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Compiled by Leon P. Zann
Great Barrier Reef Marine Park Authority, Townsville Queensland
Ocean Rescue 2000 Program
Department of the Environment, Sport and Territories, Canberra, 1995
ISBN 0 642 17399 0
Centre for Coastal Management
Southern Cross University
P.O. Box 157, Lismore, N.S.W. 2480
Historically, Australian estuaries have been the major points of human settlement. With European settlement, they formed a focal point for ocean and river shipping which was soon followed by storage, processing and supply facilities. Adjacent lowlands proved suitable for early agricultural activities while the estuaries themselves were used as efficient waste disposal systems. Ironically, the natural productivity of these estuaries ensured the supplies of fish and shellfish to sustain the very settlements that now threaten that productivity. With increasing populations along Australia's coastline and with a corresponding increase in recreational usage and urban development, all coastal resources are coming under more pressures (Yapp 1986). This pressure however, is particularly evident in estuaries near large urban centres.
Many studies have emphasised the significance of estuaries to the commercial and recreational fishing industries (see review by Hutchings & Saenger 1987, Saenger 1991). Not only are estuaries popular fishing grounds (PA Management Consultants 1985, Neumann & Hundloe 1986) but their seagrass, mangrove, saltmarsh, mud flat and sand bar habitats provide productive feeding, spawning and nursery grounds necessary for the continued viability of fish and invertebrate populations. There is a distinct need for the adoption of sensible management strategies and for the identification of research priorities. These steps will ensure that the economic value of estuaries can be utilised while they maintain conservation values not only for fish but for all plants and animals dependent on the estuarine environment.
Before sound decisions can be made in relation to estuarine management and research, it is necessary to assess the resource and, using a consistent approach, review relevant available knowledge (United States Department of the Interior Fish and Wildlife Service 1970, Field, Alexander & Broutman 1988). Such an estuarine inventory has recently been completed for Australia (Bucher & Saenger 1989, 1991) with the support of the Australian National Parks and Wildlife Service and the Australian Recreational and Sport Fishing Confederation. The following account draws mainly on the information from that inventory which contains records for 783 estuaries and enclosed marine waters around Australia as shown in Table 1. For that inventory, estuaries were defined as semi-enclosed bodies of water and their adjacent wetlands that have input from both marine tidal inundation and terrestrial runoff. Limitations to this applied definition are that (1) the permanent water body must be wide enough to be represented on a 1:100 000 topographic map by double lines for at least 1 km upstream of its seaward entrance, and (2) the estuary must represent the discharge point of a catchment of at least 15 km2. All other terms and categories as used below are also defined in Bucher and Saenger's inventory (1989).
As noted above, Australian estuaries are being increasingly used for a range of activities even though there is high public awareness of the importance estuarine wetlands hold for conservation. Attitudes to estuarine wetlands were surveyed in northern New South Wales during 1986 (unpublished survey by the Centre for Coastal Management based on 423 random interviews). The results showed that over 80% of respondents thought that coastal wetlands were important, many believing that the prime value for the wetlands was in providing habitats for fish and birds. Nearly 40% of the respondents regularly used wetlands for a variety of purposes (fishing, boating and aesthetic appreciation) and nearly 70% felt these systems were threatened by (in order) urban development, pollution, drainage and mining.
All estuarine uses have some resultant impact although clearly, some are less intense than others (Table 2). With future increases in usage, it will be essential that some of these activities are regulated, while others will need to be re-located to non-estuarine areas if estuarine values are to be maintained in terms of their productivity and the services they provide (Saenger 1991).
Australian estuaries occur over a wide range of geological and climatic conditions and consequently display a great variety of form. The largest estuary has an area of 1120 km2 (Victoria River, Northern Territory) and the smallest covers only 0.11 km2 (Merrika River, New South Wales). Enclosed marine waters range in area from 46 km2 for Raffles Bay, Northern Territory to Shark Bay in Western Australia which is the largest at 13 887 km2. Associated catchment areas range from 15 km2 (five unnamed estuaries in Queensland) to the Murray-Darling Basin in South Australia which is 1 062 530 km2.
Table 3 shows the total areas of open water, intertidal mud/sand flats, mangrove, seagrass and saltmarsh areas in the estuaries of each State. Western Australia contains the most open water and mud/sand flat habitat (53% and 46% respectively of the national total), presumably because many areas experience large tidal ranges. Queensland and the Northern Territory support the most mangroves and saltmarsh (a combined total of 78% of Australia's mangroves and 76% of saltmarsh) while Victoria has the most extensive seagrass beds so far recorded (58% of the total, mostly recorded from Western Port and Corner Inlet).
Based on the total mangrove areas (11 617 km2) as determined by Galloway, Story, Cooper & Yapp (1984) it is worth noting that 70.5% of Australian mangrove areas are associated with estuaries and only 29.5% occur on open shorelines. The percentage of estuarine saltmarshes is likely to be smaller than that of mangroves but no data are available.
