his is a great study from the Natural Resources Defense Council.

Guide to Mercury in Sushi
Women who are pregnant or planning to become pregnant should be especially careful about eating sushi. Many of the fish chosen for sushi are the apex predators of the fish food chain, which means they can bear high concentrations of mercury. The following list highlights sushi choices highest and lowest in mercury.

HIGHEST MERCURY
Avoid
Kajiki (swordfish)
Saba (mackerel)

HIGH MERCURY
Eat no more than three 6-ounce servings per month
Ahi (yellowfin tuna)1 Buri (adult yellowtail)2
Hamachi (young yellowtail)2 Inada (very young yellowtail)2
Kanpachi (very young yellowtail) Katsuo (bonito)2
Maguro (bigeye, bluefin* or yellowfin tuna)1
Makjiki (blue marlin)* Masu (trout)
Meji (young bigeye, bluefin* or yellowfin tuna)1
Shiro (albacore tuna) Toro (bigeye, bluefin* or yellowfin tuna)1

LOWER MERCURY
Eat no more than six 6-ounce servings per month
Kani (crab)
Seigo (young sea bass)*
Suzuki (sea bass)*

LOWEST MERCURY
Enjoy these fish
Aji (horse mackerel)2 Akagai (ark shell)
Anago (conger eel) Aoyagi (round clam)
Awabi (abalone) Ayu (sweetfish)
Ebi (shrimp)* Hamaguri (clam)
Hamo (sea eel) Hatahata (sandfish)
Himo (ark shell) Hokkigai (surf clam)
Hotategai (scallop)* Ika (squid)
Ikura (salmon roe) Kaibashira (shellfish)
Kaiware (daikon-radish sprouts)
Karei (flatfish) Kohada (gizzard shad)
Masago (smelt egg) Mirugai (surf clam)
Nori-tama (egg) Sake (salmon)
Sawara (spanish mackerel)2
Sayori (halfbeak) Shako (mantis shrimp)
Tai (sea bream) Tairagai (razor-shell clam)
Tako (octopus)
Tamago (egg) Tobikko (flying fish egg)
Torigai (cockle) Tsubugai (shellfish)
Unagi (freshwater eel) Uni (sea urchin roe)

* Fish to avoid for reasons other than mercury: Fish and other types of seafood are marked with an asterisk above if any of their populations are depleted due to overfishing or if the methods used to catch them are especially damaging to other sea life or ocean habitats.