“Don’t eat that halibut! It’s a code red!” It sounds strange now, but it might become a familiar refrain soon enough. Color-coded seafood is about to hit store shelves–at least in Whole Foods, which is launching the first in-store color-coded sustainable seafood rating program this week. Created in conjunction with Blue Ocean and the Monterey Bay Aquarium, the system relies on three colors–green (“best choice”), yellow (“good alternative”), and red (“avoid”)–to alert customers about overfished species.

For now, at least, Whole Foods allows customers to buy seafood from red-listed fisheries. But the store claims that it will eliminate red-listed swordfish and tuna by Earth Day 2011, and all red-listed seafood will be cut from store shelves by 2012.

Whole Foods’ campaign has the potential to make an impression on consumers who might never have thought about the sustainability of their seafood choices. The chain is ranked as the ninth largest food and drug store in the U.S, with $8 billion in sales in 2009.

So while it’s true that Whole Foods caters to a specific niche of health-conscious customers with money to burn, the chain still wields considerable power over its suppliers.

If Whole Foods wants to eliminate red-listed seafood, suppliers will listen–especially since chains such as Walmart and Trader Joes are also phasing out overfished species. At the very least, the color-coded program has the potential to nudge customers who want to eat sustainably but don’t know how to start.