Physical Characteristics of Spawning Redds of Sea-Run Form of Amago Salmon Oncorhynchus masou ishikawae in a tributary of the Nagara River System, Central Japan

Abstract

We searched for the spawning area of the sea run form of amago salmon Oncorhynchus masou ishikawae which is an endangered fish in a small tributary of the Nagara River system in central Honshu, and surveyed the location, size, water depth, current velocity and substrate size of the spawning redds. The redds (n=22) were observed only at the pool ends and riffles. Averages of the major and minor axis of redds were 129.5±44.9 (SD) cm and 85.0±28.9 cm, respectively. Average water depth was 61.5±16.1 cm. Averages of surface and bottom velocity were 42.0±15.5 cm/sec and 25.9±10.7 cm/sec, respectively. Fine pebble (16-63 mm) was the preferred substrate size. Characteristics of redds observed in our study resembled those of the O. masou group reported in previous studies. It is considered that to conserve wild stocks of the sea run form of amago salmon in the Nagara River, maintaining the physical characteristics of the spawning area mentioned above is important.