Fish habitat and protection in the Barwon-Darling and Paroo Rivers
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Summary
Following European settlement, native fish species in many south-eastern Australia rivers have undergone reductions in both distribution and abundance, introduced species such as carp and gambusia have become widespread and habitat degradation has been identified as one of the main contributing factors.At present, there is no set of principles for the assessment of fish habitats in the large dryland rivers of the Murray-Darling Basin, despite that fact that they account for 83% of all lowland rivers in Australia. The current study developed a framework that could be used to assess fish habitat in such rivers and then applied that framework to determine habitat associations in the Barwon-Darling and Paroo Rivers and to report on the condition of those habitats and the fish assemblages in them.
The proposed framework takes into account the very large size and spatial variability of large dryland rivers. It uses a hierarchy which spans scales from large habitat units to catchments and recognises that a river system can be broken down into discrete sections called functional process zones (FPZ) based on its flow and sediment characteristics. Downstream gradients of habitat change were evident between the 2 rivers, along the length of each river and between FPZs.
The Paroo River is a largely unmodified river with less wet channel than the Barwon-Darling. As the amount of wet channel increases downstream in the Paroo River, the habitat of this river changes to more closely resemble that of the lower Barwon-Darling. The Paroo River has low gradient smooth banks and numerous mid-channel bars, whereas the Barwon-Darling River has benches and vegetated point bars along its generally steep, incised banks. The riparian zone in the upper reaches of the Paroo River has medium density trees which are sometimes clumped and overhang the channel but the lower reaches have only scrubby riparian vegetation. The upper Barwon-Darling is characterised by tall riparian trees of medium density while the lower reaches vary between sparsely vegetated sections and sections forested with tall trees. Structural woody habitat within the Paroo River includes medium to small items of varying complexity but the Barwon-Darling generally has much larger structures along with abundant small branches. Within each FPZ of both rivers there was a consistent downstream reduction in structural woody habitat and smooth bank, and a downstream increase in water depth and the amount of matted bank.
An electrofishing survey of the six discrete habitat units identified in the framework (large wood structures, smooth banks, irregular banks, banks matted with tree roots, mid-channel areas and deep pools) was done in the Barwon-Darling River (there was insufficient water in the Paroo to repeat the fish survey there). The fish assemblage varied significantly between habitat types and at the larger river scale. Golden perch, Murray cod, and carp were strongly associated with large wood, but golden perch and Murray cod exhibited higher habitat specificity than carp. Bony herring were more commonly found in shallow edgewater patches. Along the river, a regional difference in the fish assemblage was found at scales corresponding to FPZs. These regional differences involved changes in the relative abundance of species rather than the addition or replacement of species. The Collarenebri to Brewarrina zone was dominated by carp with few native species compared to other zones. However, the Brewarrina to Bourke zone contained a higher abundance of golden perch, Murray cod and bony herring.
Although strong associations were consistently found between the number of fish and the amount of structural woody habitat, the availability of this habitat type was not a good predictor of differences in fish assemblages along the length of the Barwon-Darling River. It is likely that a combination of habitat types and the unobstructed passage of fish along the river corridor may be important in structuring the fish assemblage at the river scale. Therefore, decisions regarding the reintroduction of suitable fish habitat (e.g. snags) cannot be made in isolation from processes that affect the fish assemblage at larger scales such as barriers to fish migration, river regulation and landforms.
The findings of the study were used to propose an action plan to protect the fish community of the Barwon-Darling River and to facilitate its recovery. One potential way of gaining community support for habitat rehabilitation in the Barwon-Darling River, and thus maximising the chance of success, is through the establishment of a model demonstration reach. The main stretch of river channel from Brewarrina to Bourke was identified as the best candidate for such a demonstration reach. If this was done in a scientific way but also utilising adaptive management, agency collaboration and strong community engagement, it would provide a good example of how river rehabilitation can be achieved in a semi-arid zone. Along with other demonstration reaches being implemented throughout the Murray-Darling Basin, it would give a truly basin-wide approach to restoring fish communities in inland NSW.