Markedly reduced prevalence of Schistocephalus solidus in pelagic three-spined sticklebacks (Gasterosteus aculeatus) from Lake Constance.

Host total length, body mass and prevalence of Schistocephalus solidus were assessed in pelagic three-spined sticklebacks (Gasterosteus aculeatus) from Upper Lake Constance, a prealpine lake in Central Europe, based on 987 individuals collected by pelagic trawl in late autumn 2024. In addition, 62 fish sampled using gillnets in late autumn 2025 were examined using the same biometric and parasitological procedures. The 2024 population was dominated by small-bodied individuals, with a mean total length of 5.26 ± 0.55 cm and a mean body mass of 1.56 ± 0.51 g. Infection was rare: 16 of 987 fish were infected in 2024, corresponding to a prevalence of 1.6%, and parasite index (PI = parasite wet mass / host wet mass) values ranged from 0.013 to 0.302. In 2025, prevalence remained similarly low at 3.2%. Compared with earlier reports from Lake Constance, the late-autumn 2024 sample indicates markedly lower prevalence of S. solidus in the pelagic population, and the 2025 sample showed similarly low prevalence.