US officials ordered Japanese seaweed and barnacles to be stripped Thursday from a tsunami-wrecked dock washed up on an Oregon beach, to guard against \"possible invasive species\" from Japan. The 66-foot (20-meter) long rectangular structure, made of concrete and metal, landed on a high tide early Tuesday on Agate beach, near the town of Newport some 100 miles (160 kilometers) southwest of Portland. The Oregon Parks and Recreation Department (OPRD) contacted Japanese diplomats, who confirmed it was from the March 11, 2011 tsunami, and had drifted 5,500 miles (8,850 kilometers) across the Pacific. The wreck is believed to be the biggest piece of debris from the Japanese tsunami to make landfall on the US West Coast so far. In an update Thursday morning, the OPRD said: \"The dock sitting on the sand at Agate Beach is covered with marine organisms. Some are native, but others are specific to the waters of Japan. \"As a precaution against possible invasive species, the Oregon Department of Fish and Wildlife is coordinating a group of volunteers to remove the organisms from the dock starting Thursday morning.\" Salt water-dependent organisms will be removed from the beach, it said. \"Among the exotic species are different kinds of mussels, barnacles and marine algaes. One invasive marine algae in particular -- Undaria pinnatifida, commonly called wakame -- is present on the structure.\" Japanese officials confirmed that the dock -- 66 feet long, 19 feet wide and 7 feet tall -- came from the port of Misawa, in Aomori Prefecture in the northern part of Japan. A Japanese-language metal plaque was dated June 2008. The wreck has been checked for radioactivity -- the killer earthquake and tsunami triggered a disaster at the Fukushima nuclear plant on Japan\'s east coast -- but proved negative, said OPRD spokesman Chris Havel. Various debris from the Japanese tsunami have begun washing up on the US and Canadian west coast, and experts predict a surge of flotsam in the coming months. The OPRD spokesman said the dock was bigger than either a trawler scuttled off Alaska in April for safety reasons before reaching land, or a shipping container with a Harley-Davidson inside that washed up on a Canadian beach at the start of May.