R. Wager and P. Jackson
Environment Australia, June 1993
ISBN 0 6421 6818 0
Australia encompasses a wide variety of freshwater habitats and boasts a relatively diverse fauna. Hence processes responsible for the decline of native fishes are often unique to particular species or communities.
Whilst threatening processes for individual species have been discussed by a number of authors (Ingram et al. 1990, Harris 1988, Rowland 1989), others have grouped threatening processes for all species into broader categories (Pollard et al. 1990, Michaelis 1985, Lake 1971, Cadwallader 1978, Koehn and O'Connor 1990 and Lawrence 1991). These groupings are convenient for listing threatening processes, but are arbitrary. The processes are inexorably linked, and rarely is one factor alone responsible for the decline of a species. For example, water quality problems can be exacerbated by reduced flow rates, sediment load or changes in water temperature.
As agricultural and industrial development and urbanisation continue to increase, so does the demand for water (Brown et al. 1983). Many rivers, particularly in the south east of the continent, have been severely affected by present water demands. It is significant that almost all control and diversion systems in operation in Australian rivers were designed and are operated for non-environmental purposes (McMahon and Finlayson 1991).
Altered flow volumes
Changes in flow volumes are most apparent immediately below impoundments or at major water abstraction sites.
Dams, weirs and barrages (collectively known as impoundments because they collect and confine water) are at times capable of capturing and entirely containing floods. The impounded water is stored and may be redirected to irrigation schemes so that downstream flows within the natural stream channel are reduced. This causes a reduction in the amount of the channel which is submerged and hence a reduction in the amount of habitat available to aquatic organisms (instream habitat). Changes in general channel morphology may also result, further altering instream habitat.
In smaller streams flow reduction may be caused by water abstraction or by water storage in farm dams. These effects may be most detrimental to stream ecology during periods of low rainfall.
Increased flow volumes may also be detrimental to fish populations. Summer irrigation flows in the South Australian reaches of the Murray River cause turbidity levels to remain high. In the past flow would have essentially ceased, allowing suspended sediments to settle and the water to clear. Increased turbidity may have induced changes in fish communities; for example, the replacement of Murray cod (Maccullochella peelii) with golden perch (Macquaria ambigua) as the dominant native species (B. Pierce, pers. comm.).
Flow volumes are increased temporarily in streams with catchments that include urban areas or where the natural vegetation has been cleared. Hard, unvegetated surfaces cause most rainfall to flow over the surface rather than seeping into the substrate. This causes temporary flooding of streams rather than a steady increase and fall in flow volume.
Exploitation of subterranean water from the Great Artesian Basin of central Australia has increased since European settlement in the 1880s. This has caused a reduction in flow rates of natural artesian springs in the south central, south western and north western parts of the Basin. For example, Elizabeth Springs are located near Springvale homestead (80 kilometres south east of Boulia). The original flow rate of these Springs could have been as high as 158 litres per second and Spring Creek may have flowed about 130 kilometres from Elizabeth Springs. Current flow rates from the springs appear to be variable. Measurements since the late 1800s range from about 29 to 0.8 litres per second, with the length of Spring Creek ranging from approximately 30 to 0 kilometres (Habermehl 1982). This results in considerable reduction of habitat available to fishes.
Altered seasonality of flows
On regulated streams impoundments have the potential to capture seasonal flood waters. Captured wet season floodwaters are often stored and released for irrigation during the dry season. This seasonal flow reversal can markedly affect the reproductive success of many species. Changing the season of water flows below impoundments may remove necessary stimuli or habitat requirements for spawning. Species included in this group are: golden perch in the Murray-Darling and Lake Eyre Drainages (Lake 1967), common galaxias (Galaxias maculatus) in south eastern Australia (Pollard 1971), and black catfish (Neosilurus ater) and Hyrtl's tandan (Neosilurus hyrtli) in north Queensland (Orr and Milward 1984).
Australian aquatic fauna and flora appear well adapted to hydrologic variability. This variability has lead to evolutionary adaptations, usually by opportunistic use of the environment (Nicholls 1989).
Reduced frequency of floods
Impoundments mitigate flood levels proportional to their supply level at the time of the flood. Larger impoundments can reduce the incidence of minor flooding and mitigate the extent of major floods.
