Efficacy of a side-mounted vertically oriented bristle pass for improving upstream

17 Globally, populations of diadromous anguilliform morphotype fish, such as eel and 18 lamprey, have experienced substantial declines, partly as a result of habitat 19 fragmentation caused by river infrastructure. In the UK, a new configuration of 20 hydraulically unobtrusive bristle pass (side-mounted and vertically oriented) has been 21 developed to help upstream moving European eel (Anguilla anguilla) negotiate gauging 22 weirs. The efficacy of vertically oriented bristle passes remains untested, despite their 23 potential as a low-cost low-maintenance solution to improve habitat connectivity at low24 head structural barriers worldwide. This study assessed the ability of small (82 – 320 25 mm) and large (322 – 660 mm) European eel and adult (291 – 401 mm) river lamprey 26 (Lampetra fluviatilis) to pass upstream over an experimental Crump weir installed in a 27 large open-channel flume with (treatment) and without (control) side-mounted vertically 28 oriented bristle passes under three different hydraulic regimes. Both species were highly 29 motivated to explore their surroundings and move upstream during the trials. Under 30 flooded control conditions, passage efficiency (the total number of times fish passed the 31 structure as a percentage of total attempts) and passage success (the number of fish that 32 passed the structure as a percentage of those that attempted) were high, delay was short, 33 and number of failed attempts before passage was low for both species. When 34 difference in head was at its greatest (230 mm) and velocity and its variation 35 downstream were high (maximum u and σ: 2.43 ms and 0.66 ms, respectively), the 36 upstream movement of small eel and lamprey was blocked, and passage efficiency and 37 success for large eel low (4.6% and 17.2%, respectively). For large eel that successfully 38 passed, delay was long, and number of failed attempts before upstream passage was 39 high. When bristle passes were installed, passage efficiency for small (91.5%) and large 40 eel (56.7%), and passage success for large eel (76.5%) and lamprey (36.7%) was higher, 41 while delay and the number of attempts before passage was lower for both species. 42 Bristle passes helped European eel and river lamprey pass a small experimental Crump 43 weir, although interspecific variation in efficacy was evident. 44