Radiation levels in most kinds of fish caught off the coast of Fukushima haven't declined in the year following Japan's nuclear disaster, it has emerged.

Researchers believe that deposits of the chemical cesium on the seafloor or leakage from the damaged reactors is continuing to contaminate the waters - and has the potential to threaten fisheries for decades.

Around 40 per cent of bottom-dwelling fish such as cod, flounder and halibut tested off Japan's northeast coast remain unfit for human consumption.