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Farmed fish and seafood has sort of a bad reputation in some green circles. Sometimes -- as with, say, Atlantic salmon -- it's for good reason. But there are a number of cases where farming can actually be a good thing -- for water quality, the health of the fish or seafood in question, and the aquatic environment as a whole.
As with lots of food-related issues, location has a lot to do with determining how green the process is; while a farm in the U.S. might be a good option, one from, say, southeast Asia might not, even if the species being farmed is the same. With that in mind, these are the greenest ways to go when it comes to farmed seafood.
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Like all bivalves, oysters act as small filters in the water, so they can actually help improve the water quality of the ecosystem where they're farmed. And, because oysters aren't much for swimming, the chances of 10's of thousands of them making a break for it are pretty slim. And, with ocean acidification causing big problems for shellfish, farming them might be the only way to get oysters before long.
MusselsSometimes even sharing the same habitat, mussels are a greener seafood choice when farmed for many of the same reasons oysters are; they're easy to contain, they filter the water on the farm, and they can be farmed in a variety of locations. And, because they're pretty low on the oceanic food chain, mussels won't accumulate heavy metals or industrial chemicals like PCBs or dioxins in high levels.
Bay scallopsThe last of the bivalves on this list, bay scallops can also be farmed with a minimal environmental impact. A slight difference is that many scallops swim free (though some do attach themselves to substrates), so there's slightly more work involved in keeping them from swimming away. They can't swim very fast -- they do so by rapidly opening and closing their shells -- but this locomotion requires careful netting around the farm.
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Unlike salmon, many varieties of trout spend their entire lives in freshwater environments, making it easier to farm them in a nearly closed system, away from outside influences and environmental variables that can harm the fish or their habitat. The system of tanks and ponds allows for careful control; there's nowhere to escape, and sickness and disease can be more easily isolated and dealt with.
Arctic charLike trout, farmed Arctic char are mostly raised in man-made pools or tanks that are separated from natural water sources. Char are well-suited to this farming method, since they grow well in high densities. Some farm-raised char are fed fish meal and fish oil from wild-caught fish, which can put pressure on those populations, but efforts are currently underway to develop char diets that use fewer fish and more grain. Even so, farmed Arctic char are a pretty green choice.
TilapiaOne of the original aquaculture success stories, tilapia is one of the greenest farmed fishes out there. Tilapia can be farmed in large tanks -- rather than outdoor pools and ponds -- so it can be done almost anywhere, and tilapia are herbivores, so they don't require large amounts of other fish byproducts to grow quickly; they're even known by some as "the aquatic chicken," thanks to their ability to grow quickly with low-quality inputs. In more advanced systems, their waste is used to power the aquaculture system that helps them grow, increasing the overall efficiency of the process.
CatfishThough wild catfish are known as "garbage" fish -- they're bottom-feeders that scour stream and riverbeds, eating almost anything -- farmed versions usually eat mostly or all-vegetarian pellet feed, creating a more consistent-tasting fish that can be farmed similarly to the other fish on this list. Catfish also grow well in low-management systems, requiring minimal inputs, technology, and training to be done successfully.
CrayfishFarmed crayfish have the benefit of growing to a much larger size than their wild counterparts, and much more rapidly, if given optimum conditions. These conditions are pretty easy to achieve, and are much less strict than many other aquaculture species, meaning that you may be able to farm them yourself, if so inclined.
No matter whether you choose green wild-caught or farmed seafood, the world's seafood stocks are suffering, from overfishing, pollution, habitat loss, and more; ultimately, that means that the greenest way to eat fish is to do so less frequently, and make sure it comes from sustainable sources when you do.
More on sustainable seafood
Super Green Seafood List Connects Ocean & Human Health (Slideshow)
Recirculating Marine Aquaculture: Farmed Fish Minus the Pollution
Worldwatch Institute Reports that Sustainable Fish Farms Can Feed the World
Could Farmed Fish Lead Us to 'Mad Fish'?
Is This the End of the Line for Fish?