Studies were conducted to clarify the factors causing formation of locally formed kelp beds of Laminaria religiosa in Sakazuki area on the coralline flats prevailing Southwest Hokkaido. Seasonal distribution patterns of the sea urchin, Strongylocentrotus nudus and environmental conditions were compared with those in neighboring Tomari area dominated by coralline flats. No detectable differences were observed in temperature, salinity, nutrients or light condition between the two areas. But, wave action in Sakazuki was stronger than in Tomari, and increased from fall to winter, which was the season for kelp recruitment. Kelp developed on mounds dotted on the seabed of rough topography in Sakazuki. The density of the sea urchin on the mounds rose from spring to summer as the kelp beds grew. Within the kelp bed, sea urchin density declined as wave strength increased from fall to winter, and few sea urchins were seen on the mound from winter to spring. By contrast, its density outside the kelp beds remained high throughout the year. Similarly high and stable levels of density were confirmed for the sea urchins distributed over the coralline flats in Tomari.
It is concluded that sea urchin density on mounds remains low when zoospores of the kelp are released and thus seasonally reduced grazing pressure by sea urchins on young sporophytes allows kelp beds to persist in the Sakazuki area.