Radioactivity in fish exceeding health guidelines was detected for the first time off northern Japan as Tokyo Electric Power Co. dumped tainted water into the ocean to gain control of its crippled nuclear plant.

Cesium radioactivity in sand-lance caught south of the Fukushima Dai-Ichi plant was 526 becquerel per kilogram, compared with a health ministry standard of 500 becquerel, Makato Osodo, of the fishing policy division of the Ibaraki prefectural government, said in a telephone interview.

Fukushima Prefectural Federation of Fisheries Co-operative Associations asked the company, known as Tepco, to stop dumping radioactive water into the sea. Dumping began April 3 because radioactive water on site is hindering repair of cooling pumps. Discharges continued, Japan’s Nuclear and Industrial Safety Agency said.

“Clearly, they haven’t got the site under control,” said Richard Wakeford, a visiting professor of epidemiology at the University of Manchester’s Dalton Nuclear Institute in England. “They’ve got to make difficult decisions and one of those is you get rid of the mildly radioactive liquid to make way for the really contaminated liquid.”

Fishing in Ibaraki Prefecture has been suspended since the March 11 earthquake and tsunami that damaged the plant, leading radioactivity to escape into water and air. The number of dead and missing following the earthquake and tsunami reached 27,688 as of 10 a.m. local time yesterday, according to the National Police Agency in Tokyo.

Exposure to Cesium-137, among the isotopes Tepco says were released from the plant, increases the risk of cancer, according to the U.S. Environmental Protection Agency. Japan has struggled to keep the radioactive fuel at the Fukushima reactors cool after equipment was damaged by the earthquake and tsunami, triggering the worst nuclear crisis since Chernobyl.

Tokyo Electric plunged the daily limit of 80 yen, or 18 percent, to close at 362 yen on the Tokyo Stock Exchange yesterday, the lowest since its listing in August 1951. The stock has dropped 83 percent since the day before the magnitude- 9 earthquake.

The risk to people from the deliberate discharge at the Fukushima plant is low, according to the Vienna-based International Atomic Energy Agency.

The potential additional radiation dose to a person eating seaweed or seafood caught near the Fukushima plant every day for a year would be 0.6 millisievert, the agency said in a statement. That compares to 0.85 millisievert from a year of exposure to granite that comprises the U.S. Capitol, according to the U.S. Army Corps of Engineers.

With a radioactive half-life of 30 years, cesium can build up in the meat of marine predators as they eat smaller animals, said Karen Gaines, chairwoman of the biology department at the University of Eastern Illinois in Charleston.

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