Endohelminths of marine, anadromous, and freshwater three-spined sticklebacks <i>Gasterosteus aculeatus</i> Linnaeus, 1758 (Perciformes: Gasterosteidae) in the White Sea

This study investigates the endohelminth parasites of three-spined stickleback, <i>Gasterosteus aculeatus</i>, across marine, anadromous, and freshwater populations in the White Sea (Russia). A total of 241 sticklebacks were collected in Velikaya Salma Strait during 2022-2024. Thirteen helminth species were identified using morphological and molecular methods. Genetic analysis confirmed the presence of <i>Anisakis simplex sensu stricto</i>, <i>Hysterothylacium aduncum</i>, <i>Phyllodistomum folium</i>, <i>Brachyphallus crenatus</i>, and <i>Alloopistholecithum</i> cf. <i>salmonis.</i> We identified variability in parasite communities between 2022+2023 and 2024 both within and between marine, anadromous and freshwater host populations. The most striking temporal changes were observed in the parasite communities of the anadromous stickleback population, which is probably due to the different periods of time spent by the fish in fresh water in the years studied. This study highlights differences in infection of marine, anadromous, and freshwater populations of three-spined stickleback, providing insights into host-parasite dynamics across different aquatic environments in the White Sea basin.