Chum salmon (Oncorhynchus keta) are one of the most important species for commercial fisheries in Hokkaido in northern Japan. The Okhotsk coast in eastern Hokkaido is the principal area of salmon production in Japan. Commercial catches of chum salmon have been supported by intensive hatchery programs (Miyakoshi et al. 2013). In Hokkaido, returning chum salmon are fished in coastal waters mainly via set nets that are operated from September to December. Recently, coastal sea water temperatures in autumn have been higher than the historic mean. In years with high coastal sea water temperatures (> 20°C), it has been frequently observed that the peak timing of chum salmon landing was delayed and exploitation rates by coastal set net fisheries were low. In addition, high water temperatures might affect the distribution of chum salmon in coastal areas and the number of fish caught by each set net. In order to determine the effects of sea water temperature on commercial landings, the responses of migrating chum salmon to sea water temperature needs to be elucidated. To understand migration routes, depths, and temperatures experienced by returning chum salmon in the Okhotsk Sea, we caught chum salmon in the coastal waters, applied archival tags to them, and released them in late August or early September from 2016 to 2018. We report the outcomes of the tagging experiment for chum salmon in the Okhotsk Sea. From 5 to 7 September 2016, 28 to 30 August 2017 and 28 to 31 August 2018, tagging experiments for chum salmon were conducted in the Okhotsk Sea from the research vessel Hokuyo maru (237 tonnes, Wakkanai Fisheries Research Institute, Hokkaido Research Organization). At a total of 8 or 9 sites in each year, we visually counted the number of chum salmon and fished for chum salmon at night (Fig. 1). At each site, fishing lights were used, and chum salmon that appeared were fished with a lure with raw bait (a slice of squid or Pacific saury) on the hook. The captured chum salmon were anesthetized, tagged with an archival tag on the base of the dorsal fin, and measured for fork length and weight. After recovery from anesthesia, the tagged fish were released into the sea. At the fishing sites, the vertical distributions of the sea water temperature and salinity were measured using CTD (Seabird SBE9plus, Sea-Bird Electronics, Inc., Bellevue, WA, USA).
