For ocean lovers, making the right decision at the seafood market can be daunting. Almost half a billion pounds of seafood are hauled in worldwide daily. Some of it, like Pacific sardines, is fished sustainably with low-impact methods that don't harm the overall health of the target population or damage the surrounding sea. But much fishing is destructive: Huge trawlers wreak havoc on the ocean floor; some nets scoop up nontargeted species like dolphins. And while half our fish comes from farming, aquaculture can be eco-friendly or it can be polluting. Fortunately, there are reliable sources to help us make sustainable choices.** Monterey Bay Aquarium's Seafood Watch** ranks catches in three categories: Best Choices, Good Alternatives, and Avoid. Blue Ocean Institute, an initiative of New York's Stony Brook University, offers Ocean-Friendly Substitutes: albacore tuna for Atlantic bluefin, Alaskan sablefish for Chilean sea bass. And while its standards are less rigorous than some environmentalists would like, the** Marine Stewardship Council** works with supermarket chains to label seafood from responsible fisheries with a blue eco-sticker.