This study aims to evaluate the predation pressure during daytime and nighttime on larvae of Ayu, Plecoglossus altivelis, by comparing the number consumed by three predatory fish species between light and dark experimental conditions. Four of five individuals for Zacco platypus consumed more Ayu larvae in the light condition than in the dark condition. The number of Ayu larvae consumed by all four individuals for Tribolodon hakonensis in the light condition was about twice that in the dark condition. For Gymnogobius petschiliensis, three of five individuals ate few Ayu larvae, one showed a higher and the other showed a lower feeding activity under the light condition. Because two out of three predators showed a consistently higher feeding activity on Ayu larvae under the light condition, we suggest that the diurnal predation pressure is a selective factor for not only nocturnal drift in Ayu larvae but also concentrating hatching after sunset.
