Abstract
Nocturnal activity of hatchery-reared juvenile flounder was observed in the laboratory to understand their predation risk after release. The number of individuals which were “swimming”, “nonburying” and “solitary” was counted using CCD camera with infrared lamp, and were recorded by time-lapse video-recorder. The number of swimming individuals was higher at night than in the daytime. In a tank without sand, most fish aggregated along the edge of the bottom during the day, whereas at night fish dispersed all over the bottom. In a tank with sand, about 60% of fish emerged from the sand at night, while fish buried into the sand in the daytime. These results suggest that the activity of juvenile flounder might be nocturnal than the diurnal one ever understood, and that released flounder juveniles might suffer comparatively higher predation risk during the night because of their ap pearance on the sand and the off-bottom swimming behavior.
