It is known that ayu spawn at the rapid in the lower reaches. However, suitable value of the flow depth and velocity were not known, as yet. It was pointed out that the velocity near the bed is more important than the bulk mean velocity, due to ayu spawn on the bed. At first, the suitable partial velocity for spawning of ayu is derived on the basis of the previous data and field measurements. It was found that the suitable partial velocity normalized by the burst speed of ayu is in the range of 0.04-0.25. In contrast, the bulk mean velocity and flow depth were used in environment assessment like a PHABSIM. The suitable curves of ayu were not known. The field measurements were conducted to obtain the suitable curves. As a result, it is pointed out that not only the bulk mean velocity and flow depth, but also the growth of the attached algae is necessary for PHABSIM. Further, ayu makes their territory not only in the rapid but also in the pool.
