TOKYO — The operator of Japan’s crippled Fukushima nuclear plant began releasing about 11,500 tons of radioactive water into the sea on Monday evening to make room in storage tanks for even more severely contaminated water.

Some 10,000 tons of the water being released into the ocean was being taken from a communal storage facility near the No. 4 reactor. Another 1,500 tons was being released from the vicinity of the No. 5 and 6 reactors — which have been less troubled than reactors Nos. 1 through 4. The amount of water being released is equivalent to more than four Olympic-size swimming pools.

Although the water being released had levels of radioactive iodine 131 more than 100 times the legal limit allowed for sea discharge, the government approved the release as an “”emergency”” measure so that water with 100,000 times more radiation than the water found in a normally functioning reactor can be removed from the basement of the turbine building at reactor No. 2 and stored somewhere on the site.

Even as the government asserted that the release of the radioactive water into the sea would not pose an immediate threat to humans, health ministry official Taku Ohara said the ministry was considering drawing up radioactivity food-safety standards for fish after high radiation levels were detected in a sand lance, a bottom-feeding fish, caught off the coast of Ibaraki prefecture.

Nuclear experts have assumed that radioactive iodine, which has a brief half-life, would become diluted in the ocean and decay too quickly to be detected in fish, but Monday’s finding has raised doubts about that, said Ohara.

According to the health ministry, the sand lance had 4,080 bequerels per kilogram of radioactive iodine.

“”We think the level found poses no immediate risk to people’s health, but the point is moot anyway because all sand lance caught in Ibaraki were disposed of,”” said Ohara. By comparison, the level of radioactive iodine in the fish was twice as high as the limit for vegetables. There are no standards for radioactivity in meat, eggs, fish and grains.

After more than three weeks of cooling the disabled Fukushima reactors by spraying them with thousands of tons of water using fire trucks, concrete pumpers and helicopters, Tokyo Electric Power Co. faces a growing problem of what to do with the vast amounts of contaminated water.

Removing the water from turbine buildings and other structures is vital to allow workers to restore cooling functions to the facilities. But with limited facilities for storing the water, the utility and the government are now considering options including putting it into a “”floating island”” offshore. Also being discussed is the installation of an undersea barrier, usually used to contain old spills, that might slow the radioactive water’s move offshore.

Tepco reported no success Monday in its efforts to stop highly radioactive water from seeping from a pit near the No. 2 reactor into the ocean. The utility believed that the leak was coming from an 8-inch crack and attempted to seal it with a polymer, sawdust and shredded newspaper. When that failed, the utility dumped some white bath salts into a pipe near the pit to attempt to trace the flow of the water, but the colored water has yet to show up in the sea.

Chief Cabinet Secretary Yukio Edano announced that the government would modify its protocol on restricting vegetable sales from prefectures where significant amounts of radiation have been detected. Instead of banning shipments from an entire prefecture, radiation levels henceforth will be monitored weekly at the level of municipalities, and if the detected levels fall below government limits for three consecutive weeks, then shipments will be allowed.

Farmers have been pressing the government to ease up on its restrictions, but the new procedures raised questions about whether municipalities were capable of carrying out thorough checks. Some farmers complained that the weekly testing would not be frequent enough and said their produce was still likely to go to rot.

In Fukushima prefecture, officials announced plans for monitoring radiation levels at 1,400 schools starting Tuesday; a new academic year is to begin Wednesday and many parents are worried about their children being exposed to radiation.

Fukushima officials also have begun checking radiation levels of products manufactured within the prefecture. Many businesses are nervous that their goods might be rejected by buyers unless they are certified as being free of contamination.

In the town of Namie, which sits northwest of the Fukushima plant just beyond the 18-mile perimeter within which authorities have urged people to stay indoors or consider evacuating, high levels of cumulative radiation were recorded over an 11-day period beginning March 23, the government announced.

The accumulated radiation was 10.3 millisieverts over 11 days, assuming that a person stayed outdoors 24 hours a day. With the government saying that it could easily take months to bring the Fukushima plant under control, the readings are raising fresh questions about the dangers of radiation over a longer period and whether the government’s evacuation perimeter is wide enough. Exposure to 100 millisieverts is believed to raise one’s risk of cancer by 0.5 percent.

Despite widespread concern about radiation from Fukushima, the public appears to be divided over whether the government should review its use of nuclear power. The results of an opinion poll by the national Yomiuri newspaper published Monday showed that nearly half those surveyed said they favored maintaining the current number of nuclear power plants, while almost a third of respondents said they wanted the government to cut back.

The poll also showed that voter support for Prime Minister Naoto Kan’s Cabinet has risen slightly, to 30 percent from 24 percent in March. But many voters — 69 percent — said Kan wasn’t exhibiting his leadership, and 19 percent said they wanted him to quit soon.

The National Police Agency said the death toll in the massive March 11 earthquake and tsunami stood at 12,259 as of Monday evening, with more than 15,000 people still listed as missing.

In a bit of positive news, national broadcaster NHK reported that a dog rescued Saturday by coast guard officers from the roof of a destroyed house floating more than a mile off the coast of northeastern Japan had been reunited with its owner.

The owner of the dog, a 2-year-old mutt named Ban, said she saw the news of the rescue on TV over the weekend and contacted authorities immediately.

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