Enhancing the upstream passage of river lamprey at a microhydropower installation using horizontally-mounted studded tiles

Passage performance of upstream-migrating lamprey (Lampetra fluviatilis) was compared between sections of a Crump flow-gauging weir with and without studded tiles, and at a bottom-baffle fishway, on the River Derwent, England. The effects of microhydropower operation on attraction to multiple routes were also studied. Studded tiles were fixed horizontally on the weir face near the right bank, forming a 1 m wide treatment route, neighboured by a tileless control route. A bottom-baffle fishway was present at the right bank, alongside the hydropower tailrace. Two further weir-face control routes at the left bank, in combination with those on the right side, enabled comparison of lamprey attraction relative to the weir flow. Downstream and upstream ends of the right-hand weir-face routes, and of the fishway, downstream ends of the left-hand weir face routes, and the entrance of the hydropower tailrace area were instrumented with PIT antennas (n = 9 total). Of 395 PIT-tagged lamprey, released 0.52 rkm downstream of the weir on 10 separate dates in early winter 2017 (turbine on for 21/43 days of study period), 363 (91.9%) were detected by at least one of the antennas (median [IQR] minimum delay at weir: 15.0 [7.4-21.4] days). All lamprey detected at the left-bank antennas (attraction efficiency AE: 255/395 [64.6%]) were also detected elsewhere. The fishway was ineffective (AE: 343/395 [86.8%]; passage efficiency PE: 5/343 [1.5%]). While lamprey were more attracted towards the control relative to the adjacent tiled route, a higher number of fish traversed the weir using the latter (AE tiled route: 172/395 [43.5%]; PE tiled route: 44/172 [25.6%]; AE control route: 257/395 [65.1%]; PE control route: 22/257 [8.6%]). Lamprey were attracted towards the right half of the channel when the turbine was running, as only n = 88/4190 (2.1% of total attempts) detections were made at the two left-bank control antennas in the turbine-on condition, compared to 2775/13,029 (21.3%) at the same two antennas when the turbine was off. While improved passage efficiency was achieved using surface-mounted studded tiles, further in situ evaluations are needed to optimize their performance.