A crewless fishing trawler has so far been the largest object in a vanguard of debris to reach North America since Japan's massive March 2011 earthquake and subsequent tsunami.

After becoming unmoored, the Ryou-Un Maru was driven by currents and wind nearly 8,000 kilometres eastward across the Pacific Ocean.

Its odyssey ended in early April when the US Coast Guard sank it with cannon fire. The "ghost ship," more than 50 metres long, had drifted dangerously close to busy shipping lanes off the south-east coast of Alaska.

"This is just the beginning," warned retired oceanographer Curtis Ebbesmeyer, referring to the small pieces of tsunami debris that he says have been washing ashore in Canada and the US states of Alaska, Washington, Oregon and California since last fall. "We're in the countdown for October, when I think the main mass will arrive. People need to get ready for a major clean-up."

Ebbesmeyer has been studying ocean currents for decades with the help of flotsam, including thousands of plastic bathtime toys - in the shapes of ducks, frogs, turtles and beavers - that fell off a container ship during a storm in 1992 and have since travelled through three oceans.

He and his helpers on North America's west coast are now sighting plastic containers and large styrofoam buoys of the kind used in Japanese oyster farming. Ebbesmeyer said that beachcombers had found about 400 pieces of debris between California and Alaska since October, all of it presumably set adrift by the tsunami.

Lacking solid evidence, the Maryland-based National Oceanic and Atmospheric Administration (NOAA) said it would not speculate on the flotsam's origin. A definite link to the tsunami could be established only for the Ryou-Un Maru and two smaller boats that drifted into North American waters. According to the latest computer model by NOAA staffer Doug Helton in Seattle, currents and wind could have washed the first pieces of debris ashore months ago, however.

"Most items are still out at sea, widely distributed," Helton said. "The currents are complicated. Some items moved faster than others, like fishing floats, buoys and plastic containers."

What began as a dense carpet of debris in Japanese waters has spread over a wide area on its long journey through winter storms.

"There are no accurate estimates of how much is floating today," said Dianna Parker, spokeswoman for NOAA's Marine Debris Programme.

"The Japanese government estimated that five million tonnes of debris went into the ocean. Seventy per cent of it sank right away. That would leave 1.5 million tonnes floating," she calculated.

On Earth Day weekend in late April, environmental activists in North America got a better idea of the slowly growing mess that last year's tsunami is making of the continent's western coast. Thousands of volunteers picked up trash during the annual beach clean-up. One of them was Jody Kennedy, a member of the Seattle chapter of the Surfrider Foundation, a non-profit environmental organization.

"We expect to see a lot more debris in the next few years after the tsunami," she remarked, adding that she assumed it was the duty of local residents and communities to remove it from their beaches. "The tsunami debris is a very serious issue. Hopefully we'll be looking at the even bigger issue of plastic and waste washing into our oceans every day all over the world."

"Nobody has the responsibility to clean up ocean debris," said Curt Hart, communications manager at the Washington State Department of Ecology. He said the department received calls from concerned citizens who found containers on beaches that could contain chemicals or other toxic substances.

According to Hart, it is "highly unlikely" that radioactive material or human body parts could come ashore along with other tsunami debris. A number of government authorities, including the NOAA, have set up telephone hotlines for reporting suspicious trash.

For retired oceanographer Ebbesmeyer, there are many open questions about possible environmental hazards posed by tsunami debris. "We see a higher than normal number of dead sea turtles washing up," he noted. "They drift and swim exactly where the main field of debris is coming across to America."

Ebbesmeyer criticized the scuttling of the derelict Japanese trawler off Alaska as "not a good idea." The vessel should have been towed into a harbour and scrapped, he said. The US Coast Guard had assessed the pollution risk to be small, and said afterwards that the leaked fuel oil would not endanger marine life.