Abstract
The bio-economy of the natural population of Strongylocentrotus intermedius, based on the census of the sea urchin and the seaweeds by means of quadrat work, was studied at the rocky shoal of Ikantai, southern Hokkaido. The balance sheet of population metabolism expressed as a unit of dry matter or of nitrogen for the echinoid population was that of the credit side of the balance, where 60.7g/m2/yr or 1.1g-nitrogen/m2/yr was initial biomass; 43.1g/m2/yr or 1.3g-nitrogen/m2/yr was population growth; 46.4g/m2/yr or 0.9g-nitrogen/m2/yr was immigration; and only 0.14g/m2/yr or 0.003g-nitrogen/m2/yr was recruitment. This sum total was divided into the following way on the debit side of the balance, loss of population due to emigration and natural mortality was 72.6g/m2/yr or 1.6g-nitrogen/m2/yr; gametes ejected were 2.9g/m2/yr or 0.2g-nitrogen/m2/yr; and final biomass was 51.3g/m2/yr or 0.9g-nitrogen/m2/yr. Since the population growth of the sea urchin has been sponsored by 412.4g/m2/yr or 8.0g-nitrogen/m2/yr of Laminaria angustata and by 12.2g/m2/yr or 0.5g-nitrogen/m2/ /yr of Ulva pertusa, growth coefficient was about 10% in dry matter and production efficiency was about 4% in dry matter. The annual food consumption of the echinoid population corresponds to about half of the annual growth (production) of the seaweed community.
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