BOSTON >> Those plump and tempting scallops behind the fish counter glass might be a lot smaller than they look — a sodium-based compound can bloat scallops well past their actual size. And that pollock fillet isn’t such a good deal if the price includes the layers of ice glazed onto it to keep it fresh.

This "overglazing" rips off consumers, as does so-called "soaking" of scallops, which can also alter the taste of the shellfish. At the International Boston Seafood Show this week, a top federal seafood quality officer announced his agency was increasing efforts to stop these and other types of seafood fraud.

"We’ve decided we’re going to take on the economic fraud concern," said Steven Wilson, chief quality officer at the National Marine Fisheries Service’s seafood inspection program.

Perhaps the best known kind of seafood fraud is species substitution, when sellers secretly replace a prized species with a similar tasting, cheaper fish — say, whiting substituted for grouper, or mako shark for swordfish.

But fraud involving inaccurate food weights, caused by practices such as overglazing and soaking, is far more common, Wilson said. Inspectors at his agency find some kind of economic fraud in at least 40 percent of all products submitted to them voluntarily. And in at least eight out of 10 of those cases, inaccurate weights are the problem, he said.

"If we focus on the net weight issues we’ll drop that 40 … percent to very, very minor percentages," Wilson said.

The problem with detecting the soaking or overglazing is that both involve legitimate ways to keep seafood fresh, so it’s tough to tell when someone is cheating.

The law says a package labeled as 10 pounds of fish must contain 10 pounds of fish, with the ice glaze as extra, uncounted, weight. But the only way to know whether the ice is being counted is with labor-intensive inspections that match the fish weight with the weight advertised on the package.

That happened in 2010, when an investigation by 17 states showed customers were often charged for the ice in seafood packaging, sometimes as much as $23 per pound. In the four-week investigation, 21,000 packages of seafood were removed from shelves.

"This sounds like something that is so simple, and so sort of pedestrian in the world of fraud, you would think … people wouldn’t get away with it," said Gavin Gibbons of the National Fisheries Institute, a seafood trade association. "But it is absolutely a challenge."

The soaking of scallops and other seafood, such as shrimp and even whitefish fillets, involves moisture retention agents that keep seafood fresh.

It’s tough to define how much is too much for a given species, but their use can be abused. Wilson described a scallop as "a little sponge" that can absorb as much as half its own weight in water. The truth about these bloated scallops becomes clear when they hit the frying pan, shrink and their water burns off.

"You’re paying for water that’s going to disappear when you cook the product," Wilson said.

Since about two-thirds of the water-retention compounds are sodium-based, they can also add a saltiness to the seafood that isn’t natural, but which some research has shown consumers see as normal, and even prefer, Wilson said.

"If this goes on long enough, the consumer thinks it’s normal, when it’s not," he said.

Both problems are hard to detect, so they’re also tough to stop.

Most seafood eaten in the U.S. is imported and packed outside the country, so regulators here can’t prevent any fraud. And the more fraud there is, the more industry members feel pressure to commit it to compete.

Once seafood arrives in the U.S., the Food and Drug Administration regulates it. But that agency’s resources are often consumed by more urgent concerns, such as food safety and bioterrorism, according to a report by the Congressional Research Service. The report said just 85 of the FDA’s 1,350 inspectors work mainly with seafood.

"Enforcement of economic fraud and labeling laws may be a lower FDA priority relative to protecting the health and safety of the U.S. food supply," said the 2010 report.

Lisa Weddig of the Better Seafood Bureau, an arm of the fisheries institute, said public and industry awareness can make a difference because complaints lead to action. The 2010 study on overglazing, conducted in consultation with the FDA by the different states, is an example.

She added that while the National Marine Fisheries Service doesn’t have authority to regulate seafood fraud, it can cut down fraud by awarding certifications for seafood that meets voluntary quality standards it devises.

Weddig said that because fraud affects both the value and taste of seafood, and the industry wants to say it’s delivering the maximum amount of both, it has major motivation to comply with the voluntary standards.

"If you serve poor-quality seafood to consumers, it might be the last seafood meal they eat," Weddig said. "And nobody wants that to happen."