Tuna at some of the city’s top sushi restaurants may contain elevated levels of mercury, a San Francisco-based environmental group claims.

The group, GotMercury.org, sampled fish from eight restaurants — including Haru, Masa, and Benihana — and found that 20% had mercury levels exceeding 1 part per million. Fish that contain higher levels of more than 1.0 ppm can be removed from stores by the Food and Drug Administration.

The average mercury level found in 13 samples of tuna was 0.73 ppm, the group reported.

“Eating tuna sushi in New York has moved from fashionable dining to a high-risk sport,” the report’s author, Caryn Mandelbaum, said yesterday. She said the group focused on sushi because of its increasing popularity. Common health risks associated with elevated blood mercury levels include heart attacks in adults, and developmental problems in fetuses and infants.

“We’re trying to be healthy by eating fish,” Ms. Mandelbaum said. “But if we’re not eating the right types of fish, or varying the types of sushi we’re eating, we’re actually causing pernicious health effects by trying to eat healthy.”

Some questioned the group’s methodology, asking how the analysts obtained the fish, how it was tested, and the motivation behind the targeting of specific restaurants.

Reached by telephone, the business manager of Masa, Veda Nishikawa, declined to comment. However, he said, “Maybe this company has got an agenda.” According to Ms. Mandelbaum, the fish was tested at a San Diego-based laboratory, Enviromatrix Analytical. The lab’s Web site lists the city of San Diego and the U.S. Navy as among its clients.

Last month, New York’s Department of Health and Mental Hygiene reported that one in four New Yorkers has elevated blood mercury levels, with the highest levels among Asians. Among women ages 20 to 49, the average blood mercury level is 2.64 micrograms a liter, three times the national level of 0.83 micrograms a liter, the department reported.