Attracting Fish into Tube Fishways: Linking Fish Behaviour with Flow Hydrodynamics

Anthropogenic in-stream barriers, including dams, weirs, barrages, and culverts have significantly contributed to the decline of freshwater fish populations. Although fishways are a viable solution to facilitate fish migration and fish local movement, the effectiveness of fishways is uncertain for multiple fish species and many operate as selective filters. To address this limitation, the UNSW Tube Fishway has been developed to safely lift fish across barriers >2 m in height using a water surge in a closed tube system. The Tube Fishway combines a volitional attraction phase with non-volitional transport by a rigid tube. It utilises unsteady flow in pipes to provide a surge of water, carrying fish at near atmospheric pressure through a tube and across barriers. Hydraulic energy to propel water is provided by the upstream reservoir and an inlet valve located within the inlet pipe which controls the flow through the Tube Fishway. A key component in the successful operation of a Tube Fishway is fish attraction into the enclosed entrance system. As such, a systematic understanding of fish attraction behaviour into the tube entrance is required to help optimise the Tube Fishway performance. In addition, research on attraction of Australian native fish species is essential as minimal information is available, and no data could be found for attraction of fish into pipes. To this aim, 170 experimental trials were conducted with groups of silver perch (Bidyanus bidyanus; N = 175) and Australian bass (Percalates novemaculeata; N = 150), one inland species and the other coastal, to identify the scalability of the entrance system of the Tube Fishway as well as preferred attraction flow velocities for both species. Three entrance tubes, with diameters of 0.1, 0.225, and 0.4 m, were tested under attraction velocities between 0 and 0.5 m/s. Both species varied in their performance. Silver perch were best attracted with a velocity of 0.15 m/s and an entrance tube diameter of 0.225 m. Australian bass were less well attracted compared to silver perch, showing no clear preference for any entrance tube diameter or attraction velocity. Despite the critical importance of attraction, understanding of the hydrodynamics of vertical slot entrances in relation to fish behaviour remains poor. To this goal, fish behaviour of silver perch (N = 175) and Australian bass (N = 150) were tracked in relation to hydrodynamic measures of three-dimensional velocity and turbulent kinetic energy (TKE) downstream of the slotted entrance tube that were measured with an Acoustic Doppler Velocimeter (ADV). The plain slotted entrance design produced a more symmetric flow in the centre, causing most of silver perch to approach the entrance by skirting the core of the attraction jet flow and areas of high turbulence. Australian bass used random swimming trajectories in plain slotted entrance. In contrast, the streamlined slotted entrance design resulted in an asymmetric attraction flow that guided most silver perch and Australian bass along the wall of the flume to the entrance. Both species preferred areas of low turbulence (TKE < 0.02 m2/s2) and asymmetric attraction flow along one of the sidewalls created by the streamlined entrance improved the fish attraction for silver perch. Despite the potential influence of entrance design modifications on both flow hydrodynamics and fish attraction, there has been limited research conducted on the effect of different entrance geometries on fish attraction. Considering this gap in knowledge, 66 experimental trials were conducted with groups of silver perch (N = 60) and Australian bass (N = 60) to investigate fish response to a series of different entrance geometries of the fishway, accompanied with their preferred hydraulic parameters. Four entrance geometries were tested comprising fully open, slotted, semi-circle and quarter circle entrances. For fully open and slotted entrance geometries, the attraction flows represented a straight jet that dispersed centrally downstream. However, velocity measurements for the semi-circle and quarter-circle entrances exhibited asymmetric attraction flows, angled towards the side wall next to the entrance opening. The swimming trajectories for silver perch and Australian bass varied with different entrance geometries. In the case of the semi-circle entrance, most silver perch preferred to use the jet sidewall. However, for the quarter circle entrance, silver perch displayed a preference for swimming along the middle of the channel and along the non-jet sidewall. Nevertheless, Australian bass trajectories were observed on both sidewalls for the semi-circle entrance. TKE < 0.16 m2/s2 was recommended for attracting juvenile silver perch and Australian bass. Varying attraction velocities and entrance geometries did not explain the maximum number of entered silver perch to the entrance tube although Australian bass outperformed in the no flow control experiments. The findings of this research could have broad applicability for Tube Fishways and other fishway designs, such as vertical slot/pool type fishways. However, further research is required to evaluate the attraction effectiveness for different fish species, fish sizes, and age groups