As tensions rise between the United States and Canada, there’s a new clash in the cool waters off the northeast tip of Maine, which are rich with lobster, scallops and cod.

For more than a decade, American and Canadian fishermen largely have had a friendly but competitive relationship in an oval-shaped region of the Bay of Fundy known as the gray zone. But this summer that camaraderie has been threatened, Canadian fishermen claim, as officers with the United States Border Patrol have started to wade into the area, pull up aside their vessels and ask about their citizenship.

“We don’t want this to be a great international incident, but it’s kind of curious,” said Laurence Cook, the chairman of the lobster committee at the Grand Manan Fishermen’s Association in New Brunswick. “They say it’s routine patrolling, but it is the first routine patrolling in 25 years.”

At least 10 Canadian fishing boats have been stopped by American immigration authorities within the past two weeks, Mr. Cook said, the latest escalation in a more than 300-year disagreement in the disputed waters off Machias Seal Island. Both countries claim the island, which is about 10 miles off Maine and home to two full-time residents (both Canadian), puffins, rocks and not much else, and say they have the right to patrol its boundaries.