Changes in movement, range and habitat preferences of adult grayling from late summer to early winter

Radio‐tagged adult grayling Thymallus thymallus ( n  = 22), monitored from mid August to mid December 1999 in the River Kuusinkijoki, Finland, shifted by the end of September (water temperature 10·0–14·5° C) from riffle sites to deeper and slower pool sites 0·7–1·6 km up‐ or downstream. In early winter ( c . 0° C water temperature), eight of 13 fish still under study made a further shift into new pool sites, possibly triggered by ice formation. The summer range of grayling in the riffles was smaller (mean ±  s . d . length: 75 ± 146 m) than the autumn range (99 ± 46 m) in the pools, but gross daily movements were equally short in both the seasons (18 ± 34 m and 15 ± 7 m, respectively). In late summer, adult grayling preferred water depths 80–120 cm and mean velocities >40 cm s⁻¹. In autumn, the preferred ranges were 100–240 cm and <30 cm s⁻¹, respectively. Substratum was mainly boulders in the summer sites, and gravel or pebbles in the autumn sites.