Evaluating migratory fish passage at partial migration barriers in a social-ecological riverscape

Anthropogenic partial barriers, such as low-head locks and dams, fragment social-ecological riverscapes and limit migratory fish access to historical spawning habitats, creating trade-offs between ecological conservation and human needs. Fish passage mitigation strategies at three low-head locks and dams (LD1, LD2, and LD3) on the Cape Fear River, North Carolina (USA), across two contrasting mitigation regimes (2013–2015 and 2022–2023) included (i) a nature-like fishway at LD1 (original and modified designs), (ii) conservation locking at LD2 and LD3, and (iii) environmental flow (e-flow) prescriptions (i.e., dam submergence flows) when locks were inoperable. We evaluated passage of American shad ( Alosa sapidissima ) and striped bass ( Morone saxatilis ) using acoustic telemetry and multistate models within a Bayesian framework to estimate upstream passage probabilities under varying flow conditions and management regimes. Passage probabilities for both species were higher in 2013–2015 when conservation locking was conducted. In contrast, passage declined when locks were inoperable and only e-flows allowed passage during dam submergence events in 2022–2023. Flow positively influenced passage, with strongest effects for striped bass; however, the nature-like fishway exhibited consistently low passage probability, and modifications did not improve passage probabilities. Given low passage probabilities during the recent mitigation period, improving longitudinal connectivity for diadromous fish in this river necessitates flexible, integrated operational, structural, and flow-based strategies. Possible future mitigation actions to improve fish passage could include resuming conservation locking (operational), structural interventions such as bypass channel construction and dam height lowering that extends dam submergence, and continued use of e-flows (flow-based). • Multistate models quantified passage probabilities across mitigation regimes. • Passage probabilities were highest when conservation locking occurred. • Flow strongly influenced fish passage probabilities. • Nature-like fishway showed similar passage probabilities pre- and post-modification. • Multi-approach strategies are likely needed to improve ecological connectivity.