Tracking anadromous fish over successive freshwater migrations reveals the influence of tagging effect, previous success, and abiotic factors on upstream passage over barriers

Predicting and mitigating the impact of anthropogenic barriers on migratory fish requires an understanding individual environmental factors that influence barrier passage. Here, upstream spawning migrations iteroparous twaite shad Alosa fallax were investigated over three successive in a highly fragmented river basin using passive acoustic telemetry ( n = 184). More approached passed lower reaches than further upstream, with median cumulative weir passage time 4.6 (1.8–9.2) days representing 18% their river. Returning second year had significantly higher rates tagging year, also positively influenced by previous success. Higher water temperatures levels positive impacts rates. Lower newly tagged individuals suggest reliance within-year estimates telemetry-based assessments could result conservative results, while previously successful versus unsuccessful conserved motivation and/or inherent ability to pass barriers.