Growth, Condition, and Seasonal Changes in the Population Structure of the Invasive Chinese Sleeper Perccottus glenii (Dybowski, 1877) in a River Subjected to Severe Anthropological Pressure

Managing invasive species such as the Chinese sleeper (Perccottus glenii) supports the goals of sustainable development by preserving native biodiversity. This study investigated the population structure, growth, and ecological impact of P. glenii in a small, anthropogenically altered tributary of the Vistula River (central Poland). Electrofishing surveys conducted between 2017 and 2023 assessed sex ratio, age structure, body size, condition (Fulton’s index), and growth parameters, as well as changes in the local fish community. The sex ratio was nearly balanced (♀:♂ = 1.00:0.99), and average standard length and weight were 6.54 cm/9.11 g (females) and 6.36 cm/7.69 g (males). Dominant individuals were from age group of 2+ years. The Fulton condition factor ranged from 2.54 to 2.58, while positive algometric growth was observed for both sexes. The von Bertalanffy growth model parameters (L∞ = 175.37 mm, k = 0.104, t0 = −1.711) revealed slower growth compared to other Eurasian populations. In the individual months of the study, changes in the sex structure, length, weight, and age of the fish were observed. This seasonality may have resulted from physiological changes (including fish growth and reproductive processes), their migration, and environmental changes, such as food availability and hydrochemical parameters, occurring during this period. Additionally, over the study period, the abundance and density of P. glenii increased significantly, coinciding with a marked decline in native fish species. These findings highlight the adaptability of this invasive fish and emphasise the need for targeted management strategies in degraded freshwater ecosystems.