Canadians often boast that their 5,525-mile boundary with the United States is the longest undefended border in the world. But tempers have frayed on at least one small stretch.

Machias Seal Island is a 20-acre, treeless island teeming with puffins, razorbills, terns, eiders and other seabirds, making it a mecca for birdwatchers. Both Canada and the United States claim sovereignty over the island, which is about 10 miles off the shore of Maine, and the surrounding 277-square-mile Gray Zone, where fishermen from both countries compete over valuable lobster grounds.

In late June and early July, Canadian fishermen said, U.S. Border Patrol agents in high-speed boats intercepted Canadian lobster boats in the Gray Zone.