Fresh Pond Seafood in Arlington. (Michele McDonald for The Boston Globe)

By Deborah Kotz Globe Staff

Quick quiz: Which fish has less mercury -- salmon or tuna, or are they both just as likely to be contaminated? If you’re ecologically conscious, is it better to buy farm-raised fish or deplete populations in the wild? If you don’t know the answers to these questions, you’re one of the majority, according to a new study from Harvard Medical School and Brigham and Women’s Hospital which found that consumers are usually given confusing advice when it comes to buying fish.

The paper, published Friday in the journal Environmental Health Perspectives, reviewed information on a host of websites run by government agencies and environmental activist groups and found that consumers who want to know which fish to eat often encounter contradictory advice. (That’s not even taking into account the problem of mislabeled fish, a subject of a recent Globe series.)

“When we started looking, we were unable to find one comprehensive source of advice,” said study leader Dr. Emily Oken, an associate professor of population medicine at Harvard Pilgrim Health Care Institute and Harvard Medical School. “A lot of messages communicated about which fish to eat tend to come from a particular perspective, whether it’s concern for the environment or about risk to pregnant women.”

Often, too, advice aimed at pregnant women -- to curtail their consumption of large fish that could be high in mercury like tuna or swordfish (no, not salmon) -- is taken by the public at large who don’t need to worry as much about mercury, which poses a risk for unborn fetuses whose brains are still developing. The result? Many of us turn away from fish altogether.

In the paper, Oken and her colleagues recommended that the government should develop one comprehensive website to describe the multiple impacts of fish consumption including the beneficial effects of fatty acids on the brain as well as types of fish most likely to contain high levels of pollutants like mercury or PCB’s. They’d also like to see attention paid to environmental consequences.

“Which is the ecological better fish, farm raised or those caught in the wild? It varies, and a lot depends on where it’s caught,” said Oken. “There’s no general rule of thumb.”

Oken and her co-authors urged the federal government to develop “a list of fish to eat, and those to minimize or avoid, that considers these multiple perspectives and not solely the health effects of contaminants and nutrients.”

In the meantime, Oken recommended looking for seafood certified by such organization as the Blue Ocean Institute as being caught through ethical practices in waters that are less polluted. She also recommended turning to coastal state websites like Maine, Massachusetts, and Washington, for the best advice on which fish to buy.