Abstract
Field ascending fishes and fishway hydraulic surveys were conducted at the full-cross-sectional overflow steppedpool type fishway of the Iwaki River diversion weir (width: 150 m) located about 55 km upstream of the river mouth of the class A river Iwaki River, in Aomori Prefecture. Twenty kinds of fish that ascended up the fishway in fullcross-sectional overflow state included mainly Tribolodon hakonensis (Japanese dace)(58%), Phoxinus lagowshi steindachneri (Japanese fat-minnow)(24%), Plecoglossus altivelis (Ayu) and Zacco platypus (Pale chub)(5%) and additionally, Salmo Oncorhynnchus keta (salmon). The time zone in a day with a high ascending frequency for fish using the fishway was when the water temperature was high, ranging from midday to about 20: 00. This result was quite similar to the results from previous ascending surveys of fishways located in the middle part of rivers in Aomori Prefecture. In addition, the swimming fish were found to use the orifices as an effective pass. The fishway pool hydraulics was characterized using three-dimensional and two-dimensional combined vector flow distributions. Large characteristic rotary flow areas were formed at the center of the pools, representing a calm area with the combined standard deviation of flow variation in the space of 10-15 cm·s-1, which was smaller than other areas.
