Report is part of a serious of 5 reports see (Menindee Lakes Ecologically Sustainable Development Project / [compiled and edited by Stephen Moore and Tania Midgley] – ISBN: 0734751710).
Executive summary only available.
Document not available.
The Menindee region represents a significant cultural, economic, and natural resource for the Murray Darling River Basin. The margins of the Menindee Lakes system provide one of the most comprehensive archaeological records of aboriginal habitation in Australia, and the role of the Menindee Lakes system as a water storage facility, benefits numerous communities and irrigation areas in the Murray Darling Basin. The waterways of the region provide habitat for a large variety of aquatic biota while the surrounding area supports a diversity of bird life. Since its completion in the 1960s, the operation of the Menindee Lakes system has involved the regulation of natural flows between the Darling River and each of the systems lakes. This has dramatically altered the water regime of the lakes and has more recently, raised concerns regarding environmental health and sustainability of current management practices. As environmental health is a prerequisite for achieving social, cultural and economic objectives, on a regional and national scale, managers need to incorporate environmental information into their management decision making framework (Scholz et al., 1999). It is therefore essential that the current environmental status of an area be understood. The Menindee Lakes system comprises four major lakes: Menindee, Cawndilla, Pamamaroo, and Wetherell, in order of decreasing capacity. The main control structure, the Menindee Main Weir, impounds the Darling River to form Lake Wetherell, which includes a number of smaller connected lakes (Lakes Malta, Balaka, Bijiji and Tandure). Shoreline erosion has been identified in Lakes Cawndilla, Menindee, Pamamaroo and Tandure. Impoundment of Darling River flows behind the Main Weir has increased the frequency of inflows to Lakes Malta, Balaka, Bijiji and Tandure. This in turn has increased the net influx of organic and inorganic material to these lakes. This is most apparent in Lakes Malta and Balaka, which are shallow and lack effective flushing. Whilst the linking of Lakes Wetherell, Pamamaroo, Menindee and Cawndilla in series facilitates the net sedimentation of suspended inorganic loads as water passes through each lake, relatively greater flushing may also facilitate the net export of suspended organic matter. Improved modelling of nutrient and particulate matter fluxes through the lake system is needed to clarify these issues. Preliminary surveys by the MDFRC indicate that post-regulation habitat diversity is highest and more extensive in Lake Wetherell than elsewhere within the system. Impoundment of the mainstream behind the Main Weir has greatly increased the extent of inundation, creating a large network of open and sheltered backwaters that provide habitat complexity for aquatic organisms when submerged. The permanence of water within Lake Wetherell as a consequence of regulation greatly increases its role within the system as a local refuge for both native and introduced biota when the lakes are dry, and as a source of re-colonisers during lake inundation. In contrast, habitat diversity within the other lakes of the system is low and has been adversely affected by increases in the permanency of water. Increases in the permanency of inundation of the larger lakes has resulted in the confinement of habitat complexity to a relatively narrow riparian fringe. The Menindee Lakes provide habitat for a diverse and productive community of organisms that appear to be well adapted to the unpredictable and highly variable water regime characteristics of the natural system. Evidence for this comes from the relatively stable nature of the fish community in the lakes over the last ten years despite several drying events. The development of the catchment and its associated water resources has brought about changes in the natural community with documented changes in the vegetation and fish community. Changes have also possibly occurred in other groups, but inadequate data is available to determine the extent of this change. Changes in the flow regime, invading species and catchment changes may all have contributed to the decline in the abundances of several native species. One species recorded within the Menindee Lakes system, the Silver Perch (Bidyanus bidyanus), has been accorded conservation status. Silver Perch are primarily a riverine species that undertake extensive breeding migrations. They are the subject of a recovery plan that identifies flow modification, barriers to migration and over-exploitation as the major threatening processes. Two additional species also appear to have undergone significant declines in the Menindee Lakes and appear now to be rare and patchily distributed. The two species are Murray Cod (Maccullochella peelii peelii) and Flyspecked Hardyhead (Craterocephalus stercusmuscarum fulvus). Murray Cod are an icon species whose decline appears to be related to a combination of habitat loss, recruitment failure and over-exploitation. The habitat requirements of the Flyspecked Hardyhead are poorly understood as are the reasons for