In 2007, for instance, two Chicago diners became seriously ill after being served puffer fish that was sold as monkfish.

Industry watchers suspect the main motive for fish fraud is profit, but Gibbons says it can be used to hide the country of origin and thus certain duties and tariffs. Lowell says it can also be used to sell overfished varieties to those trying to avoid them or to swap cheap imported farmed fish for wild caught varieties.

"Red snapper in sushi restaurants was often tilapia," Lowell said. "The customer might be thinking this is wild caught fish from the Gulf of Mexico, but tilapia is often farmed in parts of Asia that do not adhere to the same standards as we have for aquaculture here."

According to Oceana, more than 90 percent of American seafood is imported and yet the Government Accountability Office in 2009 reported that only 2 percent of imported seafood is inspected by the FDA and less than 1 percent is inspected for mislabeling.

Last week the FDA responded to Boxer's letter, saying it has recently invested in DNA-testing equipment and has encouraged state and local authorities to do the same. In 2012 FDA gathered approximately 800 fish samples for testing and "once we finish compiling results, FDA will have a better idea of where to conduct future sampling and enforcement efforts," the FDA letter said.

The FDA letter said the new Food Safety and Modernization Act restricts its data access only to the "immediate previous source and immediate subsequent recipient of food."

The National Fisheries Institute argues that existing laws, if enforced by the FDA, can solve the fish-fraud problem. It disagrees with advocates and others who seek additional regulatory power for the FDA.

Oceana, however, argues that the U.S. needs to adopt a reliable traceability system with bar codes or snap tags that can track the fish from bait to plate and even be read with smartphones. "Everywhere we looked for seafood fraud, we found it," Lowell said. "The takeaway is that it is everywhere and we need federal attention to stop it."

Until better enforcement or traceability becomes a reality, Lowell advises consumers to seek out retailers that welcome detailed questions about their fish. Gibbons says consumers should ask retailers whether they buy from distributors who are members of the Better Seafood Bureau, a voluntary group that pledges to abide by ethical guidelines and accept audits if they receive a certain number of unresolved complaints.

Finally, all warn that if you find certain seafood items at a price that sounds too good to be true, it probably is.

Twitter @monicaeng