Depression of Ichthyocenosis of the Seym River Caused by Catastrophic Contamination over Hostilities in September 2024

The accident discharge of wastes from the sugar factory in the russian town of Tiotkino in September 2024 resulted in heavy pollution of the Seym River and unprecedented fish kill caused mainly by the oxygen deficit. Among the dead fishes rheophiles prevailed - specimens of four species formed 73.3 %, specimens of the adventive crucian carp counted for 12.9 %, and specimens of seven limnophilous species counted for 13.7 %. Analysis of catches of juvenile fishes in October 2024 in the same sites as in 2021 showed principal modification of ichthyocenosis of the riverbed: change of the dominant species, species number in the catches decreased from 20 to 12, species diversity in terms of the Shannon index decreased 1.4 times. Total juveniles' numbers in the catches decreased 7 times, limnophilous 5.9 times, rheophilous 38.9 times, adventive 39 times, species of the Red Data Book of Ukraine 9 times. At the same time, species number and numbers in the catches in the flood-land lakes increased. The common loach <i>Cobitis taenia s. l. </i>was the only species that did not decrease numbers, at this ratio of its diploid and polyploid forms did not change after the disaster. According to assessment of the State fishery agency, total mass of dead fishes amounted to 40.3 t, this 2-8 times exceeded commercial catches in the Desna River basin before the hostilities. Considering changes of the hydrological regime over the last decade, the ichthyocenosis rehabilitation will take some years, successfully at the expense of the limnophilous and adventive ichthyofauna only.