The following is an excerpt from the Chelsea Green Guide to Sustainable Food  by Elise McDonough . It has been adapted for the web.
For most Americans, fish and seafood represent the only wild foods found in their diet. Consequently, mindful consumers shopping for sustainable selections will encounter a series of complicated considerations that go way beyond price per pound and freshness. Venture over to the fish and seafood counter in your supermarket, or even visit a local fish merchant or seafood restaurant, and your purchasing decisions will have dramatic effects on both your own health and the well-being of the planet.
Making informed decisions starts with learning all you can about different species, including their biology and reproductive cycles, geographic range, and place in the oceanic food chain—not to mention where they came from, whether they were fresh or frozen, “wild caught” or “farm raised.” Trying to be ethical, economical, and epicurean all at once can be overwhelming enough to make you shrug and reach for the canned tuna, but take heart: We’ll cover the basics here, and groups like the Marine Stewardship Council (msc.org) can provide you with more detailed information on tasty, affordable, guilt-free fish and seafood options.
General Fish Tips
When shopping for fish at market, always select the freshest possible. Fresh fish shouldn’t have an odor of excessive “fishiness,” or ammonia. The flesh should be firm, and bounce back when pressed by a finger. Fish on display should always be on ice, but not buried in ice, and the ice shouldn’t be melted.
Fresh fish is preferable to frozen, and while canned fish is convenient, the canning process is very energy-intensive. By weight, canned fish may also be more expensive than fresh fish. Smoked and dried fish are preferable to canned, and there are excellent varieties of tuna “jerky” on the market today.
Environmental Issues: Overfishing, Loss of Habitat, and Species Depletion
Despite the vastness of the oceans, the fish and seafood harvested from them are far from limitless. Alarmed by the worldwide collapse of fisheries, environmental activists estimate that 90 percent of the large, predatory species in the sea now teeter on the brink of extinction. And the commercial fishing industry’s shortsighted, industrial perspective on utilizing oceanic ecosystems means that vessels from different countries continue to compete for the same fish in the same areas, many using increasingly destructive and sophisticated technology to pull greater harvests from an ever-diminishing supply.
Instead of partnering with environmental groups to implement strict regulations necessary to protect and conserve fish species, thus preserving this precious food supply (not to mention the fishing trade itself) for future generations, most commercial fishing operations instead focus solely on harvesting as many fish as possible from the oceans before they’re all gone. Meanwhile, a lack of political action at all levels of government means that, despite the environmental risks, you will continue to find increasingly rare species in markets and on menus, including bluefin tuna, red snapper, grouper, orange roughy, and Chilean sea bass (also called Patagonian toothfish). We must all act now to reverse this dangerous trend, or else, one day, these and many other species will disappear, never to be seen or eaten again.
The consumer movement for dolphin-safe tuna provides a successful model that can be replicated to protect other at-risk species. We all deserve sustainable oceans, and safe, healthy fish and seafood.
Fish Farms: Good or Bad?
Farm-raised fish and seafood presents a dilemma for eco-conscious consumers. When it comes to buying the products of aquaculture, the environmental impact of your purchase could be benign or could be disastrous, depending on the species of fish, the location of the farm, and the methods employed in raising, harvesting, and transporting your next dinner. And so it’s important to educate yourself before things get fishy.
For instance, aquaculture farms producing Atlantic salmon greatly contribute to ocean pollution, and potentially spread diseases from farmed fish to endangered wild ones. Crowded into offshore pens, these farmed fish are fed antibiotics in order to control disease. Also, since salmon are carnivorous, they need to eat many smaller fish. When farm-raised, this means ground-up fish pellets, which give a gray coloration to the flesh (as opposed to wild salmon, which dine on crustaceans, giving their flesh its characteristic pink color). Since gray salmon isn’t very appetizing, farmed fish get fed a synthetic pigment produced by a pharmaceutical firm to replicate the natural color of wild salmon.
Also, to promote faster growth, farmed salmon are genetically altered, which is good for the industry, because the fish will not need to eat so many food pellets, but presents significant problems when these GM fish escape into the ecosystem and compete against wild salmon for food and mates. Together, all these drawbacks make genetically modified, antibiotic-laden, artificially colored farmed salmon a poor substitute for real, wild salmon, which can be obtained from responsibly managed fisheries in Alaska, or ordered online from services such as Vital Choice.
On the other hand, successful, sustainable fish farms do exist, ones that build on the symbiotic relationships between fish and plants found in nature. For instance, farmers in the Mississippi River and in Asian rice paddies raise catfish, tilapia, and crawfish in aquaculture systems that have beneficial or neutral effects on the ecosystem. Shellfish perform a vital service in filtering water from estuaries and wetlands, and so farmed mussels, oysters, and clams are also an ecologically sound purchase. Other farmed species that can be recommended include Arctic char, sturgeon, barramundi, striped bass, rainbow trout, and bay scallops. In general, fish farmed in rivers or inland ponds are preferable to fish farmed in the ocean, and choosing fish farmed in the United States also encourages local economies and reduces the energy used in bringing them to market.