Fish that have the highest concentrations of mercury include mackerel (King), marlin, orange roughy, shark, swordfish and tuna (Bigeye, Ahi). It is advised that pregnant women avoid eating these fish, according to the Natural Resources Defense Council. Fish that have high, but edible, concentrations of mercury include bluefish, grouper, mackerel (Spanish and gulf), sea bass (Chilean) and tuna (canned albacore and yellowfin).
Fish with the highest levels of mercury have concentrations more than 0.5 parts per million. Fish with low levels of mercury, such as anchovies, crab, haddock, hake, herring, mackerel, plaice, pollock, salmon, scallop, shrimp, squid, trout and whiting, have concentrations of less than 0.09 parts per million, explains the Natural Resources Defence Council. Fish with moderate levels of mercury include bass, cod (Alaskan), halibut, lobster, monkfish, tuna and weakfish, which have concentrations between 0.9 and 0.29 parts per million.
Knowledge of mercury levels is important for pregnant women because mercury accumulates in the body and can damage the developing baby's brain and nervous system, explains Mayo Clinic. The Environmental Protection Agency says that women can safely consume up to 12 ounces of seafood per week. Consuming fish during pregnancy is important for the baby's development due to the presence of omega-3 fatty acids.