Restoration of Ecklonia radicosa bed on a long-left concrete block dominated by Diadema sea urchins

Abstract

In Japan, deployment of substrata has been employed to expand macroalgal beds as the local fishing grounds in shallow waters. However, most of them were left without any management. In our study site, Uchiura Bay, central Pacific coast of Japan, unmanaged two cuboid concrete blocks (3.5 × 3.5 × 0.5 m) were left with dominance of diadematoid sea urchins and crustose coralline algae. To probe the possibility of restoration of macroalgal beds on such unmanaged blocks, we kept small number of sea urchins on one (treated) of the concrete blocks. Further, we monthly investigated dominant macroalgal species and number of sea urchin on the two blocks (treated and control) between June 2012 and December 2014. The annual kelp, Eckolonia radicosa, formed dense stands on the treated block throughout the study period, especially on the edge of the block. On the contrary, the dense stands hardly appeared on the control block because of the persistence of sea urchins with a large number (113.8 ± 5.9 inds. in average). The present study showed that restoration of macroalgal beds is possible even on the long-left concrete blocks, but continuous monitoring and management (control of density of herbivores) are needed.