This paper describes the reactions of freshwater fishes to He-Ne laser light and discuss herding effect of moving laser screen. The indoor tank experiments were conducted to examine herding effect at different fish numbers (10, 20 and 30 fish) and different species (Japanese dace Tribolodon hafronensis, carp Cyprinus carpio and rainbow trout Oncorhynchus mykiss). In outdoor tank, it was attempted to herd rainbow trout at different moving speeds of laser screen. Laser temporally dispersed fish school, and then fish again swam in school avoiding the laser. Thereby laser can drive fish into an area of tank. Fish are however considered to leam to escape through laser screen, since herding effects decreased, following repeated exposure to the stimuli of laser screen. Laser could herd fish more effectively in the dark than in the light at the indoor tank experiment. The larger fish number of Japanese dace was, the sooner laser screen could herd. Moreover, the herding effect was different among species. The proportion of herded rainbow trout decreased over 25 cm/s of laser screen moving speed. It is therefore resulted.that the herding effect depends on several factors e. g. fish school behavior and swimming performance.
