It’s a complicated issue: Fish is rich in nutrients essential to cardiovascular health, prompting the American Heart Association to recommend at least two servings of fish a week. Yet, one-fifth of the world’s stock is over-fished or depleted, with half nearing that mark.

While domestic farmed fish are generally your best bet for conscientious buying, this doesn’t always hold true. Salmon farms, for example, may pollute local marine systems while shrimp farms have a poor track record for large-scale habitat destruction. And ironically, both are heavily dependent on wild fish for feed.

In addition, children and pregnant/nursing women should limit intake to once a month or, some experts believe, avoid altogether certain types of fish due to high mercury levels, which negatively impact early brain development. These fish include king mackerel, shark, swordfish, tuna and tilefish. Furthermore, this demographic should limit other varieties to no more than 12 ounces a week.

These issues can make seafood dinners tricky. According to Environmental Nutrition (April 2003), here’s a short guide to help you make healthful and sustainable choices in the fish department:

Keepers: Arctic char, catfish (U.S. farmed), clams (farmed), crab (dungeness), crayfish, halibut (Alaskan), mackerel (Atlantic/Spanish), mahi mahi, mussels (farmed), rainbow trout (farmed), sablefish, salmon (wild Alaskan), sardines, scallops (farmed), striped bass and tilapia (farmed).

Throw-backs: Chilean sea bass, cod (Atlantic), flounder (Atlantic), grouper, halibut (Atlantic), monkfish, orange roughy, salmon (farmed), shrimp/prawns (farmed, most wild), snapper, swordfish, tilefish and large tuna (blue fin) as tuna steaks or sushi.