The vertical slot fishways built for locks 7, 8, 9 & 10 on the Murray River are world-class in functionality, passing a diversity of migratory fish species and a wide size range. However, the cost of constructing these fishways creates difficulties for their  broad-scale application across the thousands of other barriers in the Basin. A major factor in the construction cost is the gradient (slope) of the fishway channel, which determines the total length of the fishway.

This project tested two methods of increasing slope and improving fish passage by reducing turbulence within the fishway pools. The two methods used were:

1. improving energy dissipation with wall roughness and
2. reducing fishway discharge using middle sills placed in each vertical-slot baffle.  This technique focused on improving the passage of small-bodied fish (< 60 mm long) as they have poor swimming ability and are often the biologically limiting factor in increasing the gradient of fishways.

Findings:

The experiments yielded four major findings that have the potential to reduce cost and improve performance of fishways throughout the Basin:

1. The baffle in the vertical-slot design has significant potential to improve fish passage.
2. Turbulence in the fishway pool may be more limiting for the passage of small fish than the high water velocity in the slot of the vertical-slot baffle.
3. The sharp-edged narrow-slot appeared to provide improved velocity distribution.
4. The number of small migrating fish was much greater than expected.

Key Messages:

The results show that refining the vertical slot design can improve fish passage performance, but further research is needed before design changes can be applied with confidence to new fishways.  Significantly, the design approach with the most potential to reduce cost is to separate the function of passing small fish and large fish, as well as allowing for differential passage in high and low flows as has already been applied with cost savings to the Lock 10 fishway.

The passage of Carp gudgeons (Hypseleotris spp.), which are very poor in ascending fishways, is an indication that a range of small-bodied fishes can ascend modified fishways, including small threatened species such as Olive perchlet (Ambassis agassizii) and Murray hardyhead (Craterocephalus fluviatilis).  Restoring the ecological processes of dispersal and recolonisation, through effective fishways, will be an important part of recovering the populations of these species.

Martin Mallen-Cooper, Brenton Zampatti, Ivor Stuart, and Lee Baumgartner (2008). Innovative Fishways – Manipulating turbulence in the vertical slot design to improve performance and reduce cost. Unpublished final report to Murray-Darling Basin Commission.

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