More than half a tonne of dead fish has been recovered per kilometre along a stretch of the Dyje River in southern Moravia after oxygen levels crashed to 0.5 mg/l in the Poštorná relief arm early on July 14, 2026. The Moravian Fishing Association said crews collected the carcasses along a 1.5-kilometre section near the weir.
A prolonged spell of hot, dry weather triggered a severe cyanobacteria bloom, which depleted dissolved oxygen overnight. Volunteer firefighters were called to the Břeclav reach at 22:00 on July 13 and deployed six pumps to aerate the water, but the effort could not prevent the die-off downstream.
Water management authority Povodí Moravy adjusted weir operations at Břeclav and Poštorná in response to the bloom, while staff at Lednice Castle cut off the Dyje inflow to its chateau pond to block algae. Compounding the problem, the Nové Mlýny hydroelectric plant has been offline since late June, reducing water circulation.