Dzierżno Duże is an artificial water reservoir that was created on the site of a former sand mine. It is the largest “no kill” reservoir in Poland. In 2024, as a result of a golden algae bloom, the degree of oxygen saturation of the water was even more than 300 percent. 
As a result, fish were dying en masse. For several days, anglers, the military, firefighters and others pulled out all the dead animals. In total, from 3.08.2024r. to 4.09.2024 from the waters of the Dzierżno Duże reservoir fished out about 120,075 kg of dead fish - this is an estimate, some of the fish were not weighed. Among the fish pulled out were specimens of considerable size, 20 or even 30 years old. Golden algae thrive in salty water. Dzierżno Duże is a reservoir located in Silesia, a mining, industrialized part of Poland. The water in the reservoir has been heavily salinated, by draining water from mines.