Bull. Jpn. Soc. Fish. Oceanogr. 83(1), Page 28-41, 2019   Zoning of the central and western parts of the Seto Inland Sea based on the principal component analysis of sediment parameters and comparison between the 2016 and previous surveys Hisashi Yokoyama1† and Tateki Fujiwara2 1 Field Science Education and Research Center, Kyoto University, Kitashirakawa-oiwakecho, Sakyo, Kyoto 606–8502, Japan 2 Kyoto University, Professor Emeritus † E-mail: patiens07@gmail.com Total organic carbon (TOC), total nitrogen (TN), δ13C and δ15N in sediments collected from 135 stations in the central and western parts of the Seto Inland Sea in July 2016 by Ministry of the Environment, Japan (MOF) were analyzed to evaluate the bottom environment. The principal component analysis based on these parameters plus C : N ratio and six parameters provided by MOF shows that the area is divided into four zones: zone A characterized by fine and organically enriched sediments, zone B characterized by coarse sediments showing low contents of organic matter, zone C1 characterized by low δ13C and δ15N values and the nearshore location and zone C2 characterized by low δ13C and δ15N and the offshore location. The terrestrial organic matter ratio in the sediment of zone C1 was high (37%), whereas the ratio in zone C2 must be low, considering low values of the C : N ratio. The comparison in the sediment parameters between the 2016 and previous surveys shows no clear trend in the sediment parameters during the 30 years except for TOC and TN in sediments collected from zone A in Hiroshima Bay. Key words: bottom environment, sediment, decadal change, stable carbon isotope ratio, stable nitrogen isotope ratio