The supply of fresh water to Australian estuaries is highly variable both in amount and regularity. Rainfall on estuarine catchments range from an arid 242 mL p.a. (northern Spencer Gulf, South Australia) to 4321 mL p.a. (Tully and Hull rivers and Maria Creek, Queensland) (Bureau of Meteorology 1977). Runoff coefficients estimated for each catchment cover almost the entire range of values, ie from 0.01 (Port Davis Creek, South Australia) to 0.96 (Little Henty River, Tasmania). The supply of saline water is equally variable. Around Australia the tidal range varies from 1.0 m at Little River, Victoria, to 12.2 m at eight estuaries in Western Australia (Royal Australian Navy Hydrographic Service 1987). The nature of fresh and salt water supply will affect the type and extent of wetlands associated with an estuary. While the tidal range of subtropical and tropical estuaries correlates positively with the mean wetland area per estuary (ie 571 out of 783), the relative proportions of mangroves and saltmarsh within estuaries show significant relationships with average annual rainfall (positive for mangroves and negative for saltmarshes) (Bucher & Saenger 1994) (Figure 1).
Figure 1: Mean percentage of tropical and subtropical estuaries covered by mangrove and saltmarshes with annual rainfall categories. (Error bars are Stanard Errors)
Other characteristics such as estuarine configuration and geology and low temperatures (particularly in the south) add further to the overall estuarine variability.
Human activity has influenced some estuaries substantially but many remain almost unaffected. Clearance of estuarine catchments is widespread, particularly in central and southern coastal Queensland, New South Wales, Victoria and South Australia (see Table 4). However, more information on catchment clearance is needed especially for Queensland south of the Daintree River, Tasmania and south-western Australia.
Development within a catchment can have flow-on effects on water quality, either by discharge of chemical pollutants, by changes in the silt loads, or temperature of the water. Although adequate information for assessing water quality is available for few estuaries, the frequency of water quality categories in Australian estuaries, based on our current knowledge, is given in Table 5. Even so, these categories are a crude system of ranking as water quality consists of a number of parameters that cannot be combined in a single measure. This means that in most cases a subjective assessment was made and that no quantitative limits can be ascribed to each category. Some source of pollution has been identified in most of the estuaries listed in Table 5 under 'Insufficient information' and it is likely that further information about these estuaries will place them in the 'Fair' or 'Poor' categories rather than the 'Excellent' one.
To determine appropriate management, research and conservation priorities, some method of ranking estuaries in terms of their value is required. The values of estuaries are many but can be summarised under three categories: fishery, conservation and amenities. Briefly, fishery value is determined by the use of an estuary as a fishing ground or its value as fish habitat. The conservation value of an estuary may involve the presence of rare or endangered species or communities, use as a scientific study (or reference) area or educational resource, having representative examples of communities, and so on. Amenities value refers to the provision of shore access, boat ramps, jetties and port facilities. The national distribution of estuaries having fishery, conservation and amenities value are given in Table 6. Many of the value ratings are based on very inadequate information and further input may require some revision. However, estuaries are most likely to be under-rated due to lack of information than over-rated.
Real and potential threats to the fishery or conservation values of each estuary have also been identified (Table 7). Threats include such measures as pollution, catchment clearance, wetland reclamation, engineering works (eg dredging, training walls, marinas, wharves, flood mitigation, dams), boating activity, over-fishing, weed infestations and litter. Aside from boat channel silting and macroalgal blooms (which often pose a threat to fisheries and conservation values) no threats to amenities value have been identified.
Priority areas for protection would be those with high or moderate value that are, or may be, threatened. Table 7 shows the number of threatened estuaries with high conservation or fisheries value.
The overall conditions of Australian estuaries can be defined by several characteristics including the percentage of estuaries with (1) a low degree of catchment clearance, (2) high water quality, (3) high fisheries value, (4) high conservation value, and (5) a low threat to conservation values. The percentage of estuaries falling into each of these categories for each State and Territory and for Australia as a whole is provided in Table 8. The last column in Table 8 shows the extent of scientific data available on each estuary and hence the possible accuracy of the assessments.
Most estuaries in northern Queensland, Western Australia and the Northern Territory have a lot of uncleared catchment area and high to excellent water quality characteristics. On the other hand, estuaries in southern Queensland, New South Wales and Victoria have lower water quality, greater catchment clearance and a high percentage of estuaries faced with real threats to their conservation values. Equally, the most extensive (but possibly still inadequate) literature available is on New South Wales and Victorian estuaries while the least studied are the estuaries of north Queensland, Tasmania, Western Australia and the Northern Territory.
At present 62% of Australia's estuaries carry at least some form of administrative classification that restricts their use and 28.4% can be classified as having some formal conservation status (eg marine park, national park, game reserve, flora/fauna reserve). However, in only a few cases does this status apply to a significant proportion of the estuary and it rarely covers the entire estuary. More importantly, numerous classifications are used throughout Australia and there is little uniformity in nomenclature. As with the nomenclature, the reservation of estuaries in whole or in part, appears to reflect local interests and cannot be said to constitute a national system of representative estuarine reserves. Similarly, the level of surveillance and enforcement of existing estuarine reserves varies dramatically.
