Since cesium binds to the sediment on the ocean floor, the radiation has likely had the biggest effect on bottom-dwelling fish and other animals though the consequences still aren't clear, said Tatsuo Aono, a specialist at Japan's National Institute of Radiological Sciences.

WSJ: In Japan, Relief at Radiation's Low Toll

He said that some whales appeared to guard the bottom of this "bait ball", preventing the prey from sinking to unreachable depths, while other animals in the group took advantage of the centre of the ball.

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