The U.S. Food and Drug Administration (FDA) wanted to better understand the amount of mercury in seafood available to U.S. consumers and conducted an extensive survey to collect that data.

The agency is looking for contractors to conduct surveys on behalf of its Center for Food Safety and Applied Nutrition to test about 600 seafood products for mercury and methylmercury, according to a June 7 notice.

The products tested included shrimp, salmon, canned tuna, tilapia, Alaska pollock, cod, crab, catfish, pangasius, scallops and canned horse mackerel. Contractors were required to collect 50 samples per category from a variety of sources in at least two major U.S. cities, including online retailers, independent markets, and national supermarket chains. The contractor will also randomly collect additional samples to meet the 600 sample threshold.

The FDA said the investigation will not be used for direct regulatory action, but it will inform how the agency "makes decisions and guides policy on scientific issues related to food safety issues." FDA previously implemented a monitoring program to track mercury concentrations in fish from 1990 to 2010.

The new survey is part of a wider government-wide effort to update federal advice on how much seafood pregnant women and children should eat.

In 2004, the agency issued a notice advising pregnant women, women who might become pregnant, nursing mothers and young children to avoid fish high in mercury. The updated guidelines list mackerel, marlin, orange snapper, shark, swordfish, bigeye tuna and more from the Gulf of Mexico as fish to avoid.

Now the agency is looking to update that advice.

Last year, the FDA commissioned an independent study by the National Academies of Sciences, Engineering and Medicine (NASEM) to determine the role of seafood in child development. The research will support the FDA's "Near Zero" action plan, which aims to reduce the risk of exposure of infants and young children to mercury, arsenic, lead and cadmium.

"Seafood is part of a healthy eating pattern and provides key nutrients during pregnancy, breastfeeding and/or early childhood to support the development of the child's brain, spinal cord and immune system," the agency said in announcing the new research. "At the same time, seafood is a major dietary source of mercury, which is transmitted throughout the environment through natural and anthropogenic processes. Mercury can damage the nervous system, and infants and young children are more susceptible to mercury's harmful health effects."

The agency expects to complete the study within 18 months to determine whether it needs to update its recommendations. The FDA also noted at the time that it would look for the latest data on mercury levels in seafood.

"We will use updated analytical methods to collect and analyze new data on mercury levels in seafood," the agency said. "We also plan to conduct research with consumers to better understand how FDA can provide information to help families consider how seafood can be part of a healthy diet."