Quantifying a mass mortality event in freshwater wildlife within the Lower Odra River: Data

• Toxins from Golden Algae resulted in an unprecedented mass mortality of fish, bivalves, and water snails.
• In the lower Odra, there were 65 million dead Unionidae bivalves, marking an 88% decrease; the native Anodonta anatina declined by 95%; the invasive Sinanodonta woodiana decreased by 15%.
• 147 million water snails were found washed ashore in the lower Odra, representing an 85% decline.
• The lower Odra witnessed the death of 3.3 million fish (equivalent to 1,025 tons); across the entire 560 km section of the affected river, the toll was 1,650 tons, indicating a 60% reduction in fish population biomass.