抄録
As a beginning of a nationwide 'Isoyake' project (2004 to 2006), we surveyed the current status of reduction of seaweed beds in Japan by hearing from 39 coastal prefectural governments. The survey revealed that seaweed beds reduced at least from 86 areas of 37 prefectures. The most prominent trend is the increase of urchin barrens and deforestation by herbivorous fish in southern Japan; these two phenomena are sometimes coupled together. The southern type of urchin barren is inhabited by Anthocidaris, Diadema, Echinometra or Hemicentrotus, while the northern type is dominated by Strongylocentrotus. Deforestation by fish browsing is caused by Siganus, Calotomus, Priomirus and Kyphosus (the latter two are limited to exposed coasts), among which Siganus was shown to extend up to warm temperate Sea of Japan coast of northern Honshu. In Siganus, juveniles rather than adults are becoming frequently found by divers or caught by set nets. Fluctuations of Kuroshio, elevation of water temperature, change of coastal currents, increase of turbidity and local accumulation of volcanic ash are also enumerated as the causative agents of the recent seaweed bed reductions.
