Investigation of Cause of Mass Fish Death in October 2007 at Minami-Sotobori in Osaka-Jo Park

A mass fish death occurred at Minami-Sotobori (moat) in Osaka-Jo Park on Oct. 4, 2007. It was reported that more than a thousand fish died. Features of the mass death were: 1) it involved many of the fish species that live in the moat, 2) most were fish with long body length and died with open mouth, 3) dissolved oxygen was 3.1 mg·L-1 or less throughout the water column. It was judged that the fish had died of anoxia. The air temperature had decreased rapidly seven days before the fatal accident and low temperatures continued for four days. As a result, it is thought that the epilimnion was cooled strongly and sank to the bottom during this period and that the hypolimnion, which contains less oxygen, welled up. Since the hypolimnion seems to become anaerobic under organic loading for long periods, it is possible that similar accidents will happen again when water stratification breaks down for some reason.