Upstream passage and attempt behaviour at a sloping weir by migrating adult river lamprey: are studded tiles effective in improving longitudinal connectivity?

The performance of weir-mounted studded tiles for passing upstream-migrating lamprey (Lampetra fluviatilis) was compared with unmodified parts of a Crump flow-gauging weir and with use of a bottom-baffle fishway on the River Derwent, Northeast England. Equidistantly-studded tiles were fixed horizontally on the weir face near the right bank, forming a 1m-wide treatment lane, neighboured by a tileless control lane. A bottom-baffle fishway was present at the right bank, alongside a hydropower tailrace. Two further left-bank controls enabled, together with right-side controls, comparison of lamprey attraction relative to the dominant flow on the right side. Downstream and upstream ends of the right-hand weir-face lanes and of the fishway, downstream ends of the left-hand weir face lanes, and the entrance of the hydropower tailrace area were instrumented with PIT antennas (n=9 total). Of 395 PIT-tagged lamprey, over 10 release sessions in early winter 2017 (turbines on for 21/43 days), 363 (91.9%) were detected at any of the antennas (mean ± SD minimum delay: 14.8 ± 8.9 days). All lamprey detected at the left-bank antennas (attraction efficiency AE: 255/395 (64.6%)) were detected elsewhere also. The fishway was ineffective (AE: 343/395 (86.8%); passage efficiency PE: 5/343 (1.5%)). However, overall passage using the studded-tile corridor doubled (44/395 cf22/395) relative to the adjacent bare weir-face route (AE tiled lane: 172/395 (43.5%); PE tiled lane: 44/172 (25.6%) - AE control lane: 257/395 (65.1%); PE control lane: 22/257 (8.6%)). Fewer PIT detections were logged at the turbine tailrace and fishway entrance, respectively, when turbines were on (n=441 and n=700; median [range] river discharge turbines-on: 18.7 [10.5-36.3] m3 s-1) compared to turbines-off conditions (n=1005 and n=2457; discharge: 36.2 [10.4-52.3] m3 s-1). While improved passage efficiency was achieved using surface-mounted studded tiles, further in situ evaluations are needed to optimize their performance