The run-up to the Oscars are a heady time for nominees: a whirlwind of screenings, cocktails, celebrity encounters and, for the makers of this year's prize winning eco-documentary, secret meetings in the parking lot of a sushi restaurant with federal investigators.

In an action worthy of the eco-commandos of Greenpeace, the makers of The Cove, an Oscar-winning documentary on Japan's dolphin slaughter, helped break up an alleged whale meat smuggling operation at a Santa Monica sushi restaurant catering to "adventurous" eaters.

On offer at The Hump, aside from yellowtail tuna, live octopus and shrimp, and baby abalone, was what was said to be whale meat, despite a ban on the sale and possession of whales.

That went too far for Louie Psihoyos, the director of The Cove, who co-ordinated the sushi sting from the parking lot.

"These are endangered animals. They are protected species. It is one thing for the Japanese to be doing it in their own country, but I take it as a major affront that they are doing this on our shores," he told the Guardian. "When they are cut up in little hunks of sushi it's a tragedy."

A spokesman for the US attorney's office told the New York Times that the restaurant could be formally charged as early as this week. Anyone convicted could face prison or a fine of up to $20,000 (£13,340).

In the week before the Oscars, the crew from The Cove made two visits with police to the restaurant. Two women activists went inside and ordered while Psihoyos maintained audio surveillance outside.

Secretly filmed video from an earlier supper last October showed the two women ordering off the chef's special omakase menu, with a waitress bringing thick pink slices of what she said was whale meat.

The pair ate two slices of the meat, putting six others in a plastic bag so it could be sent for DNA testing. The samples were sent to an expert who established the slices were from a sei whale. The species is endangered but is still hunted in Japan under a controversial programme that allows the killing of up to 1,000 whales a year in the name of science.

The bust offered yet more positive buzz for The Cove after it took the Oscar for best documentary. The Cove is Psihoyos's first feature-length film though he says he has been doing undercover work for 20 years. It relied on remote-controlled cameras mounted in helicopters, helium balloons, and even fake rocks as well as night vision equipment to record the annual dolphin hunt in a small coastal village on Honshu island in Japan.

Fishermen, banging on the hulls of their boats to confuse the dolphins' sense of direction, head out to sea to trap the migrating shoals. They herd the dolphins back to shore, packing them into a small inlet as closely as sardines, and then stab them to death with long harpoons and clubs.

In the course of each fishing season, the fishermen kill 2,000 dolphins, selling the meat to local supermarkets for about $500 a dolphin. They can earn far more by taking somem dolphins alive and selling them to aquariums.

The film-makers have seen a surge of support for stopping the hunt since Oscar night when Psihoyos' collaborator, the former dolphin trainer and underwater stuntman Ric O'Barry, held a sign asking viewers to text in their support. The appeal led the Oscar Academy to cut off Psihoyo's acceptance speech for "activism".

Psihoyos is already at work on his next film about the widespread extinctions that will come about because of the changing chemistry of the oceans brought by global warming. The Cove is due to be released in Japan, where the government has responded coolly to the film's success. "There are different food traditions within Japan and around the world," an official statement said. "It is important to respect and understand regional food cultures, which are based on traditions with long histories."