Many species of shark have sky-high mercury levels, and shark meat has meager nutritional value, so there's little reason to eat it.
Tilefish could be the unhealthiest fish of all, with extremely high levels of mercury wherever it's harvested around the world.
Unlike its smaller, healthier cousins in the mackerel family, the king mackerel — also known as Spanish mackerel — is large enough to guarantee that it will contain a higher level of mercury.
According to Greenpeace, "Many orange roughy stocks have already crashed…and recently discovered stocks are decreasing rapidly.
Highly valued as a delicacy — the owner of a Japanese sushi chain paid $3.1 million for a 613-pound specimen earlier this year — bluefin tuna is one of the most endangered major marine species.
Conservationists urge that consumers choose tuna other than bigeye because of its diminishing numbers due to unsustainable fishing practices
Like all tuna, yellowfin — the other species marketed as ahi — is extremely nutritious, but like other large fish, it is contaminated by mercury.
Avoid tilapia from China, where concerns include illegal use of antibiotics by farmers and widespread escape of the fish from flooded farming regions