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Monitoring of key freshwater fish indicators of adequacy of baseflow in the Blackwood River

Beatty, S.J. and Morgan, D.L. (2011) Monitoring of key freshwater fish indicators of adequacy of baseflow in the Blackwood River. Freshwater Fish Group and Fish Health Unit, Murdoch University, Perth, Western Australia.

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The freshwater fish fauna of south‐western Australia is highly endemic (~82%) and under immense pressure from past habitat alterations, water quality decline and introduced species. Groundwater is known to help maintain relictual fish fauna in this region, including within the Blackwood River (Beatty et al. 2006, 2009, 2010). Previous research within the secondarily salinised Blackwood has shown that stenohaline species are now largely confined to the downstream forested reaches where fresh groundwater intrusion by the Yarragadee and Leederville aquifers is greatest. The upstream reaches of the catchment are now mostly dominated by typically estuarine (commonly euryhaline) fishes (Morgan et al. 2003; Beatty et al. 2011).

The current study builds on the longest and arguably the most comprehensive monitoring program conducted on freshwater fishes in south‐western Australia. Consisting of 32 sampling events, this study previously identified two species that appeared appropriate as indicators of river connectivity during baseflow in the Blackwood River; a period where the majority of streams mostly dry and when the influence of the fresh groundwater discharge proportionally contributes the greatest amount of discharge into the salinised river. These species were the EPBC listed (Vulnerable) Balston’s Pygmy Perch, which was found to breed in one perennial tributary (Milyeannup Brook) and the Freshwater Cobbler (the largest native freshwater fish in southwestern Australia), which was demonstrated to undergo large, but short term and localised movements through baseflow maintained riffle habitats (Beatty et al. 2009, 2010). These two species since form part of the Ecological Triggers that the Department of Water uses to monitor the adequacy of baseflow discharge in maintaining the ecological integrity of the Blackwood River. Relationships were previously found between the strength of upstream movement of the Freshwater Cobbler through riffle zones and the amount of discharge, and also a significant inverse relationship was found between the amount of baseflow discharge and conductivity within the study sites in the main channel (Beatty et al. 2009, 2010). These relationships are important as the former allowed estimates of baseflow discharge reductions through two riffle/runs that would be predicted to prevent access to those habitats by the Freshwater Cobbler, and the latter could potentially be used to predict at what point that a reduction in fresh groundwater discharge would result in the salinity tolerances of the various native species being exceeded (some of these acute tolerances have recently been determined by Beatty et al. 2011).

Furthermore, monitoring the length of baseflow habitat permanency within the perennial Milyeannup Brook and how this relates to the occupancy of this system by Balston’s Pygmy Perch aims to allow the habitat requirements (particularly pool depth) of this and other species to be determined. Microhabitat utilisation by this species within this system during baseflow conditions previously demonstrated the importance of pool habitats with the Balston’s Pygmy Perch found to only occupy the downstream <1600m of permanent habitat during March. Once the relationship between baseflow groundwater levels, discharge, stream length, are determined, they will be able to be linked to fish habitat usage to determine the groundwater levels required to maintain usage of this refuge system by Balston’s Pygmy Perch and other species.

The current report presents information on the Freshwater Cobbler and Balston’s Pygmy Perch within the major groundwater discharge zone within the Blackwood River system based on the sampling from 2006‐2009 and the additional baseflow monitoring that occurred in March 2010, and March 2011. It assesses whether Freshwater Cobbler continued to access riffle zones upstream and downstream of the major zone of Yarragadee Aquifer discharge and that the upstream extent of the Balston’s Pygmy Perch population is maintained in Milyeannup Brook.

Publication Type: Report
Murdoch Affiliation: Centre for Fish and Fisheries Research
Fish Health Unit
Publisher: Freshwater Fish Group and Fish Health Unit, Murdoch University
Notes: Report to the Department of Water, Government of Western Australia September 2011
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