Evaluation of horizontally and vertically aligned bar racks for guiding downstream moving juvenile chub (Squalius cephalus) and barbel (Barbus barbus)

Abstract Bar racks are commonly installed to divert fish away from water intakes, such as those at hydropower stations or other abstraction points. In temperate regions their effectiveness has predominantly been assessed in relation to diadromous species, such as salmon and eel. This study compared the efficacy of horizontally and vertically oriented racks (5 mm diameter and 10 mm spacing) to guide downstream moving groups of five chub (Squalius cephalus) and barbel (Barbus barbus) to a bypass channel in a recirculating flume under two discharge regimes, and with the rack angled at either 30° or 45° to the direction of flow. Regardless of treatment, the bulk flow predominantly passed through the bars resulting in a lack of a well-established sweeping flow across the rack face, and consequently many instances of entrainment and impingement occurred. Total guidance (the number of bypass entries expressed as a percentage of the total number of approaches) was low and comparable between species with means of 21.3% and 24.8% for chub and barbel, respectively. Bar orientation had limited influence on all metrics, with the exception of the number of guidance events which was higher for the vertical treatment. Interspecific differences in the number of entrainments and guidance events and the exhibition of fine-scale avoidance behaviours were apparent, being higher for chub. In conclusion, the racks used here were not suitable for guiding juvenile cyprinids under conditions similar to those tested. Accounting for interspecific differences and reducing avoidance behaviour are important factors that should be considered in advancing guidance screens for multiple species.