The U.S. has moved closer to approving a laboratory-tweaked salmon that grows twice as fast as conventional farmed fish and would become the first genetically modified animal to appear on American dinner plates.

An FDA advisory committee discussed Monday whether the fish was safe to eat and whether it posed any threat to the environment. The meeting ended without any decision about the fish. But in some ways, the FDA has already drawn its conclusions: In the run-up to the meeting, the FDA posted an analysis online that concluded...