Although fish is considered good for your health, science has been warning about high levels of mercury and other pollutants in ocean fish for years.
However, a major review from the Scripps Institution of Oceanography in La Jolla, California, found steady decreases in contaminants.
Among the contaminants that have dropped significantly over the last 40 years are mercury, DDT and polychlorinated biphenyls, or PCBs. DDT is an insecticide and PCBs are found in many plastics.
In high concentrations, the Scripps researchers note these chemicals can cause cancer, neurological disorders, birth defects and thyroid problems in humans.
Stuart Sandin, PhD, a marine biologist at Scripps, led the study. Dr. Sandin and colleagues analyzed data from more than 2,600 studies from all over the world.
"There's just a lot more complexity out there," Dr. Sandin said in an interview for an article in the Los Angeles Times. "The pollution does not stay in one place. We thought that we would find something like that. But when you start increasing the scale, that predictability goes away. We didn't find any evidence that when you're near shore versus 100 miles offshore that you had more or less chemicals in the seafood."
Fish samples in the studies were collected between 1969 and 2012. During that period, clean-water regulations and other factors have decreased the amount of agricultural and industrial contamination of the oceans.
Mercury levels in fish dropped about 50 percent. PCB levels dropped more than 90 percent.
Fish samples varied greatly in contamination levels. The location of harvest did not affect contamination levels, as even fish from the same school might have different levels of contamination.
Dr. Sandin and colleagues also found that even small fish like mackerel and sardines had contaminants. Previously, scientists believed pollutants were concentrated in the larger fish at the top of the marine food chain.
Dr. Sandin and team cautioned that while this is good news, most fish still have levels too high for frequent consumption. Eating fish two or three times a week is probably OK, they said.
The study was published in the February issue of PeerJ.
Funding for the study was provided by the Waitt Foundation and the US National Science Foundation.
None of the authors reported a conflict of interest.