A recent study done for the New York Times found surprisingly high levels of mercury in sushi -- tuna sushi -- at 20 restaurants and supermarkets in Manhattan. In some cases, the mercury levels were higher than allowed by the federal government.

But that doesn't mean you have to stop eating sushi. It just means that you should be more choosy about the type of fish you eat, health experts say.

Sushi is general considered a healthy food -- a good source of omega-3 fats, protein, and a variety of nutrients. Plus it's relatively low in calories.

But based on the New York Times tests, you may want to greatly limit the amount of tuna sushi you eat - especially sushi made with bluefin tuna. That's because you'd likely get the same results if you tested tuna anywhere in the country, says Joanna Burger, a professor at Rutgers University who analyzed the sushi.

The Times found five out of 20 places surveyed had mercury levels above 1 part per million. That's so high, the federal Food and Drug Administration could ban it from being sold.

Burger told me she was "surprised" at how much mercury was in the tuna.

"That's really risky because if you are pregnant you don't know if you're going to happen to get the sushi that happens to be made from the tuna that's quite high in mercury," she said.

Based on this study, Professor Burger has a warning for sushi lovers.

"If I were pregnant or if I had a small child, I would not eat sushi made with tuna, nor would I give it to my young child."

Professor Burger wants to make it very clear that she is not suggesting that anyone stop eating fish -- in fact, she thinks we should all eat more fish.

She just says we should go easy on the fish that tends to have high levels of mercury -- tuna, mackerel, shark, and swordfish -- and remember that goes for this fish in any form -- including sushi.

Thankfully, salmon is not on that list.

The New York Times survey found that tuna in supermarkets tended to have lower levels of mercury than the tuna used at restaurants -- a higher grade called bluefin.

Experts say there's also another reason for avoiding bluefin tuna -- it's being over-fished and its numbers are declining.