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Understanding and improving passage by diadromous species at tidal barriers is less well advanced than that for non-tidal anthropogenic river barriers. This study assessed factors affecting upstream passage of anadromous river lamprey (Lampetra fluviatilis) at a tidal weir with pool-and-weir (PAW) and bypass (BP) fishways. A Continuous Time Markov Model (CTMM) was used to analyse migration behaviours of 120 acoustic- and PIT-tagged lamprey across 2 years. The weir was a major barrier to upstream migration with a mean time of 31.0 days taken to pass the weir compared to 2.5 days for the unobstructed reach immediately downstream. River stage was the most important variable associated with weir passage, with a 5.68 (CI = 3.95, 8.17) increase in passage probability for every 1 m river stage increase. Passage was predominately over the weir directly rather than by the fishways. Monitoring the fishways using additional PIT-tagged lamprey (n = 2814) suggested poor entrance efficiency (BP2018, 28.6%; BP2019, 53.1%; PAW2018, 37.0%). Successful fishway passage was estimated as 5.4% (BP2019)–9.0% (PAW2018) of lamprey that entered. Effective fishway entrance for lamprey is probably facilitated by high fishway discharge, yet high-velocity areas may have made it difficult for river lamprey to successfully ascend the fishways. [ABSTRACT FROM AUTHOR]