Seasonal foraging activity of the sea urchin Strongylocentrotus nudus on coralline flats in Oshoro Bay in south-western Hokkaido, Japan

Bimonthly field investigations of the seasonal foraging activity of the sea urchin Strongylocentrotus nudus were carried out in Oshoro Bay of Hokkaido, Japan. Migration of the marked sea urchins transplanted to a fixed 9 m square site (81 divisions, each 1 m square) on coralline flats with the brown alga Laminaria religiosa set in the center division was monitored. The highest foraging activity was observed in July when the gonads of the sea urchins were in the growth stage, and high water temperature was 19.9-20.7°C. No foraging behavior was observed in September and November due to their maturation and spawning, while the sea urchins migrated actively. In January, neither foraging nor migration occurred due to the low water temperature of 4.0-5.0°C. A little foraging activity was observed in March when the gonads were again in the growth stage. In May 1992, when the gonad index was higher than that in July of the previous year, foraging activity was low, while active migration was observed. It is considered that the time when S. nudus forage actively, during spring and summer, alters due to annual fluctuations in their gonadal development attributable to the biomass of the algal community on which they feed.