2.2 | Study area
Mark-recapture surveys were conducted at the headwater tributary of the Ito River, Shiodomari River system, southern Hokkaido, Japan. The study reach was located between two waterfalls (both are about 2 m high). Since the waterfall prevents the upstream migration of most, but not all, sea-run migrants (Hasegawa unpublished data), the population was mainly composed of residents. The study reach was 536 m, and it was divided into 22 sections (i.e. 25 m section × 21 + 11 m section × 1). The water temperature was measured hourly with a HOBO data logger (Onset Computer Corporation, Bourne, MA) from 2 June 2020 to 7 July 2021, and the average water temperature in the study reach was 8.4 °C (min 0.0 °C–max 18.9 °C). No other fish species were observed in the study reach, apart from white-spotted charr (Hasegawa unpublished data).