Distribution of cestodes of the genus Triaenophorus (Cestoda, Pseudophyllidea) among pike (Esox lucius) in the upper course of the Dnipro River

Cestodes of the genus Triaenophorus are widespread parasites of freshwater fish throughout the Holarctic region. However, under current environmental changes and increasing anthropogenic pressure on aquatic ecosystems in Ukraine, there is a need to update the available data on their distribution. The aim of this study was to investigate the distribution patterns, infection intensity, and spatial distribution dynamics of the cestodes Triaenophorus nodulosus and Triaenophorus crassus іn the population of northern pike (Esox lucius) inhabiting the upper Dnipro River (Kyiv Reservoir). During 2024–2025, comprehensive parasitological examinations were performed on 85 northern pike aged 1+ to 6+ years collected from four sampling sites within the Kyiv Reservoir (Hlibivka, Lebedivka, Stari Petrivtsi, and Vyshhorod). The overall prevalence of Triaenophorus infection in northern pike was 50.6 ± 5.4%, with a mean intensity of infection of 5.89 ± 0.58 specimens (range: 1–15 specimens) and a mean abundance of 2.98 ± 0.43. T. nodulosus was the dominant species at all sampling sites. The mean prevalence of infection with this parasite was 48.5%, and its mean abundance (2.21) was nearly three times higher than that of T. crassus (25.7% and 0.76, respectively). The highest level of infection was recorded near the village of Stari Petrivtsi (prevalence: 66.7%; mean abundance: 4.25), indicating the presence of a stable local transmission focus. A retrospective analysis showed that, despite substantial environmental transformations and anthropogenic impacts, the overall level of Triaenophorus infection in northern pike has remained stable over the past four decades. At the same time, the occurrence of T. crassus in the northern part of the reservoir indicates changes in the structure of intermediate host populations. The obtained findings demonstrate the high ecological plasticity of T. nodulosus and the efficient functioning of trophic interactions within the ecosystem. The pronounced aggregated distribution of parasites within the host population prevents mass mortality of fish and contributes to the long-term stability of the host-parasite system.