Bull. Jpn. Soc. Fish. Oceanogr. 84(3), Page 161-177, 2020   Relationship between temperature and catch of chum salmon Oncorhynchus keta in set nets adjacent to Ishikari Bay New Port in autumn 2017 Tokihiro Konoi1† 1 Department of Marine Biology and Sciences, School of Biological Sciences, Tokai University, 1–1–1 5-jo Minamisawa, Minami-ku, Sapporo, Hokkaido 005–8601, Japan † E-mail: tkono@tsc.u-tokai.ac.jp The homing migration and catch amount of chum salmon (Oncorhynchus keta) depend on the physical conditions of the coastal water in the autumn, which are likely to change due to the recent warming trend. We analyzed catch numbers at four salmon set nets placed 1.9–14.1 km southwest off the mouth of Ishikari River. We also analyzed water temperature observed at two of the nets in September and October 2017. Between October 4–6 and 16–18, we observed the highest catches, each of which corresponded to a minimum water temperature and a maximum downward vertical gradient. Each event followed the occurrence of a maximum water level at the mouth of the river with a time lag of 6–7 days, indicating that a freshwater plume from the Ishikari River possibly attracted more salmon. A heat and volume conservation model, which was applied to the estimated amount of the river discharge, resulted in the plume increasing to 37 km in radius covering the inner part of Ishikari Bay where the set nets existed. We estimated the dilution of the river water to be 11.3% due to the extension of the plume, which was 1–2 orders of magnitude larger than the lower limit of concentration at which salmonoids can distinguish their original river water, suggesting salmon off the coast were attracted to the plume and caught. As the salmon catch increased earlier at the set nets that were situated further away from the river mouth, they may have concentrated southwest of the mouth. We also discuss the importance of seasonality in oceanic conditions on the west coast of Hokkaido to the returning of salmon. Key words: Ishikari Bay, river plume, chum salmom set net, water temperature, catch number