Biotoxicological monitoring and river renaturalization to prevent fish kills: integrating environmental and economic solutions

Climate change and river system regulations are causing environmental degradation. Water quality in rivers is deteriorating significantly. Massive fish die-offs are indicative of the problem. One example is the Oder, one of Europe's largest rivers. Due to salinity, the Oder suffered Europe's worst ecological disaster in 2022. Salty water caused a bloom of the golden alga Prymnesium parvum Carter, which synthesizes ichthyotoxins. The prymnesins damaged the gill tissue of fish and caused their death. The first reports of fish deaths in the Oder were the results of mercury (Hg) contamination of the river. This article demonstrates the impact of weather anomalies on ecological disasters in large rivers, the economic impact of river disasters between 2010 and 2024 on all continents (except Antarctica), global ecological disasters in rivers caused by ichthyotoxins since 2000, Hg concentration in fish muscles from the Oder River over the last 30 years, and the impact of river regulation on their ecological functions. The article presents the results of physicochemical and biotoxicological studies of the Oder River in the Lubusz region (Poland) conducted in 2024. The results of the study support the recommendation of the water flea Daphnia magna Straus for continuous river biomonitoring. Huge economic losses, loss of river ecosystems, fish kills, and depletion of aquatic biodiversity compel radical remedial action. Two main approaches to protecting rivers from extinction are presented. For rivers, these are: biotoxicological monitoring combined with renaturalization.