Real Simple asked Tim Fitzgerald, a marine scientist for the Oceans program of the nonprofit organization Environmental Defense, to list the healthiest picks for you—and for the environment. You can download and cut out a foldable pocket-size guide at www.realsimple.com/fishguide.

* Because this fish is high in mercury or other contaminants, those who are pregnant, nursing, of child-bearing age, or feeding young children should avoid it entirely.

Twice a Week:

Catfish, U.S.

Caviar, U.S. farmed

Char, Arctic; farmed

Clams, farmed

Crab, Dungeness and stone

Herring, Atlantic

Mackerel, Atlantic

Mussels, farmed

Salmon, wild Alaskan

Scallops, bay; farmed

Shrimp, U.S. farmed

Striped bass, farmed

Sturgeon, farmed

Tilapia, U.S.

Trout, rainbow; farmed

Once a Week:

Halibut, Pacific

Sablefish (a.k.a. black cod), Alaska or Canada

Tuna, albacore (a.k.a. white); canned or fresh; U.S. or Canada*