On the deterioration process of Ecklonia and Eisenia beds observed in 2013 at Gounoura, Iki Island, Nagasaki Prefecture, Japan. -Initiation of the bed degradation due to high water temperature in summer and subsequent cascading effect by the grazing of herbivorous fish in autumn-

Abstract

In 2013, summer water temperature off Iki Island was the highest among the eight years from 2006 to 2013. Deterioration of kelp (Ecklonia spp. and Eisenia bicyclis) beds occurred around Iki Island. Summer water temperature was higher on the western coast than the southern coast of the island, and within the same area it was higher in shallower zone than in deeper zone. In late August and early September, fronds of kelp were lost, and the lower part of the stipe had lost its color and was bent. Deterioration of kelp progressed more rapidly in the areas with relatively higher water temperature. Until October, kelp beds in deeper zones or along the southern coast of the island, where water temperature was relatively low, remained with less damages. But in October to December, the kelp beds in these areas were heavily grazed by herbivorous fish. Juveniles of the kelps appeared in late December and their density was proportional to the amount of fronds that remained at the previous reproductive season (October to November). In this process of the kelp bed deterioration in Iki Island, higher water temperature and grazing by herbivorous fish affected the kelp beds with a time lag and spatial difference, rather than occurring simultaneously.