New research shows that 84 percent of the world's fish contain levels of mercury that could be harmful, but there are certain kinds of seafood that should be avoided in particular.

According to a recent Yahoo Health article, scientists have pinpointed the reason why such deep-water fish as bigeye tuna have more mercury than the marine animals that feed closer to the surface of the ocean, like yellow fin tuna.

A piece published in Nature Geoscience showed the work of University of Michigan and University of Hawaii scientists, who discovered that 80 percent of methylmercury - the metal's extremely toxic form - can be found in the ocean deep, which explains why fish who feed on the floor of the sea have such immense levels of it in their systems.

"We knew this was true, but we didn't know why," co-author to the study Brian Popp told Hawaii 24/7, adding that the team used a mass spectrometer to study mercury levels in three prey fish species and six species of predators in the Pacific Ocean.

The study predicts that the levels of this toxin will surely rise, and may double by 2050 in the Pacific Ocean. Children are the most susceptible to methylmercury poisoning, since their brains and nervous systems are not yet fully developed. Exposure to this toxin for babies in the womb could lead to stunted memory, attention, language, fine motor and mental skills, the EPA reported.

The fish that contain the highest amount of mercury include the marlin, Pacific Bluefin tuna, swordfish and king mackerel. A handful of shark species were also found to have higher-than-normal mercury levels. Another new study points to other deep-water animals, like the lantern fish, moonfish and bigeye or skipjack tuna, as potentially dicey fish to ingest.

The healthiest fish to eat, according to the Biodiversity Research Institute, include anchovy, sardines, flounder, salmon, cod and haddock. They said that these choices can be eaten every week. Flying fish and mahi-mahi have the least amounts of mercury in their meat.