According to a report in Science Daily, one in four New York City adults has elevated blood mercury levels. The data was collected as part of the New York City Health and Nutrition Examination Survey, the first such survey ever conducted in a U.S. city.

The levels are closely tied to fish consumption, with the highest levels reported in Asian and higher-income New Yorkers who eat more fish. While the levels pose little health risk for adults, concerns have been raised about the health of infants and young children.

Health officials have been quick to emphasize that fish is an important part of a healthy diet. It is high in protein, low in calories, and rich in nutrients. But according to Daniel Kass, Assistant Commissioner for Environmental Surveillance and Policy, “Young children, breastfeeding mothers, and women who are pregnant or planning pregnancy should eat fish that are lower in mercury and limit fish that are higher in mercury.”

Recommendations regarding which fish are best to eat can be found in a brochure titled “Eat Fish, Choose Wisely.” Young children and pregnant or breastfeeding women should not eat the following fish, which are too high in mercury: Chilean sea bass, grouper, mackerel, marlin, orange roughy, shark, swordfish, tilefish, and tuna (fresh, steaks, or sushi).

Only one serving a week should be eaten of the following fish, which are fairly high in mercury: bass, bluefish, eel, halibut, lobster, monkfish, sablefish, scorpionfish, sheepshead, skate, snapper, sea trout, and tuna (albacore or canned white). This guideline is based on an adult serving size of 6 ounces. A serving for a child should be smaller. To help you estimate serving sizes, the brochure describes a 4-ounce serving as about the size and thickness of a deck of cards.

Young children and pregnant and breastfeeding women can eat up to two servings a week of fish that are low in mercury. These fish include: butterfish, catfish, cod, crab, croaker, flounder, haddock, jacksmelt, mackerel, mullet, mussels, perch, scallops, shad, sole, squid, trout, tuna (canned light), and whitefish.

Up to five servings a week can be eaten of fish that are very low in mercury. These fish include: anchovies, clams, crayfish, hake, herring, oysters, pollock, salmon, sardines, shrimp, tilapia, and whiting. The brochure notes, however, that some farmed salmon can contain high levels of contaminants other than mercury.

Finally, pregnant women should not eat raw fish, in order to avoid harmful bacteria. Unless you’re a cat, in which case a serving size is one goldfish – raw!