their decline. Despite the declines in some species, several economically important species remain in the Menindee Lakes system, including Golden Perch (Macquaria ambigua) and yabbies (Cherax destructor). A healthy population of Golden Perch has been recorded in each of the Menindee Lakes with juvenile fish being recorded regularly in the upper lakes and Lake Wetherell. The population dynamics of yabbies are less well studied, but anecdotal evidence suggests healthy populations occur in the lakes after flooding. One of the critical processes underpinning the fish community of the Menindee Lakes system is fish recruitment. Our knowledge of fish breeding requirements is far from complete, however habitat availability and an adequate food supply are both believed to be important. While we cannot yet adequately describe the habitat required by all life stages of all native fish, habitat is created and maintained by flow regime. Within the main channel, flow creates the dynamic mosaic if habitats and changes in flow may lead to the reduction or loss of particular hydraulic habitats either immediately or over time as channel morphology changes. On the floodplain, flood frequency and duration determine the availability of habitat. Flooding and drying create a mosaic of wetlands and patches of both deep and shallow water that may provide feeding or refuge habitat for fish. The flow regime also determines the composition and distribution of vegetation, which may also provide critical habitat for fish at different life stages. Food availability is determined by the productivity of the system. Productivity is determined by the cycling of organic carbon and nutrients through the food web which are also affected by the water regime. Algal production as well as the production and decomposition of floodplain species are affected by water depth and wetting and drying cycles. This plant material then becomes food for invertebrates and small fish that form the bulk of the diet of most native fish. Wetting and drying cycles are also critical to the life cycles of zooplankton. Zooplankton are important because they represent a critical food resource for larval fish, and access to this abundant food supply may be critical to successful fish recruitment. Zooplankton resting eggs remain within dried sediments awaiting the arrival of flood waters. During floods zooplankton hatch out and their populations grow rapidly in the productive flood waters providing an abundant source of food for birds and small fish. Loss of the wetting and drying cycle leads to decline in zooplankton abundance with the associated loss of food for more charismatic fauna. Fish occur in all of the major lakes within the Menindee Lakes project area. The abundance and distribution, however, varies considerably between species. A total of 17 species of fish have been previously recorded from the Menindee Lakes project area, with an additional 7 species expected to occur within the project boundary but as yet have not been recorded. Carp Gudgeons (Hypseleotris spp.), Australian Smelt (Retropinna semoni) and Bony Bream (Nematalosa erebi) historically have been recorded in high abundances, particularly from Lake Pamamaroo. Three species of exotic fish have also been found to occur within the region. Carp (Cyprinus carpio), Eastern Gambusia (Gambusia holbrooki) and Goldfish (Carassius auratus) have all been recorded to have a broad distribution throughout the system. One species that occurs within the project area has a conservation significance in NSW. Silver Perch has been listed as vulnerable. Three species that are expected also have a conservation status, under the ASFB. There is some difficulty in identifying macroinvertebrates to species level, therefore not all of those that have been recorded within the Menindee Lakes project area have been identified to species level. Many macroinvertebrates within this area have only been identified to Family or as broad as Class. A total of 37 different macroinvertebrates have been recorded within the project area. This diversity may however be fewer as the taxonomy of some that have only been identified to family level may overlap with those identified to species level. The diversity includes a variety of decapods, particularly shrimp and yabby, and dipterans, the majority of which are non-biting midges. A high abundance of sponges and microinvertebrates (zooplankton) have also been recorded within the Menindee Lakes project area. No species of macroinvertebrate recorded from the Menindee Lakes project area have a conservation status on either a local, regional, or national level, although this may be a reflection of both the poor taxonomic resolution and the fact that very few invertebrates are considered for listing. Records on the abundance and distribution of aquatic reptiles and amphibians from within the project area are very limited. Three species of reptiles have been recorded in the project area, the broad-shelled river turtle (Chelodina expansa), the eastern long-necked tortoise (Chelodina longicollis) and the Murray turtle (Emydura macquarii). All of these species were recorded in the Great Darling River Anabranch. No other aquatic species are likely to occur within the study area, however many terrestrial reptiles have been