The difficulties associated with jurisdictional and administrative fragmentation of estuaries are similar to those of other tidal areas (Saenger 1987). However, given the detailed estuarine database (Bucher & Saenger 1991) available the start on a national network of representative and significant sites to cover the physical, hydrological and biotic variation in estuarine types merely requires coordination between Federal and State governments.
Major gaps in our knowledge of estuarine habitats appear to be in seagrass distribution (except in Queensland, New South Wales and Victoria) and intertidal flats. Similarly, more information on catchment clearance is needed especially for Queensland south of the Daintree River, Tasmania and south-western Australia. Water quality data for most Australian estuaries are inadequate and need to be collected systematically as part of a national monitoring program. Inventories of estuarine biotas are adequate for southern Queensland, New South Wales, Victoria, South Australia and south-western Western Australia while the least studied are the estuaries of north Queensland, Tasmania, north-western Western Australia and the Northern Territory. However, with a few exceptions (eg Hodgkin 1978, Saenger, Stephenson & Moverley 1980, Staples 1980, Moverley, Saenger & Curtis 1986) the biotic dynamism (natural fluctuations, resilience, etc) of Australian estuaries is virtually unknown.
Australia's marine and estuarine areas - a policy for protection. 1986. Aust. Comm., Int. Union Cons. Nat., Nat. Res., Occas. Pap., 1. 30 pp.
Australian Water Resources Council 1976. Review of Australia's water resources, 1975. Canberra: Department of National Resources. 273 pp.
Bucher, D. & Saenger, P. 1989. An inventory of Australian estuaries and enclosed marine waters. 7 volumes, summary report and computer database. Lismore, NSW: Centre for Coastal Management. 1920 pp. Prepared for the Australian Recreational & Sport Fishing Confederation and the Australian National Parks & Wildlife Service.
Bucher, D. & Saenger, P. 1991. An inventory of Australian estuaries and enclosed marine waters: an overview of results. Aust. Geogr. Stud., 29: 370-381.
Bucher, D. & Saenger, P. 1994. A classification of tropical and subtropical Australian estuaries. Aquat. Cons., 4: 1-19.
Bureau of Meteorology 1977. Rainfall statistics, Australia. Canberra: Australian Government Publishing Service. 510 pp.
Field, D., Alexander, C. & Broutman, M. 1988. Toward developing an inventory of U.S. coastal wetlands. Mar. Fish. Rev., 50(1): 40-46.
Galloway, R.W., Story, R., Cooper, R. & Yapp, G.A. 1984. Coastal lands of Australia. Canberra: CSIRO Division of Water and Land Resources, Natural Resources Series No.1. 53 pp.
Hodgkin, E.P. 1978. An environmental study of the Blackwood River estuary, Western Australia, 1974-75. Report No.1 to the Estuarine and Marine Advisory Committee of the Environment Protection Authority, Western Australia. 78 pp.
Hutchings, P. & Saenger, P. 1987. Ecology of mangroves. St Lucia, Queensland: University of Queensland Press. 388 pp.
Moverley, J.H., Saenger, P. & Curtis, M.A. 1986. Patterns of polychaete recolonization in Queensland subtropical estuaries following severe flooding. Hydrobiologia 134: 227-236.
Neumann, R & Hundloe, T. 1986. The characteristics of recreational fishing in the Moreton Region. Res. Rept, Griffith Univ. Inst. Appl. Envir. Res., 1. 34 pp.
PA Management Consultants 1985. National survey of participation in recreational fishing. Report prepared for the Australian Recreational and Sport Fishing Confederation, Canberra. 47 pp.
Royal Australian Navy Hydrographic Service 1987. Australian national tide tables, 1988. Canberra: Australian Government Publishing Service. 223 pp.
Saenger, P. 1987. Managing coastal wetlands: headlong into muddied waters. Paper presented to the Environmental law conference, Coolangatta, Queensland, 21 November 1987. Unpubl.
Saenger, P. 1991. Australian estuaries - our main productive resource. pp. 161-167, in G. Pike (ed.), Proceedings, First Australian recreational and sportfishing conference, Canberra, 5-7 September 1986. Canberra: Australian Recreational and Sportfishing Confederation.
Saenger, P., Stephenson, W. & Moverley, J. 1980. The estuarine macrobenthos of the Calliope River and Auckland Creek, Queensland. Mem. Qd. Mus., 20: 143-161.
Staples, D.J. 1980. Ecology of juvenile and adolescent banana prawns, Penaeus merguiensis, in a mangrove estuary and adjacent off-shore area of the Gulf of Carpentaria. Aust. J. Mar. Freshwat. Res. 31: 635-652.
United States Department of the Interior Fish and Wildlife Service 1970. National estuary study, Vols 1 and 2. Washington: Government Printer. 769 pp.
Yapp, G.A. 1986. Aspects of population, recreation, and management of the Australian coastal zone. J. Coastal Zone Manag., 14: 47-66.
The technical paper by Dr P. Saenger was reviewed by Dr. Paul Adam, University of NSW, and Jon Brodie, Great Barrier Reef Marine Park Authority, Townsville.