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Risk of inter-basin water transfer to the aquatic fauna of the Denmark and Hay Rivers. Report to the Water Corporation of Western Australia.

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Abstract

This report describes the fishes and freshwater crayfishes of the Denmark and Hay River catchments and provides a risk assessment to these fauna of the proposed extraction of water from the Mitchell River (a tributary of the Hay River) and subsequent transfer to the Quickup River Dam (within the Denmark River catchment). Three sites for the pumpback facility (and dam) are under investigation, one in the vicinity of the present gauging station site (Site 1), one just upstream of the Denmark - Mt Barker Rd (Site 2) and one just upstream of the confluence with the Hay River (Site 3) (see Plate 1). The project aims to divert peak flows from the Mitchell River and pump the water to Quickup River Dam (opened in 1990) in the adjacent Denmark River catchment. The size and shape of the interception structure is yet to be determined, but early estimates for the scheme are based on a concrete weir <5 m high to produce a reservoir of up to 20 ML capacity. A total of 22 sites in these catchments were sampled for fish and freshwater crayfishes. These data were collated with additional historical information on the fishes and crayfishes of the rivers. Marron, Gilgies and Koonacs were found in both catchments, while the Restricted Gilgie was only recorded within the Hay River catchment (predominantly within the Mitchell River). Seven species of fish were recorded during the current study, including five strictly freshwater species that are endemic to the south-west region of Australia (i.e. Western Minnow, Western Mud Minnow, Western Pygmy Perch, Balston’s Pygmy Perch and Nightfish), one introduced freshwater species (Eastern Mosquitofish), and one typically estuarine species (i.e. Swan River Goby). Additionally, Morgan et al. (1998) recorded a second estuarine species from the Hay River and the larval Pouched Lamprey from the Denmark River catchment. Both Balston’s Pygmy Perch and Western Mud Minnow are listed as Schedule 1 under the Wildlife Protection Act 1950 and Balston’s Pygmy Perch is listed as vulnerable under the Environment Protection and Biodiversity Conservation Act 1999. Potential impacts to the ecology of the system will depend largely on the selection of the site to be regulated on the Mitchell River. Threats to fish and freshwater crayfish include altered flow and environmental regimes, introduction of parasites, impact on spawning migrations and transfer of potentially genetically discrete populations between catchments. The site that poses the least threat to maintaining the quality of freshwater fish and crayfish populations is Site 1, which is already regulated by the Mitchell River gauging station. The highest threat to the aquatic fauna would be if a weir was constructed near to the mouth of the Mitchell River (Site 3), as this could potentially limit the access to the river for fishes that utilise the Hay River during periods of low flow, then move into the Mitchell River for spawning, and the subsequent offspring utilise it as a nursery area. Fishway construction at any new dam would reduce some of the impacts to fish migrations.
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... Such utilisation of large water supply dams by native freshwater fishes in the region has previously been documented (e.g. Beatty et al. 2003;Morgan et al. 2008). ...
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