To restore macroalgal beds in sea urchin-dominated barren areas, concrete blocks were placed and seeded with germlings of several Sargassum species at a wave-sheltered shallow site off Sakinoyama, Kasasa, Kagoshima, Japan in 1999. This resulted in a remarkable extension and long-term persistence of Sargassum beds, without removing urchins. Previous studies suggested that thin sediment cover might exclude sea urchins and allow the recruitment of sediment-tolerant species such as Sargassum duplicatum. Field studies were added to understand the mechanisms for the establishment of Sargassum in more detail. Transect surveys suggested that sea urchins including long-spined Diadema spp. having high mobility do not move into the area with >50% of even thin sediment cover, although shorter-spined Echinometra sp. translocated to small blocks within sand-covered areas moved over sand. Survivorship of caged Sargassum germlings on cobbles translocated to sediment-free urchin barrens were considerably lower than the previous observations in sedimented Sargassum beds, suggesting that thin sedimentation might prevent herbivory not only by urchins but also by mesograzers. However, outplanting S. duplicatum germlings suggested that thin sediment cover might not ensure their survival due to other factors such as feeding by sandy bottom organisms. Maintenance and recent reduction of Sargassum stands are also discussed.
