Abstract
As potential competing species for the economically important resource kelp (Saccharina japonica var. ochotensis), the spatial distribution of two Sargassum species (Sargassum siliquastrum and S. boreale) on Rishiri and Rebun Island in northern Hokkaido, Japan, was observed by diving surveys (1993-2014). Oceanographic conditions of this coastal area were analyzed based on satellite images and particle-tracking experiments. We confirm the extension of the distribution of S. siliquastrum to the more northern areas on Rishiri (Kutsugata, from 2010) and Rebun islands (Funadomari, first found in 2014). The vertical distributions of S. siliquastrum and S. boreale were recorded along with environmental conditions in those habitats. One possible reason that an increase in sea surface temperature in August, the reproductive season of S. siliquastrum, and changes of the Tsushima Warm Current paths in the northern Sea of Japan might have affect their horizontal and vertical distribution.
