CNN has a nice report about a question that I’m often asked, “Which is healthier — farmed or wild fish?”

These days, it’s hard to know what underwater life you should be eating. There’s talk of great benefits from fish-originating omega-3 fatty acids but worries about contamination and concerns about the environmental impact of farmed fish.

PETA, People for the Ethical Treatment of Animals, even launched a campaign last year to discourage people from killing and eating fish, suggesting that they be called “sea kittens” instead.

In answering a recent reader question about the relative benefits of farmed and wild salmon, CNNHealth’s nutrition expert, Dr. Melina Jampolis, urged the reader to “limit farmed salmon consumption to once a week at most if you are unable to find fresh, wild salmon.”

The answer, which also quoted a spokeswoman for the nonprofit Environmental Working Group, generated a flurry of questions and comments. In response, CNNHealth took a deeper look at the issue.

“It’s really high time that people have a new perspective on farmed salmon from a nutrition standpoint,” said Gavin Gibbons, spokesman for the National Fisheries Institute Inc., the largest seafood trade organization in the United States.

Salmon is rich in omega-3 polyunsaturated fat and is a good source of protein while being low in calories and saturated fat. Omega-3 fatty acids have been shown to reduce the risk of sudden cardiac death and are associated with better cholesterol levels.

Six ounces of East Coast Atlantic salmon has more DHA and EPA omega-3 fatty acids than the same weight of wild salmon, shrimp, chicken or beef (which has none).

Because of these benefits, the American Heart Association says, people should eat fish twice a week, especially fatty fish such as salmon.

But studies have found that some species of fish are contaminated with methylmercury, dioxins and polychlorinated biphenyls (PCBs).

A 2003 report from the Environmental Working Group showed that farmed salmon in the U.S. has the highest levels of PCBs, toxic man-made chemicals. And a widely publicized study in the journal Science in January 2004 suggested that farmed Atlantic salmon had higher levels of PCBs and other toxics than wild Pacific salmon.

Amid public concern, the importation of farmed Atlantic salmon to the United States went down by 20 percent in early 2004.

But subsequent research has found that the health benefits of both farmed and wild salmon exceed potential risks, said Eric Rimm, associate professor of epidemiology and nutrition at the Harvard School of Public Health.

Rimm was a co-author of a study in the Journal of the American Medical Association in 2006 that said the PCB levels in farmed salmon were not a cause of concern compared with the benefits.

“It’s clear that if there is any risk, the benefit is still in the range of 300 to 1,000 times greater from the fact that you’re getting the omega-3s,” he said.

Jampolis, citing the more recent research, agreed that the benefits of eating any salmon outweigh the risks, especially with heart disease being the leading cause of death in the United States and the fact that salmon is one of the best sources of heart healthy omega-3 fatty acids.

She continues to recommend trimming the skin and fat as much as possible and using cooking methods such as grilling and boiling to reduce fat, as this is where the toxic chemicals are stored.

Farmed fish receive a diet that often consists of smaller fish, such as sardines, and if they eat contaminated food, the fish themselves retain that toxicity.

In recent years, fish-feed makers have done a better job of regulating themselves, and the levels of some contaminants have gone down, Rimm said. Farmed salmon is not the main source of PCBs for the average person; in fact, the majority of these chemicals that we ingest daily probably comes from other animal products such as beef and chicken, he said.

Because farmed salmon are fed more, they contain more omega-3 fatty acids than wild fish, which tend to burn off these fats, he said.

Anyone concerned about contamination issues should try to find out where their fish came from and read about any potential problems in that area, said David Love, project director at the Center for a Livable Future at Johns Hopkins School of Public Health. He called for more widespread testing of contaminants in fish in the United States and better labeling practices so consumers know how and where their fish were caught.

Fish eaters should also consider eating smaller fish on the food chain, such as anchovies, mackerel and sardines, because they live shorter lives and don’t have as much opportunity as larger fish to pick up toxics, Love said. Since they are lower on the food chain, they are a more sustainable choice, he said.

The environmental impact of eating particular kinds of fish is an important matter but not so straightforward, experts say. To produce one farmed salmon, you have to feed it more than its weight in smaller fish, which leads to a net loss of fish from the sea and potential ecosystem disruption, Love said.

“It may not hurt my health, and it may not hurt your health, but on a population level, you can see some issues,” Love said.

Some farmed fish may also receive antibiotics that, if spread in the human population in large quantities, could lead to antibiotic resistance, meaning bacteria would no longer respond to these drugs, he said.

But there are also farmed fish produced in environmentally friendly situations, Rimm said. In some cases, it may take more energy to capture wild fish than to keep them in a pen, leading to a negative environmental impact.

Eating different kinds of fish is good for both your individual health and for the environment, experts say. From the health angle, it minimizes your risk of contamination from any one fish group, Jampolis said. From the environmental perspective, it would be detrimental to the ecosystem if everyone ate just one kind of fish en masse, Love said.