The Downstream Mortality of Native Fish Project (brochure)
Baumgartner L (2007) The Downstream Mortality of Native Fish Project. Brochure prepared by the NSW Department of Primary Industries (Brochure No. 8161) as part of a project funded by the Murray Darling Basin Commission (Project No. MD744). 4 pp.
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Summary
There are two types of weirs constructed in NSW, undershot and overshot. Undershot weirs discharge water beneath a series of steel gates and are associated with strong turbulence and high water velocities. Fish that pass through undershot weirs are often injured by either physically striking the structure or as a result of stress associated with the turbulent conditions. Overshot weirs release water over the top of a crest, creating a plunging effect. Fish choosing to pass through overshot weirs must swim over the crest and then fall downstream to continue their migrations. This may result in injuries or death, especially at very high structures. There is little information regarding the nature and extent of potential impacts, making it difficult for fisheries managers and water supply authorities to improve weir operations.
In 2005, NSW DPI conducted some small-scale experiments using Murray cod and golden perch larvae to determine any harmful effects caused by a low-level weir (1 m high). The study showed that 95% of Golden perch and 52% of Murray cod died whilst passing through undershot weirs1. In contrast, mortality arising from passage through overshot weirs was less than 1.5% for both species. This preliminary experiment clearly showed that the larvae of some native fish can be killed or injured by undershot weirs. In contrast, construction of overshot weirs would minimise potential impacts on native fish. The initial work was limited because experiments were undertaken at a very small structure and only provided data on two species.
To obtain results that are more widely applicable to the management of other structures throughout the Murray-Darling Basin, NSW DPI and the Murray Darling Basin Commission have recently commenced a more comprehensive, 3-year research project. The project, being undertaken at the Narrandera Fisheries Centre, will assess downstream movements of native fish through both types of weir but under a wider range of flows and river conditions. Specific aspects of weir operation, such as discharge and tailwater depth, will be manipulated to determine any adverse effects on fish. Researchers will do trials with more species than were used in the initial experiment to develop weir operating protocols that minimise the risk of injury or mortality. Additional species examined will include Australian smelt, bony herring, flyspecked hardyhead and several gudgeons.
This brochure provides the background to the project and explains the research procedures to be used. It is targeted for distribution to construction authorities, water managers and interested community members.