Not just for adults! Evaluating the performance of multiple fish passage designs at low-head barriers for the upstream movement of juvenile and adult trout Salmo trutta

Longitudinal connectivity in salmonid streams is vital for juvenile as well as adult fish, yet most upstream passage studies consider only larger adults. Upstream passage of juvenile and adult brown trout Salmo trutta at low-head (<3 m) structures on two River Ribble tributaries (NW England) was investigated using Passive Integrated Transponder (PIT) telemetry during summer-autumn 2013 and 2014. The efficiency of a Servais low-cost baffle (LCB) fish pass was evaluated for the first time, along with two pool-weir (PW) passes, an embedded rock ramp (ERR) and an open culvert (C), the latter a man-made structure within predicted swim speed, acting as an experimental control. We used a combination of naturally migrating trout and displacement experiments. Resident fish were displaced from above to below structures, utilising their homing instinct to instigate their ascent of the structure, with up to 91% of displaced trout attempting to pass. Approximately 30% of parr morphotype trout released at their capture locations attempted to pass upstream of structures in both streams. Passage efficiencies of up to 82% for the LCB pass design were similar to the PW (up to 79%) and better than the ERR (71%), but below that for C (96%–100%). Significant differences occurred between fish passes in time to passage, and number of attempts to pass, with all but PW1 having significantly longer time to passage than the control culvert. Median time to passage at PW2 decreased almost fifty fold between 2013 and 2014, following modification to equalise step heights at the structure. Logistic regression demonstrated a strong body-length effect on passage success at passes, with 50% probability of successful passage (82–132 mm) varying, but not significantly, between passes. We conclude that small trout, including juveniles, can and do exhibit functionally significant upstream movement and that greater consideration should be given of their passage needs as well as for large, adult trout.