Assessment of fish biodiversity and trophic status of freshwater reservoir ecosystem

Abstract:

Nearly 40% of freshwater fish species worldwide face some level of threat from habitat degradation and changing water quality, yet baseline biodiversity data for many small to medium-sized reservoirs remain unavailable. This research examined fish species richness, relative abundance, and trophic status in a freshwater reservoir near Kagoshima, Japan over a 12-month sampling period from April 2022 to March 2023. Fish were collected monthly from four sites using gill nets, cast nets, and electrofishing. A total of 1,847 individuals belonging to 23 species across 9 families were recorded. Cyprinidae was the most dominant family, accounting for 38.2% of total catch. The Shannon-Wiener diversity index ranged from 1.73 to 2.91 across sites and seasons, with the highest values at the outlet site during summer. Trophic status was evaluated using Carlson's Trophic State Index based on Secchi depth, chlorophyll-a, and total phosphorus. The reservoir was classified as mesotrophic with seasonal shifts towards eutrophic conditions during late summer. Correlation analysis revealed a significant positive relationship between chlorophyll-a concentration and the abundance of planktivorous species (r = 0.74, p<0.01). The data suggest that seasonal nutrient loading from upstream agricultural land influences both primary productivity and fish community structure in this system.