will have an appetite for the fish if it is approved. Genetic engineering is already widely used for crops, but the government until now has not considered allowing the consumption of modified animals. Although the potential benefits and profits are huge, many people have qualms about manipulating the genetic code of other living creatures.
Critics call the modified salmon a "frankenfish." They worry that it could cause human allergies and the eventual decimation of the natural salmon population if it escapes and breeds in the wild. Others believe breeding engineered animals is an ethical issue.