Abstract
Leaping behavior of Ayu (Plecoglossus altivelis altivelis), which is often seen in a pool-type fishway and groundsel, is discussed using video-recording of their migration above and below the water surface. The video was recorded through four types of experiments/observation prepared to clarify their leaping characteristics, i.e., 1) laboratory experiment using a testing flume, 2) on-site experiment using a slope temporary installed in an irrigation canal, 3) on-site experiment in an existing fish laddar and 4) field survey at a pool of groundsel. As a result, though many of the Ayu seem to accerelate toward the water surface just before leaping, their swiming speed varies widely and speedup and slowdown are repeated inconsitently, which would appear that the accerelation does not contribute directly to the leaping. It also becomes apparent that the swim required for the leap is within an extremely short period just below the water surface. In addition, the angle and the height of the leap finally attained depend on the depth of the pool and the fish tends to remain in the pool when the depth is too large. It indicates that the pool-depth of 20cm might be enough for Ayu to leap over small falls, which was conventionally considered insufficient.
