TOKYO — Former “Flipper” trainer Ric O’Barry was released by police late Tuesday after being arrested for not carrying his passport while in Japan to protest a dolphin hunt due to begin this week.

The activist had been held since Monday night when police in western Japan stopped him based on a tip that he might be driving under the influence of alcohol. He passed a breath test but was detained after he could not produce his passport.

O’Barry was released as “he no longer had to be kept in custody,” said Toshio Fujita, a police official in the town of Shingu, where the activist was detained. Fujita did not give more details of the reason for his release.

O’Barry, 75, starred in “The Cove,” a 2009 Oscar-winning documentary about Taiji’s dolphin hunt.

Many dolphins are killed and some are captured for sale to aquariums during the western town’s annual, six-month hunt. This year’s was scheduled to start on Tuesday but postponed because of bad weather, according to Kyodo News service.

O’Barry and other activists are unwelcome visitors to Taiji. Sea Shepherd Conservation Society USA said earlier that two of its members were refused entry to Japan on arrival.

Police have tightened security in town during the dolphin hunt season, when Taiji’s foreign population increases, Fujita said.

Lisa Agabian, a spokeswoman for Sea Shepherd, said by email that in past years the group’s activists have been followed and harassed by the police.

O’Barry was stopped while heading to his hotel from dinner at a restaurant, his son and fellow activist Lincoln O’Barry said by phone from Los Angeles.

Japanese law requires that foreign visitors carry their passports or another acceptable form of ID. No charges have been filed yet against O’Barry, and police refused to say whether they forwarded the case to prosecutors, who have about three weeks to decide to file or drop the case.

The elder O’Barry, an American who trained dolphins for the 1960s “Flipper” TV series, had a change of heart and now campaigns against both capturing and killing the animals.