Regional variations of Saccharina japonica forests distribution around the Tsugaru Strait by acoustic method

Abstract

As the global warming, commercially important species Saccharina japonica disappeared, what is called sea desertification occurred in some coastal waters around the Tsugaru Strait recently. For the sustainability of coastal ecosystem and resources use, understanding the distribution and variation of S. japonica forests is necessary. Surveys for the distribution of S. japonica forests by acoustic method were conducted in four coastal waters: ① Osatsube, Hakodate, the Pacific Ocean side of Hokkaido; ② Ishizaki, Hakodate in Tsugaru Strait of Hokkaido; ③ Okoppe, Oma, in Tsugaru Strait of Aomori Prefecture; ④ Shiriyazaki, Higashidori, the Pacific Ocean side of Aomori Prefecture. Thickness and spatial distributions of seaweed were obtained by acoustic method and density of sea urchins by visual observations. In Osatsube coastal water, main species was S. japonica and which forests were most among survey regions. In Ishizaki and Okkope coastal waters, Sargassum spp. was also luxuriant except S. japonica. In Shiriyazaki coastal water, S. japonica dominated and a greater number of sea urchins were observed than other regions. In regions where little benefit from Oyashio Current, Sargassum spp. grew thickly when sea urchins were rare whereas sea urchin barren occurred when sea urchins with high density.