Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Jonas Ball, Sinclair Knight Merz Pty Limited, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06750 7
Aquatic ecosystems (continued)
Pressures on aquatic ecosystems (continued)
Exotic species include non-native species and native species that have been relocated to areas outside of their natural range. The threats posed by introduced (or exotic) species to global biodiversity are immense (IUCN 1996). The introduction of exotic aquatic fauna and flora into Australia has in many instances had a profound effect on native species and their communities. While a number of impacts from exotic species are relatively well documented, undoubtedly there are many more impacts that are yet to be recognised.
Although a range of aquatic species are exotic to Australia, the most damaging are aquatic vegetation (weeds) and fish. The environmental costs of aquatic weeds are considerable (Boulton & Brock 1999) and include direct impacts caused by competition with native flora, and indirect impacts such as changes in the vegetation structure, movement of nutrients, light penetration, hydrology, water quality (such as dissolved oxygen levels) and stream morphology. In a number of inland Queensland river systems, the riparian zone of 96% of sites was found to be affected by aquatic weeds (Queenlsand EPA 1999).
Introduced fish species are also a significant problem in many inland waters. During a two-year survey of fish communities in the rivers of New South Wales, 18% of the total fish catch was introduced species (Harris & Gerhke 1997). Eight intentionally and 27 accidentally released exotic fish species have become established in Australian waters (Kailola et al . 1999). Eight introduced fish species are considered to have a significant current or potential impact (Clarke et al. 2000).
European Carp (Cyprinus carpio) is a particularly invasive species and is generally found in many disturbed lowland river systems in the eastern states. Over the past 40 years the distribution of carp in the Murray-Darling Basin has increased substantially (Figure 25).
Figure 25: Carp distribution in the Murray-Darling Basin
Source: Modified from Shearer and Mulley 1978, MDBC 2001.
European carp compete with native fish species for resources, but can also degrade stream habitats by disturbing bottom sediments and destroying aquatic plants. Several pest management programs are in place to reverse the impact of the carp on Australia's waterways (see European carp - Cyprinus carpio).
Case Study 9: Trout
Fish from the family Salmonidae (commonly called trout and salmon) are a popular angling species that are native to the cool and cold waters of the northern hemisphere. Five species have been introduced into Australia. Of these five species, brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) have the widest distribution with self-sustaining populations. These populations are also supplemented by hatchery released stocks.
Predation and competition for resources are the main mechanisms by which trout affect other aquatic species. Trout are opportunistic feeders, consuming a wide variety of animals including aquatic macroinvertebrates, small fish and terrestrial invertebrates (McDowall 1996). A number of native fish species are also prey for trout. Trout have been observed to feed selectively on small fish in preference to other food. Predation by trout is considered to be the main threat to a number of endangered native species such as the barred galaxias (Galaxias fuscus) (Raadik 1995).
Brown trout are aggressive and territorial and may compete with native fish for habitats that offer the best cover or feeding positions. Overlaps in diet between trout and native species such as Macquarie perch, river blackfish and trout cod have led to a decline in the abundance of these species (Arthington 1991). In river systems where trout are present, native fish are often restricted to the upper reaches of the rivers in areas that are inaccessible to trout (e.g. Raadik 1995).
Additional risks posed by trout include their potential to transmit disease, particularly through the restocking of waters of trout from hatcheries. The decline in the distribution of some macroinvertebrates and amphibians has also been caused by the establishment of trout (e.g. Fletcher, 1986).
Emphasis for control of trout species centres on recovery plans for threatened native fish species. This can include measures such as exclusion structures on streams to prevent trout access to areas where threatened species are found (Clarke et al. 2000).
Incidences of large-scale loss or distress of wildlife are often highly visible events and are generally caused by extreme water quality conditions such as low dissolved oxygen levels, high concentrations of toxicants such as pesticides and blue-green algal toxins. Although other animals such as birds and macroinvertebrates may be affected by extreme water quality conditions, the effects are most visible and significant on fish.
For many fish kills it is difficult to determine the cause of mortality as the specific water quality conditions that caused the event may no longer be present once the fish kill is reported and investigated. There also may be more than one environmental factor that contributes to the event. Chronic environmental stress can reduce the capability of some fish populations to survive abrupt changes in water quality (NSW EPA 1997).
Between 1990 and 1999, 146 fish kills were reported in New South Wales not including fish kills in farm dams (NSW EPA 2001). For approximately half the fish kills, the cause of mortality was unable to be determined. Where mortality could be determined, pesticide contamination and low dissolved oxygen levels were responsible for most of the fish kill events. Low dissolved oxygen levels can be caused by the release of deoxygenated bottom water from water storages, decay of algal blooms and the natural oxidation of organic matter washed into inland waters after rainfall. All the pesticide-related fish kills were in agricultural areas where pesticide use is high. Fish kills could often be linked to over-spraying or the release of pesticide contaminated run-off from irrigated agricultural land (e.g. tailwater from cotton farms).
Between 1978 and 1990, 45 fish kills were reported in the Northern Territory (NT DLE 1999). Most fish kills occurred after first large rainfalls of the wet season, which washed accumulated pollutants and organic material into inland waters.
- The natural flow regimes of many regulated river systems have been modified due to water extractions and the use of rivers as irrigation and water supply channels. Many of Australia's native flora, fauna and habitats are dependent on natural flow regimes for sustenance and reproduction so changes in flow patterns may have a significant effect. Aquatic ecosystems in the highly regulated river systems of the Murray-Darling Basin and a number of smaller eastern coastal river systems are likely to be under pressure from modification of natural flow regimes.
- Increasing salinity of inland waters is a major threat to many aquatic ecosystems. Already aquatic ecosystems in western Victoria and south-west Australia are affected by salinity and it is predicted that many wetlands and river systems in the Murray-Darling Basin will be severely affected by increased salinity by 2050.
- Cold-water pollution and pesticide contamination are other significant water-quality problems that affect aquatic ecosystems. Cold-water pollution is likely to occur downstream of most water storages and up to 3000 km of rivers in New South Wales alone could be affected. Pesticide contamination of inland waters has been implicated as the cause of fish kills in agricultural areas. The long-term effect on aquatic ecosystems of low levels of pesticides (and other chemicals such as endocrine disrupters) in the environment has not been determined.
- The introduction and spread of exotic flora and fauna continue to have a significant effect on inland aquatic ecosystems. For example, many riparian zones are weed infested, and competition and predation by introduced fish species have been implicated in the declining populations and distributions of some native fish species.