Story highlights U.S. Coast Guard official says Japanese trawler has capsized

It sank about 180 miles west of Southeast Alaska, an official says

Authorities say it is a navigation hazard

The boat is part of a giant debris field making its way toward North America

The U.S. Coast Guard sank a Japanese fishing trawler off the coast of Alaska on Thursday, more than a year after it was swept away by a tsunami off the east coast of Japan, authorities said.

The trawler capsized hours after the Coast Guard set fire to it, causing it to begin taking on water, said Petty Officer 1st Class David Mosley, a Coast Guard spokesman.

The sinking of the Ryou-Un Maru brought about the end of a journey for the rust-stained ship that drifted across the Pacific Ocean as part of a giant debris field generated by a tsunami that struck Japan following a 9.0 earthquake on March 11, 2011.

The Coast Guard determined the trawler posed a threat to other ships navigating the area, and had been broadcasting the unmanned, unlit vessel's location to mariners to alert them to the hazard.

The U.S. Geological Survey advised the Coast Guard where the ship's sinking would have the least environmental impact.

"The potential for a pollution incident is unknown at this time, but officials have limited concerns about any biological threats due to the length of time the vessel has been at sea," the Coast Guard said in a statement.

The Coast Guard opened fire on the Ryou-Un Maru at 1 p.m. Thursday.

The ship drifted from Canadian waters into U.S. waters on Saturday, and was located about 180 miles west of the Dixon Entrance in Southeast Alaska, the Coast Guard said.

The trawler was first spotted floating near British Columbia by a Canadian military air patrol, and it was then determined that it had been adrift since the tsunami, Canadian officials said last month.

The Japan Coast Guard identified the owner of the vessel after being contacted by Canadian officials, who were provided the identification number on the hull.

The vessel, which was used for squid fishing, was moored at Hachinohe in the Aomori prefecture when the tsunami hit, Japanese authorities said.

A Canadian vessel, the Bernie C, had expressed an interest in salvage, which delayed the operation, but officials deemed that it would be too unsafe to tow or attempt to salvage the vessel, said Coast Guard spokesman Kip Wadlow.