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Synopsis of the studies on the ecology of fishes in the Blackwood River and the importance of groundwater intrusion

Beatty, S.J. and Morgan, D.L. (2009) Synopsis of the studies on the ecology of fishes in the Blackwood River and the importance of groundwater intrusion. Murdoch University. Centre for Fish & Fisheries Research, Western Australia.

Abstract

The collation of recent studies in this report represents the most comprehensive review freshwater fish studies relating to a specific river in south‐western Australia. The key finding of this group of studies is they have revealed, for the first time, that groundwater appears crucial in maintaining the relictual fish fauna in the South West Coast Drainage Division’s largest river system (by discharge), and second largest river system overall; the Blackwood River. Combined, these data represent an important review of arguably one of the region’s most important river systems, and highlight the value in long term monitoring of a diverse range of aspects relating to the ecology of these fishes.

This ecological monitoring is important in order to predict and quantify the effects of environmental change predicted for this and other rivers of the region. Potential reductions in river flow may occur as a consequence of reduced rainfall, groundwater extraction and climate change. For example, average annual rainfall in the south‐west of Western Australia has declined 10% since 1970, however the nonlinearity between rainfall and stream‐flow has contributed to a >50% reduction in average flows into public water supply dams (State Water Plan 2007). The ranges of predicted average annual rainfall declines are: by 2030 between 3‐22% and 0‐22% for the extreme south‐west (i.e. the location of the Blackwood River) and the remainder of the region, respectively (Suppiah et al. 2007, who summarised the best‐performing 15 models performed for the IPCC 2007 4th Assessment Report). By 2070, models predict a range of annual average rainfall decline to be between 7‐70% and 0‐70% for the extreme south‐west and the remainder of the region, respectively (Suppiah et al. 2007).

Future reduction in groundwater discharge into the main channel of the Blackwood River is predicted due to allocated water extractions from the Yarragadee Aquifer (Strategen 2006) and reduced recharge from lower rainfall expected from climate change in this region (Hughes 2003). Furthermore, due to secondary salinisation associated with wide‐scale clearance of vegetation for semi‐intensive agriculture, only ~44% of flow in the largest 30 rivers in the south‐west of Western Australia is fresh; the remainder being brackish or saline (Mayer et al. 2005). The levels and trends on stream salinity and salt loads are variable by catchment with those rivers with catchments originating in cleared, lower rainfall areas (<1000mm) generally having fresher flow in lower reaches (due to freshwater inputs from forested streams and groundwater) than in their headwaters (Mayer et al. 2005).

Until recently, there was little information available on the freshwater fishes in the Blackwood River, with most fish related research restricted to detailing the estuarine fauna (see Lenanton 1977, Hodgkin 1978, Valesini et al. 1997). Morgan et al. (1998) provided distributional data (40 sites) on the fishes in the lower reaches of the Blackwood River and a major tributary, the Scott River, and Morgan & Gill (2000) demonstrated that for the various fish habitats in the lower south‐west region that there were significant differences in the fish fauna associated with salinised systems, compared to fresh habitats. Prior to this salinisation in the Blackwood River was believed to have resulted in a decline in the State’s largest freshwater crayfish, the Marron (Cherax cainii) (Morrissy 1978), while salinisation in general was identified as becoming a major threat to the ecology of the region’s larger river systems (e.g. Hart et al. 1991, Bunn & Davies 1992, Pen 1997, Morgan & Gill 2000). The Blackwood River system is only one of two in the South West Coast Drainage Division that contains all of the eight endemic fishes of the southwest, and as the recent research demonstrates, some species have undergone range reductions in the river and are now highly restricted in distribution.

Publication Type: Report
Murdoch Affiliation: Centre for Fish and Fisheries Research
Series Name: Murdoch University Report to Department of Water
Publisher: Murdoch University. Centre for Fish & Fisheries Research
Copyright: 2009 Murdoch University. Centre for Fish & Fisheries Research
URI: http://researchrepository.murdoch.edu.au/id/eprint/5985
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