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Hundreds of marine animals have made an epic 7000-kilometre trip across the Pacific Ocean from Japan to the US. They simply hitched rides on the myriad boats and debris swept up by the massive Tohoku earthquake and tsunami of 2011.

A marine wildlife census under way since 2012 has now documented 289 species arriving this way, including fish, mussels, barnacles, sea slugs, anemones, sea stars, crabs, clams and sponges – all native to Japanese waters.

Some fish survived for years in water-filled troughs on fishing boats, which somehow avoided overturning. They include and , some of them up to 60 centimetres long.

“They had just enough food to keep going, but they were pretty emaciated on arrival,” says of Williams College in Massachusetts, who set up the census. Since he knew when the tsunami happened, he could tell how long the debris stayed at sea, which stowaways thrived, and how long it took them to reach the US.

Many of the stowaways were adrift for years, clinging to upturned boats, tsunami buoys and other flotsam.

“The largest were two large, floating fishing docks, both as long as tennis courts and half the width,” says Carlton. “The first dock arrived in Oregon in June 2012 and had 100 species on it.” That came from the Japanese port of Misawa, followed by another which reached Washington state in December that year. Two identical docks from Misawa remain missing.

“It’s the biggest rafting event ever aboard anthropogenic material, and the first time we’ve been able to track material on this scale,” says Carlton.

Arrivals are starting to tail off, but Carlton says the invasion is not over. “We’re waiting for the spring 2018 pulse to see what comes in.” What we know of is probably a fraction of what actually arrived, as much debris was cleared without analysis before the census began.

The sheer number of species surviving arduous trans-Pacific journeys is impressive. But the census also shows how potentially invasive species can travel further and for longer than before, because human-made materials – like fibreglass, used in many fishing boats – last longer than natural ones like wood or .

If climate change increases the number or strength of storms and typhoons that lash built-up coastlines, ever more species will be swept around the world, Carlton says. The current spate of hurricanes probably carried more debris out to sea, he adds.

“What’s of most concern is that growth in coastal infrastructure, ongoing plastic pollution, rising sea levels and extreme events make situations like the one documented more likely,” says of Monash University in Australia.

None of the Japanese species has yet become established or invasive in the US. “But after they arrive, it can take several years before we can detect them, so until they grow to a detectable point, we don’t know how many will settle,” says Carlton. “It’s like ecological roulette.”