Most of us are probably aware that eating fish -- which is low in saturated fat and high in protein, omega-3 fatty acids and such nutrients as selenium and vitamins D and B2 -- is an important part of a healthy diet.
But it can be difficult to reconcile the knowledge that eating fish helps prevent heart disease, stroke and cancer, reduces hypertension and aids brain development with reports about elevated mercury levels in tuna and swordfish, and with recalls of fish tainted with listeria, salmonella and other dangerous bacteria.
And how to balance the experts who recommend eating 8 ounces of seafood a week against those who raise health concerns about the rapidly growing number of fish raised in aquaculture farms?
"It's not something that's been exhaustively researched, but from the few studies out there on specific fish or incidents, we can put them together and get a picture that there is possibly a real health risk to people if they are eating a lot of (farmed seafood)," says Meredith Moore, senior research and policy analyst for the nonprofit advocacy group Food and Water Watch.
Moore cites the heavy use of chemicals -- including pesticides and antibiotic and antifungal drugs -- in many aquaculture operations as a major concern. "There's a lot that gets dumped into these facilities in order to try to keep fish healthy in really crowded conditions ... and those chemicals or residues can end up on or in the fish," she says.
In addition to potential health problems from direct exposure to such toxins, Moore notes the documented rise of antibiotic-resistant bacteria in and around aquaculture facilities and in farmed seafood itself. "There still would have to be crossover from fish pathogens to human pathogens," she notes. "But that's a real risk in the future."
Several studies have also shown that farmed seafood contains higher levels of organic pollutants than wild-caught fish. These include dioxins, PCBs and metals such as mercury and lead, which have been associated with carcinogenic, reproductive and other developmental health issues.
Researchers attribute that, at least in part, to the diets of captive fish. "A lot of formulated feeds used on farms are ground-up smaller fish (from the ocean) that are then given to these fish to raise them, and somewhere in that process, they are consolidating the amount of environmental toxins," Moore says. For example, one study found that farm-raised salmon contained higher levels of organic arsenic than wild-caught fish, while another concluded that farm-raised blue fin tuna in Japan had higher concentrations of mercury than did wild counterparts.
We have no clear idea of how widespread such problems are, says David C. Love, science director of the Public Health and Sustainable Aquaculture Project at the Johns Hopkins Center for a Livable Future. That's because most farmed fish sold in the United States is raised outside the country.
"Imported seafood is about 80 percent of (the seafood) that's eaten in the U.S. It's about 50-50 farmed versus wild-caught, but the FDA only inspects about 2 percent of what's imported, which is almost nothing," says Love, adding that when there is oversight, what's documented is often worrisome.
Indeed, Love co-authored a 2011 study that found "a lot of veterinary drug residues in commonly eaten seafood, particularly shrimp and particularly from Southeast Asia." The imports with the highest rate of violations? Shrimp or prawns, eel, crabs, catfish or pangasius, tilapia and salmon. The countries most likely to be cited were Vietnam, China, Thailand, Indonesia, Taiwan, India and Malaysia.
It remains to be seen what the health implications of this are, since there have been no good long-term studies on the topic. "It's difficult to say whether there are health effects from the low-level residues of drugs of all sorts in seafood," Love says. "We might have long-term or chronic health issues associated with that, but it's not obvious, and with very low-level residue, it's really hard to study."
Health questions aside, one consideration for seafood lovers is that the nutritional profile of farmed fish isn't always as beneficial as that of wild-caught species, according to Andrew Weil, a physician who is director of the Arizona Center for Integrative Medicine. "When fish are penned, they don't get normal exercise, so they don't build up as much muscle protein as normal and may have lower protein levels, and the healthy-fat content of oily farmed fish may not be as good as that of wild fish -- it depends on what it's being fed," he says.
So what's a health-conscious consumer to do?
"It's important to be aware of where your seafood is coming from," says Love, who adds that this information is required by law to be available to consumers and should be prominently displayed in grocery-store seafood cases. (To complicate matters, Love notes that there are "a lot of gray areas" in labeling; for example, he says, "about half of salmon caught in Alaska that are (marked and sold as) wild were raised in a hatchery and then released into the wild.")
"Personally," says Moore, of Food and Water Watch, "I try to eat as little farmed or imported seafood as possible. I eat local and wild as much as I can. But the U.S. actually has pretty good fisheries management and oversight, so if you're eating domestically farmed fish, you're pretty safe."
For those in search of more specific advice, the Monterey Bay Aquarium Seafood Watch offers a "super green" list of fish that are good for human health -- meaning low in contaminants and high in omega-3s -- as well as abundant, well-managed and fished or farmed in environmentally friendly ways.
Its top choices include troll- or pole-caught Albacore tuna from the United States or British Columbia; freshwater coho salmon farmed in American tank systems; farmed oysters; wild-caught Pacific sardines; farmed rainbow trout; and wild-caught salmon from Alaska.
Meanwhile, the seafood selector of the advocacy group Environmental Defense Fund advises avoiding several species, including farmed salmon, due to contaminants and green concerns.
All of that said, most experts still maintain that eating any fish -- farmed, imported or otherwise -- is better than avoiding seafood entirely.
"I would much rather see people eat farmed salmon than no salmon at all," says Weil, choosing the species that is most often cited as suspect. "The benefits of eating farmed fish -- including brain health, emotional health, inflammation control, heart health and the reduction of cancer -- moderately outweigh any risks."