A variety of organizations such as the Blue Ocean Institute, Fishwise, and the Environmental Defense Fund offer guides to sustainable seafood selection. Some even offer small cards that can slip into your wallet for on-the-go seafood decision making. However, with a little knowledge and common sense you can make smarter purchases even without a seafood guide.
A good rule of thumb for fish consumption is "the smaller the better." Typically, large slow-growing fish like Orange roughy, Chilean seabass, and Bluefin tuna are most susceptible to depletion by poorly managed fisheries. These species breed later in life and, if caught and consumed early in their life cycles, may never have the opportunity to reproduce. Anchovies, sardines and other fish at the bottom of the food chain don't play quite as crucial a role as they reproduce quickly, more plentifully and earlier in their lives. This makes them a far more sustainable choice. Also, because of their shorter lifespan and eating habits small fish typically take in less mercury than larger fish making them a healthier choice as well.
Avoiding carnivorous fish is another step consumers can take that has a two-fold benefit. Carnivorous fish such as tuna, swordfish, and mackerel have become hugely popular since fish became a "super-food" thanks to the omega 3s they produce. These large carnivorous fish, however, get there sustenance by feeding on small fish lower on the food chain. This means that they are ingesting more mercury and hence storing more mercury in their fat. By avoiding these carnivorous fish the savvy consumer can lower their mercury intake while aiding in making fish supplies more sustainable.
Substituting shellfish for fish fillets is one of the greenest seafood decisions the consumer can make. Most shellfish, such as oysters, clams and mussels, are raised on shellfish farms that have a very minimal environmental impact. Even with shellfish, though, some choices are better than others. Farmed crawfish, for example, is an excellent substitute for lobster. Although lobsters are plentiful in the ocean, they are often harvested at minimum size and have often not yet had a chance to reproduce before they are caught.
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