Mass Fish Die-Off in Osaka Castle Moat

Aug 14, 2025 | News On Japan

OSAKA - More than 100 carp and other fish were found dead in the outer moat of Osaka Castle on the morning of August 13th, in what experts believe was caused by a lack of oxygen in the water.

A visitor reported to the Osaka Castle Park Center around 9 a.m. that “a large number of fish are dead and there is a foul odor.” Staff confirmed the deaths in the east outer moat and noted a strong smell. The fish carcasses were seen floating on the surface.

This is not the first such incident at Osaka Castle. In the summer of 1994, about 2,000 crucian carp and carp were found dead in the north outer moat, with oxygen depletion identified as the cause. At that time, air was pumped into the water to address the shortage.

According to Nobuyuki Aiko, director of the Biodiversity Center, the current situation also points to hypoxia. “I believe it’s oxygen depletion. Fish use the limited oxygen, and so do microorganisms. In summer, oxygen consumption increases, leading to competition for oxygen, and fish are unable to get enough,” he explained.

The Osaka Castle Park Center plans to collect the dead fish and conduct water quality tests starting on the morning of August 14th.

Source: YOMIURI

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