Higher-than-normal radiation levels found in fish caught off Japan's east coast more than a year after the Fukushima nuclear disaster could indicate the plant is still leaking, new research says.

Marine chemist Ken Buesseler of the Woods Hole Oceanographic Institution reviewed official Japanese data on caesium levels in fish, shellfish and seaweed collected near the crippled nuclear plant.

Buesseler concluded the lingering contamination may be due to low-level leaks from the facility or contaminated sediment on the ocean floor, according to his research, published Thursday in the US journal Science.

He said that while the vast majority of the catch off Japan's northeast coast is well within safety limits, some fish caught near Fukushima are considered unfit for consumption under Japanese regulations.

"To predict how the patterns of contamination will change over time will take more than just studies of fish," said Buesseler, who led an international research cruise in 2011 to study the spread of radionuclides from Fukushima.

"What we really need is a better understanding of the sources and sinks of caesium and other radionuclides that continue to drive what we're seeing in the ocean off Fukushima."

A huge tsunami, sparked by a massive 9.0-magnitude undersea earthquake, swamped the Fukushima Daiichi nuclear plant in March 2011.

Reactors went into meltdown, spewing radiation over a large swathe of Japan's agriculture-heavy northeast, in the planet's worst atomic disaster for a generation. Around 19,000 people were killed or remain missing.

Scientists estimate that the vast bulk of that radiation found its way into the ocean, either by direct releases of contaminated cooling water in the early weeks of the disaster, or through the water cycle.

The study called it the "largest accidental release of radiation to the ocean in history".

Contamination levels vary across fish species and are not declining, the study showed, though Buesseler found that demersal, or bottom-dwelling fish, consistently showed the highest caesium counts from the damaged nuclear plant.

Demersal include cod, conger, flounder, halibut, pollock, rockfish, skate and sole.

The scientist stressed that the levels of radiation found in most fish caught off Japan's northeastern coast mean they remain safe for consumption, even after the government tightened the rules in April this year.

An abstract from Buesseler's paper on the website of the journal says: "Although offshore waters are safe with respect to international standards for radionuclides in the ocean, the nuclear power plants continue to leak radioactive contaminants into the ocean."

Fears about food safety, which was once a given in Japan, have only slightly abated in the wake of the Fukushima disaster, with many consumers still eschewing products from the affected area despite government reassurances.

Japan's powerful farming and fishing industries have suffered both at home and abroad, with exports of farm products taking a major hit in 2011, falling 7.4 percent compared to the previous year.

No one is officially recorded as having died as a direct result of the Fukushima nuclear disaster, although tens of thousands of people have been forced from their homes.

Buesseler and Mitsuo Uematsu of the University of Tokyo are organising a scientific symposium in Tokyo on November 12 and 13 to present the latest findings on how the nuclear disaster has affected the ocean and marine life.