Some fish and environmental experts have raised concerns about the health of farmed fish raised in cramped conditions on unnatural diets.

BALTIMORE — About half of the seafood people consume around the globe now comes from farms but efforts to make fish a sustainable food source by raising it in a tank instead of letting it grow wild may mean it’s losing its main nutritional selling point.

Omega-3 fatty acids found naturally in fish have been shown to improve cardiovascular health and possibly are staving off other maladies such as cancer. Levels of the fats, however, likely are being altered by a shift at the farms from feed made from fish meal and oil to plant-based feed, according to an analysis by Johns Hopkins Bloomberg School of Public Health researchers.

Some fish and environmental experts have raised concerns about the health of farmed fish raised in cramped conditions on unnatural diets. The nutritional content of the fish, however, hasn’t thoroughly been explored, said Jillian Fry, director of the Johns Hopkins Center for a Livable Future’s Public Health and Sustainable Aquaculture Project.

Her study didn’t determine exactly how much had changed in the flesh of popular farmed fish such as salmon and tilapia, though researchers who scrutinized aquaculture and public health data believe it could be significant. Fry said more precise measurement of the fatty acids in farmed and wild fish is needed so the public — and food policymakers — know what they’re eating.

“We have enough information to say it’s clear that taking fish oil and meal out of fish feed changes the fatty acid profile of the fish,” said Fry, who’s also a faculty member at the Bloomberg School. “It’s not clear how that impacts human nutrition, but it’s something we need to pay attention to. Dietary guidelines are developed based on hitting a target amount of omega-3s, and, if the fish profile changes, and there is evidence it is, the advice needs to change.”

For years, Americans have been encouraged to eat more fish, though the question of what to buy has become murky as the dangers of mercury in some fish, pollution and overfishing have become better known. Farming and plant-based feed were supposed to be an answer to depleted ocean stocks, but now questions about nutritional impacts will likely complicate consumption habits further, Fry said.

In the meantime, fish farming is expected to continue growing to meet demand.

Americans ate just over 4.8 billion pounds of seafood in 2009, the latest year available, or about 15.8 pounds of fish and shellfish per person, according to the National Oceanic and Atmospheric Administration. The United National Food and Agriculture Organization reported that globally another 40 million tons of seafood annually will be needed by 2030 just to keep pace.

Most U.S. consumers are eating less than half the recommended servings of fish, which is about 8 ounces a week, according to the U.S. Department of Agriculture. Dietitians don’t want to see that drop further because of concerns over nutritional value.

There are other sources of omega-3s, including nuts, vegetable oils and leafy greens. But they aren’t the right kinds of fatty acids for optimal health, said Kerry Strom, a dietitian at MedStar Franklin Square Medical Center. Fish contain eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), linked in studies to improved heart health, fetal brain and eye development, and other benefits.

She said when people do eat fish they tend to get it fried and may not know what type it is, and some have more omega-3s than others. She recommends salmon and tuna because they’re common and they have more EPA and DHA than other varieties and are familiar to most people.

While omega-3 supplements are available, it’s not clear if they are absorbed in the body the same way as those found in food. Still Strom said those may be necessary for people who won’t eat fish.

“Farm-raised fish fed alternative feed is still good for you, and fish is the best way to get omega-3 fatty acids,” she said. “I tell people to try and get two servings a week. That’s a concrete number people can understand. And a serving is the size of your palm or a deck of cards.”

Fish get their fatty acids by eating algae in natural bodies of water. When bigger fish eat smaller fish they acquire even more omega-3s. That chain is interrupted when all fish in a tank are given soybeans, corn or wheat-based food.

Twice as much soybean meal was used in commercial aquaculture feed in 2008 than fish meal, Fry said, and crop-based ingredients are projected to continue rising.

That could have more negative impacts on the environment from use of land and water resources and runoff of nutrients and fertilizer into waterways, according to the study, which was done in partnership with the University of Minnesota’s Institute on the Environment and McGill University and published March 11 in the journal Environmental International.

“Seafood has become quite a complicated topic,” Fry said. “The advice from our perspective is not to avoid farmed fish. Taking everything into account, the advice to eat fish has not changed.”