ABRUPT INCREASE IN POPULATION SIZE OF THE SEA URCHIN Mesocentrotus nudus AND KELP DEFORESTATION OFF THE PACIFIC COAST IN NORTHERN MIYAGI AFTER THE 2011 TOHOKU TSUNAMI DISASTER, JAPAN

Yukio Agatsuma*, Hikaru Endo, Ayaka Horikoshi, and Masakazu N. Aoki
 
 Graduate School of Agricultural Science
 Tohoku University
 Sendai, Miyagi 981-8555
 agatsuma@bios.tohoku.ac.jp

Mesocentrotus nudus is the most commercially harvested sea urchin in Japan. This species spawns from September to October in northern Japan. Following the Tohoku-Oki Earthquake on 11 March 2011, the juveniles of M. nudus recruited heavily off the coast of northern Miyagi Prefecture. The recruitment level in Shizugawa Bay and Onagawa Bay was markedly high compared to that from a long-term monitoring research in southwestern Hokkaido (Agatsuma et al. 1998). At a permanent survey site in Sizugawa Bay, adult kelps Eisenia bicyclis survived in low densities at the depths of 5.9 m and 7.5 m after the attack of the huge tsunami with 15.9 m height. However, the kelps began to decrease since February 2014. Increased adult sea urchins at 2-years of age invaded into the lower margin of the kelp bed, and denuded the area by April 2014. Currently, more than 50% of the kelp area disappeared. In this bay, densely aggregated sea urchins bit the proximal part of kelp stipe, fell the thalli down and then grazed lateral blades, and branches or the stipe in order. Finally, only the tough holdfasts remained on the sea bottom (FIGURE 1). Deforestation of adult E. bicyclis by sea urchin herbivory was firstly observed. In Onagawa Bay, the density of M. nudus increased to > 20 individuals/m2 at the depths of < 7 m from September 2012 to July 2014. The predominant brown alga Sargassum yezoense decreased the coverage to result in subtidal "barren", possibly, due to intensive grazing of M. nudus.