Since peak flood levels determine the extent of flood plain inundation, flood mitigation reduces the area of floodplain available to fish. Inundated floodplains provide increased food and habitat for fish. As peak flood levels are lowered the availability of these is reduced.
Reduced inundation of floodplains along the Murray River due to river regulation has had detrimental effects on species such as golden perch, silver perch (Bidyanus bidyanus) and Murray cod which rely on floodplain zooplankton for fry survival (Murray-Darling Basin Ministerial Council 1987).
Research by the South Australian Department of Fisheries indicates reduced flooding of the lower Murray River favours introduced species, particularly gambusia (Gambusia holbrooki) and carp (Cyprinus carpio). The inverse has also been shown (B. Pierce, pers. comm.).
Floodwaters also redistribute organic matter within the system and maintain gravel permeability by removing fine sediments.
Altered river levels
Upstream of impoundments river levels are determined by the height of the impoundment and are stable for considerable distances upstream. The aquatic environment is effectively changed from lotic (flowing water in streams) to lentic (still water of lakes, etc) and community composition changes.
Increased rate of change of water levels
Within regulated rivers fluctuation of water levels can be more extreme than would have occurred naturally. Fluctuating water levels cause rapid alterations in the amount of available habitat and this can be deleterious to many species. Rapidly decreasing water levels may result in the eggs or fry being stranded. Rapid closure of valves can result in fish being trapped or stranded either within the channel or on floodplains (Bishop and Bell 1978).
Catchment deforestation and overgrazing
Catchment vegetation is extremely important to the ecology of streams. Within catchment areas vegetation promotes seepage of rainfall into the substrate. Runoff is limited and occurs relatively slowly. Removal of vegetation in the catchment (through forestry activities, clearing for grazing and agriculture, or urban development) increases the runoff rate resulting in increased erosion and flooding. Vegetation also filters suspended sediments and absorbed pollutants from runoff water.
Removal of riparian vegetation
Natural riparian vegetation (streamside vegetation) is vital to the functioning of stream ecosystems. Its removal or modification can adversely impact every aspect of stream habitat.
Riparian vegetation maintains bank stability and channel morphology through the binding ability of root systems. It also provides a buffer zone which filters sediments, organic and inorganic nutrients, and agricultural chemicals in runoff from the surrounding catchment.
Most instream habitats, including fallen trees and branches (snags), leaves, and bark are derived from riparian vegetation. Undercut banks are also maintained through its presence.
Streamside vegetation is often the major source of energy input, particularly in upland streams. Continuous input of organic material, including leaves, twigs, and terrestrial insects, provides an important food source for aquatic invertebrates and higher animals, and replaces energy lost by downstream transport.
Removal of riparian vegetation increases the amount of sunlight reaching the stream. This allows the proliferation of weed species. Arthington et al. (1983) have documented the adverse effects of para grass (Brachiaria mutica) and floating weeds on native fish habitats in streams around Brisbane. Choking of streams and consequent loss of fish habitat is common in northern Australia.
Increased water temperatures and increased water temperature fluctuations may also result through loss of shading due to the removal of streamside vegetation.
Erosion and siltation
Poor land management practices within catchment areas account for widespread erosion. This is primarily instigated by the removal of riparian and catchment vegetation. Specific sources of erosion include domestic and feral stock watering points, mining operations, road works and construction sites. Within streams erosion modifies the morphology of channels and banks and increases the sediment load. An increased sediment load has a variety of effects including: infilling of deep holes and channels; smothering of riffle areas and the associated invertebrate fauna (an important food source for many species); smothering of demersal fish eggs deposited in gravel substrates or attached to underwater surfaces. For example, more than 80 percent of the freshwater species in Victoria produce demersal eggs which are susceptible to smothering by fine sediment (Hall 1991).
Siltation has been an important process in the decline of Macquarie perch (Macquaria australasica). It is notable that the only significant riverine populations of Macquarie perch occur in upstream areas with minimal siltation and where deep pools interspersed with riffle areas are still present (Cadwallader and Backhouse 1983).
Snags (dead timber) are the main instream habitat attribute in lowland streams. Jackson (1978) stated that snags provide spawning sites for river blackfish (Gadopsis marmoratus). Species in the Gobiidae, Eleotrididae, Percichthyidae and other families may also spawn in or on snags. The presence of snags in a river also increases habitat diversity, providing shelter and rest areas for both adult and juvenile fish. Log jams trap and hold organic debris. Invertebrates break down the organic matter allowing nutrients to be cycled through the ecosystem.
