TORONTO — Warning of an ecological “emergency” on the high seas if “pirates” continue to plunder dwindling fish stocks, Canada Tuesday said it would begin policing the North Atlantic beyond its 200-mile limit, boarding and seizing offending vessels if necessary.

“If we do not protect these fragile stocks now, we — not we as a nation, but we as a planet — may lose them forever,” said Canada’s fisheries minister, Brian Tobin, as he announced the government’s strategy to fight overfishing by renegade foreign trawlers.

“These vessels break every rule in the book,” Tobin said at a televised news briefing in Ottawa after the government introduced the measures in the House of Commons. They provide for the use of force in seizing vessels and up to $1.5 million in fines. “We will use these powers when we must, and we must when all other means fail.”

Tobin and other officials emphasized Canada’s continuing commitment to international law and to strengthening multilateral agreements to protect endangered fish stocks. But Canada’s assumption of police power in an international zone would be unprecedented and does not find obvious support under the United Nations Law of the Sea, the unratified common law of the deep.