Key words: Fish passage, fish migration, fisway, prioritisations, northern Murray-Darling Basin
Threats and Impacts: Barriers to migration have been identified as a major contributor to the decline of native fish species within the Murray-Darling Basin. The Murray-Darling Basin Authority have made significant investment in improving fish passage along the Murray River and associated anabranches through the Lake Hume to the Sea program and the Living Murray Initiative. Despite the improvements along the Murray River, this investment has not been matched in the Northern Murray-Darling Basin. At present, the movement of fish within and between river systems north of Menindee Lakes remains significantly restricted by dams and weirs without adequate fish passage.
Broad aim and methods: This project set out to develop concept designs and engineering costings for the highest priority weirs in the Northern Murray-Darling Basin.
A review of literature was undertaken initially to assess the likely composition and migratory requirements of the fish fauna in the Northern Basin. An analysis of available options for fish passage was undertaken, and justification provided for the preferred options in terms of the ecological, hydraulic and technical design constraints associated with each weir.
Assessment of structures within the northern Basin identified 12 priority sites within four sub-catchments. These structures were identified as priority sites due to their impact on migrating fish fauna, their potential benefit-cost ratio, and the river length that would be reinstated should the fish passage be provided at the site.
Of the 12 priority weir sites identified, five were investigated for feasibility of fishway installation and identification of fishway designs that would be directly applicable to five of the other sites plus generic types of weir (e.g. sheet pile with rock-fill face) in the northern Basin.
Of the two remaining sites, one has existing detailed design and cost estimates (Bourke Weir), while the second (Chinchilla Weir and gauge) requires further investigations – the costs of which were not possible within the budget for this project.
Fishway concept designs were developed at key representative sites which were specifically designed to suit the fish assemblage and semi-arid ecology of the northern Basin. Designs considered constructability, materials, regional context, maintenance and ownership, and allowed the development of cost estimates, with contingencies, to enable the financial and practical scope of the project to be assessed.
Figure 1 – Priority structures identified in the Northern Murray-Darling Basin
Figure 2 – Cunnamulla Weir, one of the priority sites identified for fish passage remediation in the northern basin. Photo courtesy of Scott Nichols
Findings: The river reaches where the weirs are located were noted to have high ecological value, with known native fish populations, high quality fish habitat, and long river reaches that would be reinstated for migration, either because of few nearby barriers or because of nearby weirs with fishways.
Spanning river systems in both NSW and Queensland, 12 high priority sites were identified, together with concept designs and investment costs to fix the top five barriers to fish passage. These weirs were chosen because of their anticipated high benefit/cost ratio.
The project identified that there are two feasible approaches to rehabilitating fish passage in the northern Basin:
- provide fish passage at the top 11 priority structures to reinstate 2,086 km of river channel. The total cost was estimated at $14.56 m.
- provide a strategic, holistic, program re-establishing broad-scale river connectivity of over 3,242 km. The total cost was estimated to be approximately $70 m.
The key features that make a fish passage program feasible in this area are:
- the main-stem barriers are not numerous (42 for a broad-scale program reinstating over 3,200 km of river).
- most of the barriers are low-level weirs between 1.5 m and 4.5 m high, with the exception of only eight structures.
- most of the sites are relatively easy to work with.
Lessons learned and future directions: This project has provided a clear direction for strategic investment to deliver substantial improvements in fish passage connectivity, reducing fragmentation of fish populations in the northern Murray-Darling Basin. Fishway concepts were specifically designed to suit the fish assemblage and semi-arid ecology of the northern Basin and considered the feasibility of construction, materials, regional context, maintenance and ownership.
Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy.
Contact: Scott Nichols, Fisheries New South Wales, (02) 66261396, firstname.lastname@example.org, 1243 Bruxner Highway, Wollongbar, NSW 2477