Effects of removing Diadema on barren ground in Haidaura Bay, Mie Prefecture

Abstract

Effects associated with removing the sea urchin Diadema spp. from an area of barren ground were examined in Haidaura Bay in Mie Prefecture, central Japan, from December 2009 to December 2012. Diadema spp. were harvested using scoop nets and spears from boats, or crushed by divers using metal rods. Three experimental areas in which initial Diadema spp. densities were high (8.8 ind./m2, area A), moderate (4.0 ind./m2, area B) or low (2.0 ind./m2, area C) were identified in the bay. Diadema spp. density, seaweed species present, and algal coverage were monitored at three-month intervals in each area. During the experimental period, 119,557 Diadema individuals were removed from the experimental sites. Diadema spp. density in areas A, B and C decreased to 0.4, 0.5 and 0 ind./m2, respectively. Algal coverage increased from 0 to 30.0% in area A, from 2.3% to 48.3% in area B, and from 2.6% to 21.8% in area C. Algal coverage was observed to increase when Diadema density decreased to fewer than 2 ind./m2.