Rehabilitating Elwood Canal and Elster Creek, Victoria:
 
An Assessment of Impacts on Native Freshwater Fishes
Continued
   
 
Discussion
Impacts on Aquatic Fauna
Numerous factors impacting on environmental condition, and more specifically aquatic fauna assemblages, were identified within the study area. In general these disturbances relate to the entire catchment although some site-specific disturbances are also present. The disturbances impacting on the freshwater aquatic fauna in Elster Creek and Elwood Canal are a result of high-density urban development within the catchment. Several processes were identified potentially impacting on aquatic fauna within the catchment, including:

-barriers to fish migration;
-introduction of exotic species;
-stream channelisation;
-clearing of riparian vegetation; and
-altered flow regimes.
Each of these disturbances is discussed in more detail below.
 
Barriers to fish migration
In the coastal drainages of south-eastern Australia up to 70% of native freshwater fish species may migrate between fresh and estuarine/marine waters at some stage during their life cycle (Harris, 1984). A similar proportion of migratory species is expected for the fish assemblage in the Elster Creek Catchment (see Table 2). In Victoria however, nearly 2200 artificial structures in rivers and streams have been documented as potentially restricting fish movement (McGuckin and Bennett 1999). These structures include barriers such as stream gauging stations, on-stream dams and weirs, fords and culverts.
Unimpeded passage of fish throughout streams is crucial for spawning migrations, recolonisations, general movement and habitat selection (Koehn and O'Connor 1990) and is therefore considered important in sustaining biodiversity in stream fish assemblages. Fish movement is protected by Victoria legislation through a number of Acts (Water Act 1999, Fisheries Act 1995, Flora and Fauna Guarantee Act 1988, Conservation, Forests and Lands 1987) and "…the prevention of passage of aquatic biota as a result of instream structures…" is identified as a potentially threatening process (Flora and Fauna Guarantee Act 1988).
Numerous barriers (fords, culverts and drop structures) which likely restrict movement (migrations and localised movements) of fish were identified in the lower reaches of the Elster Creek Catchment. These barriers included drop structures and culverts under most road crossing throughout the catchment and a syphon mechanism within the Elsternwick Golf Course. Barriers to fish passage represent a substantial risk to migratory fish species and unimpeded passage for native fish should be considered in any rehabilitation plan for the catchment. Species that are expected to occur in the Elster Creek catchment that may be impeded by barriers include the endangered australian grayling, which need to migrate to and from the sea to complete its life cycle (McDowall 1996).
Introduction of exotic species
The effects of exotic species on native fish populations may include direct predation, competition for food and habitat, the spread of disease, and habitat modification. Although only Gambusia were collected in this study, additional species which may already be present in the system or that may potentially be introduced include yellow-fin goby, goldfish, carp and weatherloach. The future risk of introduction of exotic species and their impact on populations of native fish species should be considered in any rehabilitation program.
If carp or goldfish become present in the Elster Creek Catchment, they should be removed as they can destroy aquatic vegetation (Specziar et al. 1997) and compete for native fish habitat particularly that utilised by Southern Pygmy Perch (Papas et al. 2000) and Yarra Pygmy Perch. Carp spawn at lower temperatures than gudgeons, and may exclude them from their preferred spawning areas in vegetated habitats (Koehn et al. 2000). Carp have been shown to have a dietary overlap with Australian smelt and this together with their ability to spawn at lower temperatures, may give them an advantage over other species by giving their larvae and juveniles access to food earlier than other species which spawn later (Roberts and Ebner, 1999; Koehn et al. 2000).
Changes to stream channels
Stream modification works (e.g. piping, concrete/bluestone/earthen channels) have extensively altered the natural drainage network within the catchment. The most obvious effect of these disturbances is a reduction in the diversity and availability of instream physical habitat components (e.g. rocks, logs and aquatic plants). Other effects may include changes to hydraulic characteristics (e.g. increased homogenous flow, rapid rise and fall of water surface levels) and creation of barriers to fish movement. These changes may impact upon fish communities present within the catchment through a lack of spawning sites and food resources, exposure to predators, standing or displacement of biots due to rapid changes in water surface level and the prevention of localised and large-scale movements required for habitat selection, spawning and recolonisations.
Altered flow regimes
Alteration to the natural flow regimes of rivers and streams is considered a major threat to the health of waterways, aquatic flora and fauna communities and the maintenance of essential (instream) ecosystem processes (SAC 1992). For example, alterations to key components of the natural flow regime including flow magnitude, seasonality and variability are regarded as key factors responsible for the decline in distribution and abundance of many native freshwater fish species throughout Victoria (Koehn and O'Connor 1990). Alteration to natural flow regimes has been identified as a potentially threatening process under the Victorian Flora and Fauna Guarantee Act 1988 (SAC 1992) and has been nominated as a key threatening process under the Environment Protection and Biodiversity Conservation Act 1999.
Natural flow regimes may be altered through diversion of water for irrigation or stock and domestic use, catchment dams and on onstream storages, regulated releases from major headworks and catchment imperviousness in urbanised catchments or subcatchments, and stream channel modification.
Catchment imperviousness and stream channelisation are likely to affect natural flow regimes in the Elster Creek catchment through changes in catchment runnoff characteristics and conveyance of high flow events. Rapid rises and falls in water level and stream discharge are characteristic of these reaches and may physically displace aquatic fauna and habitat (e.g. large woody debris) and conversely, result in some biota becoming stranded when water levels recede rapidly. These affects are exacerbated in areas where the stream channel has been modified to reduce. These channelised sections are designed to convey flood waters downstream and as a result areas of low water velocity in which aquatic biota can take refuge are rare. These stream modifications have also resulted in a reduction (possibly elimination) in the frequency of flood plain inundation. Flood plain habitats and off channel wetlands are important habitats for some species (pygmy perch, dwarf galaxias) and the inundation of floodplains is thought to provide a rich source of nutrients for instream ecological processes.
Although the degree of alteration to the natural flow regime was not investigated in this study, alteration the natural flows should be considered in any rehabilitation plan for Elster Creek catchment
 
Degradation of riparian vegetation
Riparian (streamside) vegetation provides important links between the aquatic and terrestrial environments. For example, streamside vegetation acts as a buffer zone for the instream environment and filters sediment input as well as other diffuse source pollutants e.g. nutrients, pesticides etc. Streamside vegetation also contributes to instream habitat (leaf litter, stick fall) and provides an important source of terrestrial invertebrate prey for aquatic biota. Much of the streamside vegetation in the Elster Creek catchment has been removed and critical ecological process (see above) reliant on it have been disrupted. Intact and healthy riparian vegetation is critical for instream biota and should be considered in any rehabilitation plan for the catchment.
Summary
Although the conclusions of this study are limited by the lack of temporal replication in survey effort, the diversity of freshwater fish species recorded in Elster Creek and Elwood Canal during the two surveys described here is low compared with the species diversity expected in the catchment under natural conditions. We recorded six species of native fish compared with 13 species which are expected to occur in the catchment under natural conditions. Numerous factors impacting on environmental condition, and more specifically aquatic fauna assemblages, were identified within the study area. These include: barriers to fish migration; introduction of exotic species; stream channelisation; clearing of riparian vegetation; and altered flow regimes. We recommended that these issues be considered as priorities for any future plans to rehabilitate Elster Creek and Elwood Canal for native freshwater fish species.
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