Cook said at least 10 Canadian boats were stopped and interrogated about whether they were carrying drugs and illegal immigrants. The incident comes at a low point in US-Canada relations. The United States in May slapped tariffs on imports of Canadian steel and aluminum, prompting retaliation from Canada on the same metals and a range of other US exports. US President Donald Trump has lashed out at Canadian Prime Minister Justin Trudeau, calling him "dishonest and weak". War of words: Trump with with Canadian Prime Minister Justin Trudeau during an awkward G7 Summit in early June. Credit:AP The two countries are also in the midst of tense negotiations over NAFTA, the North American agreement Trump has called "the worst trade deal ever made".

Canada's Foreign Ministry said in a statement it is investigating the incidents that it said "occurred in Canadian waters." "Canada's sovereignty over Machias Seal Island and the surrounding waters is long-standing and has a strong foundation in international law," the statement said. US Customs and Border Security said US Border Patrol agents were simply conducting "regular patrol operations" to enforce immigration and other American laws in "the jurisdictional waters of the United States." It said it only boards Canadian vessels with consent. An Atlantic puffin comes in for a landing on Machias Seal Island. Credit:AP/File The US State Department maintained in a statement Machias Seal Island belongs to the United States and has since 1783.

The dispute dates to the signing of the Treaty of Paris between Great Britain and the newly independent United States. The US was granted all islands within "20 leagues" of the Maine shore except for the islands that were owned by the British colony of Nova Scotia, which later became part of Canada. The two sides have differed ever since over whether Machias Seal Island was historically part of Nova Scotia. When Washington and Ottawa went to the World Court to settle the boundary differences over the adjacent fishing grounds of Georges Bank in the 1980s, they deliberately left out Machias Seal Island and the Grey Zone. The British first erected a lighthouse on the island in 1832, and Canada inherited it when it became a country in 1867. Even though the lighthouse is now automated, Canada employs lighthouse keepers on site to maintain its sovereignty claim. Machias Seal Island off Maine, US, disputed also by Canada. Credit:Google Maps There are four boundary disputes along the countries' border, but the fight over Machias Seal Island is the "only one that actually involves sovereignty over a piece of territory," said Stephen Kelly, a retired American diplomat and a research scholar on boundary issues at Duke University. The other disputes are squabbles over marine boundaries.

Both countries insist the waters in the Grey Zone are rightfully theirs, so for years there has been an arrangement whereby each country imposes its fishing rules on its own vessels. That means Coast Guard and fisheries vessels from the United States and Canada do not impose their rules on boats from the other country. Kelly, who has a summer home on the Maine coast, said the recent presence of US Border Patrol in there was unusual. "I don't know what they were doing out there. It's hardly a well-known smuggling path for illegal aliens," he said. Kelly said there are occasionally disputes between lobstermen in the zone because there are different rules for the fishing season and the size of lobsters, for example. "The Mainers have to throw back lobsters that are too small or too big," he said. "The Canadians don't have a limit on the big lobsters."

While there are occasional tensions between lobstermen, the two countries have developed cordial relations over the birdwatching trade. There is a division of the business between a US boat that sails out of Cutler, Maine, and a Canadian boat that makes its port on Grand Manan Island, New Brunswick. At the height of the season, which runs from late May to mid-August, each boat takes a maximum of 15 tourists for a daily five-hour trip to the island, which is staffed by the Canadian Wildlife Service and researchers from the University of New Brunswick. Even though the Canadians control the island, they do not ask visitors from the US boat for identification when they disembark. "We've been colleagues and friends for many years," said Captain Andrew Patterson, who runs Bold Coast Charters out of Cutler, Maine. "I've been doing this for 30 years." Patterson said he and his Grand Manan rival work cooperatively with Canadian wildlife officials who man the island to protect the seabirds. "It's a US island, but we let Canada use it and pay the bills," he joked. Patterson noted the latest incident occurred just after the G7 summit in Quebec where Trump was "kind of rude with the Canadian Prime Minister." Since that time, there have been no other incidents.

Yet he worries tensions could be raised if there is ever more than lobster and birdwatching at stake. "God forbid if oil or natural gas were ever found here," he said. Washington Post