When boat QNG 90675 limped into the Vietnamese port at Quang Ngai on December 12, it showed the signs of a confrontation at sea with a Chinese coast guard ship. According to local officials, the coast guard boarded the boat, forced the crew of seven to stand with their hands behind their heads, and confiscated their catch and fishing gear, before cutting the boat’s nets and casting them into the sea.

The boat had been fishing in the Paracel Islands in the South China Sea, as Vietnamese fishermen have for centuries, inside what Hanoi claims as the country’s exclusive economic zone in what it calls the East Sea. With coastal areas of Vietnam overfished, the fishing grounds around the islands abound in species such as tuna and flying fish.

But the Paracels also lie inside China’s self-declared maritime boundary — the “nine-dash line”— the swath of the sea within which Beijing claims exclusive rights. It does not hesitate to demonstrate this against Vietnam’s fishermen, sometimes in crude terms.