Snags also provide substrates for food organisms. This is particularly important in muddy waters where up to 60 percent of primary and secondary production can occur on snags (Murray-Darling Basin Ministerial Council 1987).
Removal of snags from streams has been extensively practised throughout Australia. The aim of desnagging is to aid in flood mitigation and to avoid bank erosion, although data demonstrating the effectiveness of this practice are not available. Removal of snags decreases the variety of instream habitat and results in the alteration of channel morphology (see River Engineering). Lawrence (1991), in the Murray-Darling Basin Commission Fish Management Plan, outlines environmentally preferable guidelines for the removal of snags from the Murray-Darling River System.
Barriers to fish movement
Barriers to fish movement (both upstream and downstream) can impede spawning migrations, dispersal of fish or intermixing of genes within a population. Artificial barriers may include dams, weirs, barrages, culverts, and excessive log jams due to poor silviculture practices (Mansergh et al. 1984). Many native fishes require access to estuarine or marine areas, or need to migrate upstream for spawning or maturation. Harris (1984), in a survey of eastern flowing streams between Fraser Island (Queensland) and Lakes Entrance (Victoria), showed that about half of the potential freshwater habitat was not accessible to migrating fishes due to artificial barriers.
Many rivers are subject to engineering schemes aimed at controlling the effects of flooding and enhancing the effectiveness of the stream as a drain. This process is achieved by: removal of logs and debris; removal of bankside standing trees and shrubs which are likely to fall into the stream; extraction of gravels and sediments from the channel and straightening of meanders to allow a more efficient draining process. Ultimately a straight, symmetrical concrete channel or buried drain pipe with no suitable fish habitat results. A study by Hortle and Lake (1983) demonstrated that unchannelised sites had significantly higher species richness, total fish biomass, numerical density and standing crop than channelised sites. They suggested that absence of suitable habitat (snags, areas of slack water, length of bank fringed with vegetation) accounted for the lower abundance and lower species richness observed after channelisation.
Engineering works to stabilise bank and bed erosion often have similar consequences, although environmentally sensitive schemes are possible. (See Victorian Environmental Guidelines for River Management Works.)
Alteration of habitat
Altered environments often result in modified faunal assemblages. Conditions more favourable to particular species or guilds may be created allowing proliferation of some species to the detriment of others. Introduced species are often most successful in altered or modified habitats (Arthington et al. 1983). Environmental alteration is often manifested through reduced habitat diversity, which results in lowered species diversity (see River Engineering).
Many toxic substances are pervasive and affect fish communities over a long period. Organochloride insecticides (eg DDT, dieldrin, lindane) and heavy metals are the most common pollutants implicated in stream pollution, and may bioaccumulate in body tissues of aquatic organisms. These substances may not kill fish outright, but may cause reduced vigour and changes to behaviour and growth. An overall reduction in reproductive efficiency may be as effective in reducing fish populations as acute pollution events.
Many persistent pesticides are being replaced by organophosphate insecticides and herbicides which are less persistent in the environment. These compounds are often acutely toxic to fishes (eg Endosulfan) and have been reported to cause fish mortality in the Brisbane River, Queensland (WRC 1986), and elsewhere.
Other toxic substances (from industry) may be implicated in fish kills. Mary River Cod (Maccullochella sp.) and other species, have been killed by the careless storage and spillage of water treatment wastes (Fisheries Division, Queensland Department of Primary Industries).
A variety of environmental factors may also be responsible for fish kills (see Dissolved Oxygen and Salinity).
Of the various environmental cues necessary to induce spawning in native fishes, specific water temperatures are one of the most critical. Alteration of temperature regimes in streams due to water releases from impoundments is widespread. Release of cool water from the bottom of impoundments can lower downstream temperatures leading to inhibition of maturation and spawning in fishes. Cessation of flow below an impoundment can cause increased temperatures leading to other water quality problems. Changed temperature regimes can alter growth rates and production of food within a stream. Feeding of fish is inhibited at lower temperatures.
Outflow of power station cooling water into streams can increase minimum temperatures allowing the survival and persistence of exotic species (Cadwallader et al. 1980).
