TOKYO — The government set its first radiation safety standards for fish Tuesday after Japan’s tsunami-ravaged nuclear plant reported contamination in nearby seawater measuring at several million times the legal limit.

The plant operator insisted that the radiation will rapidly disperse and that it poses no immediate danger, but an expert said exposure to the levels near the Fukushima Dai-ichi plant could cause immediate injury and that the leaks could result in residual contamination of the sea in the area.

The new levels coupled with reports that radiation was building up in fish led the government to create an acceptable standard for fish for the first time. Some fish caught Friday off Japan’s coastal waters would have exceeded the new provisional limit.

The areas hit by the tsunami make up about a fifth of Japan’s huge industry, but radiation fears could taint the country’s catch through guilt by association. Japan imports far more than it exports, but it still sent the world $2.3 billion worth of seafood last year. And in the home of sushi, the worries over contamination could deal a blow to its brand.

Over the weekend, workers at the nuclear plant found a crack where contaminated water was spilling into the ocean.