The federal government’s updated dietary guidelines highlight salmon as a valuable source of protein and healthy fats but new research from the U.S. Department of Agriculture shows the majority of salmon consumed by Americans is not as nutritious as it used to be.

Scientists found a standard 3-ounce serving of farmed Atlantic salmon now contains less than half the amount of omega-3 fatty acids DHA and EPA, which contradicts federal data used to determine nutritional value.

Laura Lee Cascada, founder and director of the Aquaculture Accountability Project, said farmed salmon are being fed more land-based crops like soybeans and corn because the wild fish needed for fish meal are declining in number.

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"Farmed salmon are not a key part of our diet and so really the best thing we can do is be skeptical at the grocery store, investigate the claims we see and try to cut back wherever we can," Cascada recommended.

USDA researchers noted their study included a small sample of salmon from Chilean producers in Maryland, Virginia and Washington, D.C., and further studies are needed to determine whether nutritional values should be updated.

The USDA’s dietary guidelines recommend Americans eat a variety of protein, including 6 ounces of seafood per week. The Trump administration has created a new Office of Seafood to boost domestic production and lawmakers have worked to pass legislation to permit finfish farms in federal waters.

Fishermen in Maine said large salmon farms there are polluting the ocean and crowding them out of areas they have historically used. Cascada pointed out floating cages can hold hundreds of thousands of fish and require large amounts of antibiotics people may not want to consume.

"When you're producing fish on this massive scale, these problems are inherent to the model. It's essentially factory farming in water," Cascada said.

The majority of farmed salmon Americans eat comes from Chile but the U.S. government tests only a small fraction of imported seafood for prohibited drugs.

Cascada noted salmon is often labeled in supermarkets as “sustainable” or “responsibly farmed” but there is no way to verify if it is true. She added Americans used to eat far less fish and other sources of protein but argued a myth has been created by the seafood industry we need more.