In what the New York Times somewhat hyperbolically calls a "clash," US Border Patrol vessels have over the past two weeks stopped at least 10 Canadian fishing boats near Machias Seal Island between Maine and New Brunswick. Canada has responded by beefing up its Coast Guard patrols in what is being termed a "disputed gray zone" between the two countries' territories. "There is no illegal immigration going on there," a bewildered Canadian fisherman told the Times. "It seems silly." Most observers see it as related to the current bitter trade dispute between Washington and Ottawa. The Canadian Broadcasting Corporation says the US Border Patrol has stopped over 20 Canadian vessels so far this year in "contested waters" in the Bay of Fundy, and "has no intention of stopping." The so-called Grey Zone consists of some 700 square kilometers of lucrative lobster waters where the Bay of Fundy meets the Gulf of Maine, although few actually live in it. Machias Seal Island is a migratory bird sanctuary maintained by the government of Canada, but is otherwise uninhabited.
A report on Canada's Global News provides some historical background. In 1621, the British Crown granted the lands that make up what is now Nova Scotia, New Brunswick, Prince Edward Island and the Gaspe Peninsula to the Scotsman Sir William Alexander, Earl of Stirling. However, those territories had already been claimed by the French as the colony of Acadia. This dispute was resolved in the 1713 Treaty of Utrecht, when the French handed over the territories that made up Acadia to the British. (The Acadians, descendants of French settlers, would notoriously be expelled in 1755, at the start of the French & Indian War.) The current conflict dates to the 1783 Treaty of Paris that ended the Revolutionary War between Britain and the nascent United States.
The 1783 treaty stated that islands within what's now reckoned as roughly 112 kilometers ("20 leagues") from the shore of Maine would belong to the United States. But that text excluded "such islands as now are, or hereafter have been within the limits of the said province of Nova Scotia." The Canadian government maintains that those terms included Machias Seal Island and that therefore it is Canadian territory, pursuant to the 1621 land grant. The US refused to recognize this. The dispute erupted again in the War of 1812.
As Atlas Obscura notes, in 1832 Canada built a lighthouse on the island and installed a small Coast Guard force for "sovereignty purposes." Both countries refused to include the island in a case brought before the International Court of Justice in 1979 to resolve other border disputes along the Gulf of Maine. The 1984 ruling awarded some two thirds of the Gulf to the US, but excluded the Grey Zone.
As The Economist notes, the US has a handful of other "managed maritime disputes" with Canada, including fishing waters at Dixon Entrance between Alaska and British Columbia, and areas of the petroleum-rich Beaufort Sea near the Arctic Ocean. So plenty of potential for more of this "silliness."
As usual in such disputes, the rights of the region's indigenous people are neatly ignored by both sides. The shores and islands where the Bay of Fundy meets the Gulf of Maine are the homeland of the Passamaquoddy nation, part of the Wabanaki Confederacy, whose territory is now divided between the US and Canada. The Passamaquoddy have tribal governments at Indian Township and Sipayik (Pleasant Point) in Maine.
After the 1723-6 Dummer's War war with Britain, Dummer's Treaty of 1727 for the first time formally recognzied Wabanaki territorial rights. The Confederacy, dormant since Canadian independence, is now attempting to reconstitute itself. MaineBiz website noted in March that the Passamaquoddy are currently attempting to negotiate with Ottawa to establish their fishing rights in Passamaquoddy Bay, an inlet of the Bay of Fundy.