Long-Term Monitoring of Fish Abundance Dynamics in the Middle Stretch of the Mesha River (Tatarstan)

The study area is located in the middle reaches of the Mesha River on the territory of the Republic of Tatarstan and is characterized by strong temporal variability in environmental factors and fish populations. The impact of environmental factors on fish species and communities was analyzed with regression and ordination methods. The dominant species were bleak, dace, chub, and roach, which together accounted for 60.7% of the total fish species. Over 15 years of research, there have been significant decreasing trends in overall density, species richness and density of individual fish species. The main factors determining the size of both the fish population and individual species were fishing effort and temperature during different months. The index of long-term similarity of fish populations was only 7.5%.