Portable experimental Denil and Steeppass fishways (0.3 m wide, 0.m deep, and 0.9 mlong) were installed on a divergent weir on the Okitsu River in Shimizu, Japan. The number ofupstream migratory Ayu (Plecoglossus altivelis) that exited the Denil was six times higher thanthose that used the Steeppass fishway. The mean body length of fish that passed the Steeppasswas greater than those that passed the Denil. In the Steeppass, fish tended to migrate throughregions of low velocity and fewer air bubbles. These regions were located at the bottom of thefishway between the baffles. The region of low velocity near the 'V' notch of the Denil Fishwayproved advantageous for small fish. For juvenile ayu, both velocity relative to fish body lengthand turbulence intensity had an effect on upstream migratory behaviour.