Telemetry‐Based Assessment of Non‐Size Selective Passage in a Vertical Slot Fishway Modified With Brush Blocks

ABSTRACT Fishways are widely used to restore river connectivity, yet their effectiveness often varies among species and is frequently size‐selective, limiting passage for smaller individuals. This study evaluated the effects of flexible brush blocks on hydraulic conditions and fish passage performance in a vertical slot fishway (VSF) at the Cataloluk Small Hydropower Plant (Tekir River, Türkiye). Hydraulic conditions were quantified using three‐dimensional velocity measurements before and after brush block installation. Passage of Capoeta damascina and Alburnus kotschyi was monitored with passive integrated transponder (PIT) telemetry. Brush blocks reduced flow velocity and turbulence, eliminated recirculation zones and created microhabitats within pools. Active migration was concentrated within a thermal window of 14°C–19°C, commencing when water temperature reached approximately 13.5°C in May (monitoring period: 292 days, mean 13.2°C, range 4.6°C–24.8°C). Overall passage efficiency reached 61.7%, compared with approximately 15% at the same site before the retrofit (66.1% for C. damascina , 51.9% for A. kotschyi ). Mean inter‐antenna ascent time was 17.25 h for C. damascina (range 0.09–198.22 h) and 241.53 h for A. kotschyi (range 0.38–1075.49 h), indicating that the fishway served as both a migration corridor and a resting habitat. ANOVA test showed no significant differences in total length between tagged fish and those detected at the upstream antenna ( p > 0.05 for both species), indicating that passage was no longer size‐selective—to our knowledge the first field demonstration of size‐selectivity elimination through a hydraulic retrofit in an operational VSF, confirmed by PIT telemetry. These results demonstrate that low‐cost hydraulic retrofits can substantially improve fishway performance and provide a practical, evidence‐based strategy for restoring inclusive connectivity in fragmented river systems.