Altitudinal niche partitioning between white-spotted charr (Salvelinus leucomaenis) and masu salmon (Oncorhynchus masou) in a Japanese river

Abstract

White-spotted charr (Salvelinus leucomaenis) typically occupy the upstream reaches of Japanese streams, whereas masu salmon (Oncorhynchus masou) are generally found downstream. Temperature varies predictably with altitude; thus, it is thought to be an important determinant of the altitudinal niche partitioning. We examined (i) the species composition and habitat availability (e.g., water temperature and velocity) in reaches along an altitudinal gradient (elevation: 0–270 m, gradient: 0.6–11%), (ii) microhabitat use at the individual level, and (iii) swimming stamina against a fixed water flow velocity using stamina tunnel tests in the Ohkamaya River, Hokkaido, Japan. The proportion of white-spotted charr increased in an upstream direction from 5 to 95%, whereas summer water temperature and average velocity increased downstream (temperature: 15–18°C, velocity: 17–40 cm s−1). Underwater observations revealed that white-spotted charr used slow velocity microhabitat more than masu salmon under sympatric and allopatric conditions (charr: 7–13 cm s−1, salmon: 15–23 cm s−1). Masu salmon swam twice as long as white-spotted charr against a fixed-velocity (66 cm s−1). Our results suggest that velocity was an important determinant of the observed altitudinal distribution patterns of masu salmon and white-spotted charr.