Abstract
We tracked eight matured chum salmons (Oncorhynchus keta) using acoustic and radio telemetry to understand their upriver migration around ca. 120 to 180 km site starting from the mouth of the Ishikari River in autumn 2002-2004. This study area was divided into the natural levee area, the gorge area, and the basin area including the past major spawning sites until ca. 1940s. Three tagged fish reached to the basin area, and two fish reached to the natural levee area. The maximum duration of the upstream migrationwas 17 days, and the maximum migration distance was 56 km. Mean migration speed in the basin area was 6.6 km/day, where the staying behavior was frequently observed, was approximately three times slower than those of the natural levee area (19.5 km/day) and the gorge area (14.8 km/day). The basin area had better conditions of substrates, depth, flow velocity and subsurface flows for O. keta spawning than the other areas. In addition, groundwater upwelling for egg survival and hatching occurs adequately in the basin area. Their migration patterns reflected geographical differences among three areas. Therefore, the observed slower migration in the basin area was probably due to their pre-spawning behaviors including searching for spawning sites and mates. The faster migration in the natural levee area and gorge areas was due to utilization of these areas as corridors.
