Soon after the Great East Japan Earthquake and the subsequent accident at the Fukushima Daiichi nuclear
power plant, we studied the reproductive biology of freshwater teleost bluegill (Lepomis macrochirus) in a pond in the
city of Iwaki, Fukushima Prefecture, about 50 km south of the nuclear power plant. The breeding period of the bluegill
was abbreviated and the frequency of spawning decreased. The radiation air dose rate and level of radioactivity in the fish
were less than would be expected to trigger behavioural alterations or to suppress reproduction. The occurrence of aftershocks
and two episodes of torrential rain is the likely reason for changes in the reproductive behaviour of bluegill in the
pond. These results suggest that our observations can be explained by a combination of natural disasters, rather than the
effects of nuclear radiation.