Spatiotemporal variation in gastrointestinal helminth infections in farmed pacu (Piaractus mesopotamicus) reared in floating cages in a lotic freshwater system.

This study assessed the spatiotemporal variation in gastrointestinal helminth infections in farmed Piaractus mesopotamicus reared in floating cages in the Middle Paraná River (Argentina) and evaluated the relationship between parasite abundance and host body condition. We examined 209 fish during two farming seasons (2021/22-2022/23) in four intensive production systems (floating cages) and collected parasitological and morphometric data. Four helminth species were identified, belonging to the classes Trematoda (Dadaytrema oxycephala, Thometrema overstreeti), Nematoda (Procamallanus (Spirocamallanus) inopinatus), and Acanthocephala (Echinorhynchus cf. jucundus). The digenean trematode T. overstreeti was found in the stomach (pylorus), whereas the remaining species were recovered from the intestine. The helminths T. overstreeti and P. (S.) inopinatus were recorded for the first time in pacu. We found a temporary increase in T. overstreeti abundance was consistent with ongoing transmission through intermediate hosts. In contrast, the higher abundance and temporal stability of E. cf. jucundus suggest that infections were primarily acquired before cage stocking and persisted throughout the production cycle. A significant negative association was detected between E. cf. jucundus abundance and host condition indices, indicating lower parasite burdens in fish with better body condition. While parasitism is traditionally viewed as a driver of reduced host condition, our results suggest that variation in host condition may also contribute to shaping parasite burdens in aquaculture systems. These findings underscore the value of integrated parasite and host-condition monitoring and the need for biosecurity protocols to reduce parasite exchange between farmed and wild fish populations in open-water aquaculture.