Sharing the Catchments of Southern Sydney: Biodiversity of the Georges River Catchment
The significance of biodiversity, and the need for planning to deal with it, is recognised in a suite of international, national and NSW policies and statutes with the aim of ensuring that floral and faunal resources are managed in a sustainable way.
Within the catchment of the Georges River, there is no doubt that the development of land to accommodate population growth to the south and south-west of Sydney has modified biodiversity to some degree. Catchment clearing, freshwater impoundment, discharge of sewage and sand mining are some of the many practices that have contributed to the alteration of habitats and the reduction of water quality and, in turn, have led to changes in plant and animal communities. The impacts arising from these changes have never been identified or quantified. Relative to the pre-colonial era, some of the freshwater reaches of the catchment have been highly modified. For example, the Liverpool Weir, built in 1836, fixed the upper boundary of the estuary, modified the salinity regime in the mid-reach of the river, altered tidal flow and inhibited the passage of migrating fish. Other freshwater features remain in a relatively undisturbed condition due to a variety of factors, including reservation of land for conservation and military purposes. Below the weir, the estuarine ecosystem can be broken into four geomorphic zones, each zone defined in terms of sediment type, depth and geomorphic setting. Some land along the estuarine portion of the catchment has also been reserved.
A short history of commercial and recreational fishing in the catchment of the Georges River is provided in this volume and the complete closure from 1 June 2002 of commercial fishing within the whole of the Georges River and Botany Bay system is indicated.
Maps of the distribution of littoral rainforest, paperbark, swamp she-oak, reed, saltmarsh, mangrove and seagrass have been produced for the estuarine portion of the catchment. However, there are no baseline data on the submerged vegetation above the Liverpool Weir. The littoral rainforest and saltmarsh are limited in extent with the total area of the latter apparently in decline, a phenomenon seen along the whole of southeast Australia. Threes species of saltmarsh are considered to be of special significance due to their limited distribution along the central portion of the estuary and the possibility that they may be at risk from expanding reed and mangrove, and certainly at risk from some land-use and recreational practices. Seagrass is also of conservation significance due to its role in providing habitat for fish. Following disruptions due to storms and increased wave energy brought about by dredging at the mouth of the Botany Bay, remaining seagrass patches in the bay appear to be establishing a new equilibrium with Zostera filling in bare areas once colonised by Posidonia. In the river, Zostera was located at a number of sites where it had not previously been mapped, but the history and conditions of seagrass at these sites needs to be further assessed. The condition of Posidonia in Kogarah Bay also requires further investigation.
Sampling of the estuarine portion of the river with a small haul net produced sixty-five species of fish, nineteen species of crustaceans and three species of molluscs; numbers equivalent to other NSW estuaries. Approximately half of the individuals caught were of economic importance. In each of the four geomorphic zones, the number of species was greater in seagrass than over bare substratum, providing further evidence that the conservation of seagrass is fundamental to biodiversity management in the catchment. Species such as Port Jackson glassfish, sand whiting, silver biddy, yellow-fin bream and blackfish were found throughout the estuary. Other species, such as some pipefish, goatfish and leatherjackets were more limited in their distribution.
Creeks flowing through urbanised parts of the Georges River catchment contain fewer native species, elevated abundances of introduced alien fish, higher incidences of fish with visible signs of disease, and relatively low numbers of species known to be intolerant of environmental disturbance. It is assumed that some fishes previously prevented from migration by the tidal barrier presented by the Liverpool Weir have found their way upstream via the fishway completed in 1997, but a fuller assessment of the efficacy of the fishway is needed. This present study, one of the first of its kind in an urban NSW waterway, supports the hypothesis that human-population pressure within the catchment has the potential to increase stress on freshwater habitats.
On the positive side, effluent no longer discharges to the river during dry weather, although surcharges occur during weather or as a result of poorly maintained sewerage infrastructure. Whilst some of the poor land use practices of the past have been redressed through improved management regimes, many impacts continue to result from existing activities and new developments.
A number of recommendations are provided in regard to the ambit and flexibility of planning instruments in relation to identification of critical habitat as well as recovery plans for threatened species of fish. As the river receives the end-products of activity in the catchment, and as the catchment continues to undergo extensive modification, a number of monitoring exercises for estuarine and freshwater vegetation and the fish communities of each are warranted.