Cod is king at Boston’s oldest restaurant — where more than 7,000 diners a year order New England’s most fabled fresh catch.

But the $22.95 serving of cod brought to tables, and supplied by Boston-based North Coast Seafoods, is not always the local product restaurant executives say they pay for. DNA testing commissioned by the Globe this summer showed it to be, on that occasion, Pacific cod, which is usually much cheaper — and to many palates, not as tasty.

Union Oyster House is among several well-known restaurants that blame North Coast, one of the region’s biggest seafood suppliers, for promising one kind of fish but delivering another. The Smith & Wollensky chain, Hilltop Steak House in Saugus, Paddy’s Pub in Newton, and East Bay Grille in Plymouth all claimed they received the wrong fish — after being told by the Globe that DNA testing revealed they were serving less expensive fish than advertised. But North Coast said all the restaurants got the fish listed on delivery invoices.

The pattern of problems at businesses that buy from North Coast illustrates a glaring lack of accountability in the poorly regulated seafood trade. Handshake deals and vague invoices make it easy for mislabeling — intentional or not — to take place at the expense of consumers who end up paying more for inferior seafood.

The Globe found North Coast technically did supply the fish indicated on most invoices, but some descriptions were ambiguous. Paperwork received by Union Oyster House, for example, identified shipments as “cod,” without indicating whether it was Pacific cod or more expensive Atlantic cod.

“For what we were paying [more than $8 a pound], for what we were getting, in our eyes it was white Atlantic cod, and that’s it,” said Joe Milano, the restaurant’s co-owner. “[North Coast] has a great reputation, but something is broke here.”

North Coast president Norm Stavis said the firm only promises “cod,” because the species caught off New England is in short supply. Years of overfishing and, more recently, changing ocean conditions have prompted the government to impose stringent catch limits on Atlantic cod, even though many menus across the region suggest otherwise.

“I was not aware of the restaurant’s desire to buy exclusively Atlantic cod,” Stavis said during an interview at North Coast’s headquarters in South Boston. He said what North Coast charged for the Pacific cod was fair because the special portion size and cut Union Oyster House wanted required more processing.

Money is often the motivation for fish mislabeling, but some distributors may misrepresent seafood to provide a consistent supply of fish to a business. Restaurants sometimes don’t ask many questions because diners rarely complain about the fish they are served. In the end, industry specialists say, merchants and suppliers can escape responsibility by blaming each other.

“Nobody here is taking accountability for hoisting fraud on the consumer,’’ said Tony Corbo, of Food & Water Watch, an advocacy group in Washington, D.C.

North Coast — founded in the 1950s by Stavis’s father — is one of the region’s dominant fish processing and distribution companies, with a modern facility on Drydock Avenue and another in New Bedford.

The multimillion-dollar business has about 600 employees globally, including 250 in the United States. It sells fish caught locally and internationally, as well as farmed fish, to over 1,000 restaurants, stores, and institutions nationwide.

North Coast has aggressively expanded its retail business, attracting supermarkets such as Shaw’s and Big Y with promises of sustainable seafood — species in abundant supply and caught using environmentally friendly methods. Its customer base has grown by double digits in each of the past five years, according to Rich Polins, vice president of sales.

The Globe was unable to determine how many restaurants with misrepresented fish received seafood from North Coast because some merchants declined to name their distributors. One chain with mislabeled fish supplied by North Coast took responsibility for the error and another, Dry Dock Cafe in South Boston, sold haddock obtained from North Coast that tested out to be just that.