Tracing the rise of silver carp (Hypophthalmichthys molitrix) in Thailand with eDNA

Silver carp ( Hypophthalmichthys molitrix ) is a filter-feeding cyprinid introduced globally for aquaculture that poses significant ecological risks to river systems worldwide. In Thailand, its distribution in the Chao Phraya River Basin has remained poorly documented due to limitations of traditional monitoring. This study applied environmental DNA (eDNA) analysis at 39 sites across the Ping, Wang, Yom, Nan, and Chao Phraya rivers during 2019, 2021, and 2023. Detection rates increased from 31% in 2019 to 59% in 2023, with eDNA detected at all sites by the final survey year. Bayesian occupancy modelling revealed moderate occurrence probabilities (ψ = 0.40–0.51) and consistently high laboratory detection reliability (p 11 ≈ 0.90). Temperature was the strongest predictor of detection efficiency, whereas other water-quality parameters had minimal effects on ψ, suggesting that environmental factors mainly influenced detectability rather than species presence. Comparison with Department of Fisheries records showed a contrast between long-term declines in Chinese carp catch and increasing eDNA detections of H. molitrix , indicating continued establishment and expansion of this species. Because official data still combine H. molitrix and bighead carp ( Hypophthalmichthys nobilis ) under one category, eDNA provides the species-level resolution needed to track individual taxa. These results demonstrate that molecular surveillance complements traditional fisheries assessments by offering finer spatial and temporal resolution for detecting introduced species, supporting the integration of eDNA techniques into regional biosecurity programmes.