recorded within the Menindee Lakes region. Of the species of reptile recorded in the area, none have a listed conservation status, at either the local, State or Federal level. In addition, a total of 14 species of amphibian have been recorded from the project area and one additional species that is expected to occur there has also been included. The majority of these records (11) are from the Great Darling River Anabranch. Only two of the frog species have a known conservation status, and both these are listed as endangered under the Threatened Species Conservation Act, 1995; the Growling Grass Frog (Litoria raniformis) and the Painted Frog (Neobatrachus pictus). The greatest species diversity has previously been recorded from Lake Pamamaroo. Diversity is also high in Lakes Balaka, Bijiji, Wetherell and Tandure. Silver Perch, the only fish species actually collected from the system with a conservation status, has been recorded from all these four lakes. Macroinvertebrate species diversity is also high within Lake Pamamaroo. The distribution of both reptiles and amphibians has been restricted to the Great Darling Anabranch, possibly an artefact of the sampling regime utilised. Of the 13 species of amphibians collected from the Great Darling Anabranch, one species has a conservation status; the Growling Grass Frog. Water is a highly valuable resource within the Murray-Darling Basin and the growing demand for water in the Murray-Darling Basin has lead to an examination of the management of the Menindee Lakes system to see whether water savings can be achieved. Through the assessment of the current management regime of the lakes, a variety of options for future management of the lakes have been developed. These options are summarised below. Options: 1) Lake Cawndilla – Menindee regulator: increases the drying frequency and duration for Lake Cawndilla. The construction of the regulator is likely to have a slight detrimental impact. Detrimental impacts will also extend to the connectivity for aquatic organisms between lakes, the salinity of Lake Cawndilla and the productivity of Lake Menindee. This option potentially provides improvements to the aquatic and fringing habitats. In addition, significant benefits are likely to become evident to the Great Darling Anabranch and to the productivity of Lake Cawndilla. 2) Enlarge Menindee Lake outlet: increases the rate of draw down of Lake Menindee. Similar to Option 1, there would be a slight detrimental impacts during the construction process. However, there are possible benefits to the availability of habitat and subsequently the productivity in this lake. The redesign of the outlet should consider upstream movement of fish 3) Excavate a channel in the lakebed of Menindee Lake: would increase the rate, frequency and extent of drying of Menindee Lake. There would be detrimental impacts associated with the construction of the channel and the quality of water passed in the Darling River. The passing of saline water out of Menindee Lake would, however, be beneficial to this lake. 4) Lake Cawndilla – Menindee regulator plus larger Cawndilla outlet and channel to the Darling River: builds on the previously discussed impacts of Option 1, with subsequent benefits to the water quality of Lake Cawndilla by passing the saline water into the Darling River. This would also benefit the productivity of Lake Cawndilla. 5) Pump Menindee Lake to Lake Pamamaroo: would be a variation to Option 3 with minimal local environmental disturbance. There is associated slight benefit to the salinity of Menindee Lake with detrimental effects on the salinity of Lake Pamamaroo. 6) Lake Cawndilla – Menindee regulator plus pump Lake Cawndilla to Menindee Lake: also builds on Option 1. The main disadvantage for aquatic fauna is the loss of connectivity associated with the presence of the regulator. The loss of poor quality water from Lake Cawndilla would, however be beneficial to this lake but detrimental to Menindee Lake. This option would also be beneficial to the Great Darling Anabranch as it would be relieved of some salt burden. 7) Pipeline the anabranch supply: would reduce the need to store water in Lake Cawndilla and hence increase the duration of dry spells for this lake with associated benefits to the availability of aquatic habitat upon refilling. 8) Install foreshore protection: would have detrimental effects during the works program. The benefit posed by this option is greatly dependent on the extent of foreshore protection and the material used to establish this. 9) Drain the residual pool in Lake Cawndilla: would increase the rate and frequency of drying of Lake Cawndilla. Colonisation by terrestrial vegetation during lakebed drying would increase aquatic habitat on refilling. However, the distribution of saline water would be deleterious to the Great Darling Anabranch. 10) Not evaluated 11) Install regulators on the ephemeral lakes adjoining Lake Wetherell: will alter the frequency, extent and rate of drying of the ephemeral lakes, with substantial detriment to the lakes during the works program. Like other options, this option is likely to increase the abundance of aquatic habitat upon refilling, however the operation of the levees and regulators would reduce the capacity for production of aquatic fauna.