Tests on seawater near the nuclear power plant showed that levels of radioactive iodine reached 3,355 times the legal limit on Monday, one of several peaks in recent days that have fallen rapidly as radioactive substances decayed and were steadily diluted and dispersed by ocean currents.

Officials are watching levels of iodine-131 in seawater because although it has a half-life of eight days, meaning it is half as radioactive after that time, the substance builds up in seaweed, a common food in the Japanese diet. If consumed, radioactive iodine collects in the thyroid and can cause cancer.

The International Atomic Energy Agency said iodine-131 in seawater would "soon be of no concern" presuming there are no further discharges of contaminated water from the power station into the sea.

The IAEA added that Japanese authorities have released the first analyses of fish, caught at the port of Choshi, in Chiba prefecture south of Fukushima, which found one of five to be contaminated with a detectable level of caesium-137, a far more persistent radioactive substance, though at a concentration that was far below safety limits for consumption.

Many countries, including Britain, have begun radiation testing of fish, shellfish and other fresh produce from Japan or have imposed wider bans on imports from the region. Fisheries are not entering waters within the 20km (12-mile) exclusion zone around Fukushima, said Hidehiko Nishiyama, a spokesman for Japan's Nuclear and Industrial Safety Agency.

The fate of many local seafood and shellfish farms, including scallops, oysters, sea urchins and sea snails, was sealed more than two weeks ago when the tsunami wiped out beds and destroyed fishing vessels and ports around Fukushima. In Iwate prefecture, authorities say the disaster may have wiped out businesses that account for 80% of the revenue of the region's fisheries.

At the Fukushima power plant, engineers continued the arduous task of trying to pump contaminated water from turbine rooms and trenches, which is hampering work to connect the reactor cooling systems to the national grid.

Tepco, the power station operator, plans to spray parts of the site with a resin to stop radioactive dust blowing off the site and is considering shrouding the reactor buildings with sheets to reduce radiation being released into the air.

Fish and other sea creatures are unlikely to be seriously harmed by the radioactive leaks, even in the most contaminated areas. After the Chernobyl nuclear disaster, fish in three freshwater lakes within the exclusion zone became contaminated with radioactive caesium but showed no obvious health problems, though some fish were born with reproductive abnormalities which may have been caused by radiation, said James Smith, an environmental physicist at Portsmouth University who studied fish in the area.

While fish accumulate radioactive contamination, this happens less in the ion-rich waters of the oceans than in freshwater lakes.