Fish and Dams/Weirs. In the Murray Darling Basin. Re-snagging Strategies.

Definition. Fish - Any of various mostly cold-blooded aquatic vertebrates usually having scales and breathing through gills. Dams - A barrier constructed to contain the flow of water or to keep out the sea. Weir - A low dam built across a stream to raise its level or divert its flow Murray Cod. Hume Dam. Yarrawonga Weir.

Management Strategies. “The Murray Darling Basin native fish population are currently estimated to be about 10% of their pre- European settlement levels.” The following seven strategic interventions were considered in the development of the conceptual model, as identified in the Project Brief: allocation of environmental flows; habitat restoration; abatement of cold water pollution; provision of fishways and fish passage; establishment of an aquatic reserve system; carp management; and management of other alien fish species that are threatening native fish. The Expert Panel applied the conceptual model to the following four response variables: restoring native fish; restoring listed threatened fish and fish communities; reducing the impact of carp; and reducing the impact of other alien fish species that are threatening native fish.

The negative impacts on fish communities. Threat.Threatening Process. Flow regulation. Loss of water to other uses, critical low flows, loss of flow variation, loss of flow seasonality, loss of low to medium floods, permanent flooding and high water, increased periods no flow. Habitat degradation. Damage to riparian zones, removal of in-stream Habitats, sedimentation. Lowered water quality. Increased nutrients, turbidity, sedimentation, salinity, Artificial changes in water temperature, pesticides, and Other contaminants. Barriers.Impediments to fish passage resulting from the construction and operation of dams, weirs, levees, culverts, etc., and non-physical barriers such as increased velocities, reduced habitats, water quality and thermal pollution. (changes in water temperature.) Alien species.Competition with and/or predation by carp, gambusia, oriental weatherloach, redfin perch, and trout. Exploitation.Recreational and commercial fishing pressure on depleted stocks, illegal fishing. Diseases.Outbreak and spread of EHNV (Epizootic Haematopoietic Necrosis Virus.) and other viruses, diseases and parasites. Translocation and stocking.The loss of genetic integrity and fitness caused by inappropriate translocation and stocking of native species.

The positive impacts of fish communities. There are major strategies to help the declining figures of native fish in the Murray Darling Basin. 250,000 native fish release back into the Basin. The positive outcomes to this strategy is it will help maintain the river’s system’s biodiversity balance. The release of the native fish will help to bolster and sustain populations of native fish. It will also assist in controlling the introduction of the alien fish the carp. The introduction of the Sea to Hume Fishway. The establishment of the Fishways is one factor of the Native Fish Strategy, other factors include protecting and rehabilitating fish habitat and threatened species, controlling alien fish species and managing fish translocation and stocking. The Fishway helps give native fish a way through the river Murray. The Fishway is a positive impact for the Murray Darling Basin as it helps the lifecycle and ecology of these species. The Fishway have certain measures to protect the native fish from prey, they have installed protective grid mesh covering the fish during its time in the Fishway, ensuring it is not hunted as prey by birds.

Dams and Weirs.

How the weirs effect the flow. Location.Natural.Current.Current as % of natural. Albury Yarrawonga Euston S.A border Murray mouth (Now critical – see image below.) Impact of Man on Average Flows. ( ) (GL/YEAR)

The negative impacts of Dams and Weirs. Dams and weirs both have their impacts to the Murray Darling Basins native fish. Impacts include: Dams and Weirs break-off important breeding migrations. They both allow boundless amounts of water extraction for irrigation. The release of extremely cold water, as the result of low level water outlets, which greatly reduces the river’s temperatures to the point where the native fish cannot breed. Native fish larvae also cannot survive, and sometimes native fish can not exist at all. This issue has caused what is known as “Thermal pollution” or “Cold water Pollution.” can affect rivers for up to 200 kilometres downstream of a large dam.

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