The heart health benefits from fish like salmon and mackerel seem to be weakened when farm-raised fish are fed vegetable oil instead of fish oil, new research indicates.

So the answer might be to feed them more fish oil.

However, that raises a different concern. Other studies have indicated fish oil increases the levels of pollutants in farm-raised salmon. That has encouraged some fish farmers to move to vegetable oil pellets -- which apparently decrease the heart benefits.

It is yet another fish conundrum for consumers, like the debate about whether the mercury in some fish offsets their health benefits.

Still, many experts argue that for most adults, the benefits are probably greater than the concerns about pollutants linked to cancer. They note that many more people are at greater risk of cardiovascular disease than cancer.

Wild fatty fish such as salmon, tuna, mackerel, sardines and herring are rich in omega-3 fatty acids, the healthy fat that scientists believe raises the good HDL cholesterol, lowers unhealthy tryglicerides and slows the growth of plaque, protecting the heart from disease.

However, in modern fish farming, the fish usually are fed pellets that contain a mixture of natural fish oil and vegetable oil. After a U.S. study earlier this year showed far higher levels of dioxins and other potentially cancer-causing pollutants in farm-raised salmon, some in the industry vowed to move more toward pellets with vegetable oil.

The latest study challenges that approach. At the annual meeting of the European Society of Cardiology, Norwegian scientists showed that people who ate salmon fed on pure vegetable oil or on 50% fish oil and 50% vegetable oil did not get any meaningful improvement in the relevant blood tests.

"Only 2% of the market today is wild salmon. The farmed salmon market today is very close to 50/50 feed. It's what we have in Norway, and it's more or less the same all over the world," said Dr. Harold Arnesen of Ulleval University Hospital in Norway. "The findings underline the importance of tailoring the salmon with heart protective properties."

Although experts believe that omega-3 rich fatty fish is good for the heart, the ideal amount to eat is not clear. The study indicates that if the group who ate the 50/50 salmon ate twice as much, they would likely gain the same benefit as those who ate the salmon fed with pure fish oil.

"If we are what we eat, then salmon are also what they eat," said Alice Lichtenstein, a nutrition science professor at Tufts University in Boston who was not involved in the study. "This shows there are ways of breeding salmon that can increase the fatty acids."

Lichtenstein has contended in the past that cardiovascular disease is a far bigger risk than the potential of getting cancer from eating fish tainted by pollutants.

The American Heart Association recommends adults eat fish, particularly fatty fish, at least twice a week. There is no equivalent European recommendation.