Over the past few years, native fish of the Murray-Darling Basin (MDB) have possibly faced more threats than at any other time. Fortunately, fish scientists and river managers are working together to build a comprehensive knowledge bank about fish life-history requirements that can now be used to design water for the environment strategies for amazing fish outcomes.
Against a backdrop of increasing water scarcity, a vision for water for the environment is to restore essential components of the natural flow regime to protect and recover native fish populations.
The CEWH’s Flow-MER Fish program, along with other current environmental flow studies, is working to uncover ways to deliver water to restore native fish communities. To understand the work being done, lets focus on Australia’s best-known riverine fish, the Murray cod, a large (commonly to 1.2 m long) enigmatic species with high cultural, recreational and conservation values. In recent years, a number of populations have been variably impacted by the cumulative effects of drought, water extraction, anoxic blackwater, loss of baseflows, and fire – culminating in unprecedented fish kills.
To recover these populations, we need a framework to help us design and implement flow regimes that specifically support the life-history of Murray cod. Scientists are currently testing important components of water for the environment to help river managers to design flow regimes which support Murray cod recovery, as well as enabling the transferability of learnings to other rivers.
For Murray cod recovery, four major flow elements have been identified:
- Elevated discharge throughout spring to enhance availability of, and access to, preferred nesting habitats (usually snags and undercut banks in lotic rivers and creeks) as well as enhancing productivity to provide prey resources for both adult and young fish.
- No major changes in water level or velocity during the core spring and early summer nesting and larval period so that fish can complete their nesting phases and larvae can be retained.
- An annual summer baseflow to enable young fish to access productive littoral (shallow edge) habitats for rapid growth, as well as maintaining good water quality conditions.
- An annual winter baseflow to maximise habitat availability, maintain connectivity and reduce density dependence pressures, for example, the number of fish competing for limited resources and the predation of young fish.
Using the fish life-history approach:
This life-history based approach has enabled the CEWH and other environmental water managers to deliver flows for Murray cod in 2020, in many locations throughout the Murray-Darling Basin, including a 400 km reach of the Murray main channel (Yarrawonga to Torrumbarry), ~800 km of the lower Murray River, and a 500 km reach of the lower Darling River. These flow regimes, supported by water for the environment, were designed to enhance ecological outcomes for native fish. A number of smaller flow deliveries were also implemented in the lower Goulburn River, the mid/lower Murrumbidgee, the Gwydir and Lachlan rivers. In some rivers, the Flow-MER fish team have found high numbers of Murray cod larvae present in spring 2020.
The survival of these larvae through to juveniles will be investigated across all target rivers during the 2021 annual Flow-MER fish sampling at seven selected areas throughout the Murray-Darling Basin.
In a few locations, where water for the environment has been used for several years, the Murray cod response, for both wild spawned and stocked fish, has strengthened recreational fisheries – such as in the lower Gunbower Creek. These results have encouraged broad community support since the Murray cod flow regime can usually be applied in rivers without any change in delivery services for other stakeholders.
Monitoring through the Flow-MER Fish theme, along with researchers in other studies have demonstrated solid native fish spawning in spring 2020. Further monitoring in 2021 will enable the survival of these young fish to also be evaluated. Researcher’s will analyse Basin-scale native fish outcomes using a suite of standard (i.e. boat electrofishing and larval nets) and more sophisticated techniques, such as otolith microchemistry and a consolidated dataset of thousands of tagged fish from around the MDB. Under the Flow-MER Program, researchers will also develop sophisticated mathematical fish population models to test various flow management scenarios around the Murray-Darling Basin, to prioritise the optimal flow regimes.
Over the past few years, native fish populations had to endure some dire environmental circumstances, however, we are learning more about how to help our fish, and through Flow-MER we are helping to guide flow management to maximise fish population recovery by implementing permanent flow regimes where possible. Murray cod spawning in response to watering, highlights the successful integration of river management and fish science to help achieve our native fish recovery vision.
