The measurements also showed high levels of cesium and were taken outside the discharge canal for the plant's Nos. 1, 2, 3 and 4 reactors.
Readings from a short distance away, outside the Nos. 5 and 6 units' discharge canal, showed lower but still high radioactive iodine levels some 284 times above normal.
These high levels suggest there may have been some sort of leakage directly into the ocean -- unlikely to be because of atmosphere emissions or rain alone, said an official with the Tokyo Electric Power Co., which operates the nuclear plant.
The International Atomic Agency reported online Saturday that radioactive iodine and cesium was detected 30 kilometers (19 miles) offshore, but it said that these levels differed only slightly from the previous day.
That said, its potential effect on Japan's fishing industry -- even if consumers stay away, for simple fear of contamination -- remains a major concern. So, too, is the fact that authorities have yet to pinpoint the exact source of the radiation, and thus to determine if it's stopped