Australian waters are generally well mixed and oxygenated but in some cases dissolved oxygen levels can become critically low. Water released from the lower levels of deep water impoundments is often oxygen deficient and may travel several kilometres downstream before oxygen levels are adequate to support fish. The input of nutrients from sewage treatment plants or agricultural lands can lead to conditions of low oxygen concentration. The nutrients cause algal blooms. Decomposition of dead algae creates a high demand for oxygen and often depletes surrounding water of dissolved oxygen. The effects of oxygen depletion are often exacerbated by reduced water flows.
Effluent water from hydro electricity schemes may be supersaturated with oxygen (or other atmospheric gases) and this has been reported to cause mortality of fishes (A. Sanger, pers. comm.).
High levels of suspended sediments often coincide with catchment and bank erosion. Light is unable to penetrate turbid water and consequently growth of algae and aquatic plants is reduced, leading to reduced production. Invertebrate communities, which are a major dietary component of many species, may also be smothered. It is known that high sediment loads in the water can affect the respiration of aquatic species and assist in the downstream movement of absorbed pesticides. High levels of suspended sediment may also alter fish behaviour. For example, the predatory efficiency of visual predators may be reduced. The presence of purple spotted gudgeons (Mogurnda adspersa) in the Murray-Darling Drainage is strongly correlated to low turbidity (B. Pierce, pers. comm.). Although little data are presently available, there is evidence that high concentrations of suspended sediment can be lethal to fish (Koehn and O'Connor 1990).
Eutrophication is the enrichment of a waterbody with nutrients, leading to increased production. This is a naturally occurring process with nutrients derived from catchment sources including terrestrial plants, silt, and materials leached from soils and rocks. Human activities have accelerated eutrophication in many waterbodies. Increased amounts of nutrients are derived through: increased erosion in catchment areas; input of sewage, including household and industrial detergents; run-off from fertilised land or crops; input of domestic animal wastes from grazing lands or feedlots, and storm water run-off from residential and industrial areas. Excessive eutrophication may be detrimental. Increased production of aquatic plants and algae temporarily increases oxygen production, but the associated decomposition of dead vegetable matter may cause depletion of dissolved oxygen. These conditions are unsuitable for survival of fishes and other aquatic fauna. Other effects of eutrophication may include: blooms of toxic algae; increased turbidity; and changes in community composition. The effects of eutrophication may also be increased through reduced water flows. This is particularly significant in regulated streams or where water demand for off-stream uses is high. In these situations streams may be reduced to a series of stagnant pools with poor water quality unsuitable for survival of fishes and other fauna.
Although all waters in Australia are saline to some extent, agriculture and other developments can increase the salt content of stream waters. Particularly in the Murray-Darling Drainage, but in many other drainages, water used for irrigation leaches salts from soils. When this water drains back into rivers it has an increased salinity. Over time the salinity of the river water may increase. This may effect the composition of fish communities.
In Victoria there is evidence that stream salinities are being affected by a combination of land clearing, ground water drainage, and flow regulation (Anderson and Morison 1989). Many streams in south western Victoria are now saline due to ground water seepage and the process is also affecting tributary streams in northern Victoria (Macumber and Dyson 1988). The situation is exacerbated by reduced flow rates in lower reaches. Anderson and Morison (1989) have recorded accumulations of saline water in deeper pools of the Wimmera River. These 'saline pools' have low dissolved oxygen levels and effectively reduce in-stream habitat for fish. Flushing of 'saline pools' during increased flows may result in fish kills downstream.
In south west Western Australia stream salinities have increased in recent times due to land clearing, primarily for agriculture. The Swan-Avon, Moore, and Blackwood Rivers are all unsuitable as domestic water supplies. In this region soil salt content increases from the coast inland and clearing of natural vegetation in catchment areas has the potential to dramatically increase stream salinities (G. Allen, pers. comm.).
Introduced exotic species
Between 20 (McKay 1989) and 24 (Allen 1989) exotic species have been recorded from Australian waters and 19 or 20 of these are likely to breed in the wild (Arthington, pers. comm.). Several of these have been implicated in the decline of native species although it is difficult to distinguish this from other threatening processes. Predation, utilisation of similar resources, aggressive behaviour, introduction of exotic pathogens, and habitat modification have been suggested as detrimental factors associated with introduced exotic (and introduced native) species.
The effect of salmonids on some native species has been documented (Jackson and Williams 1980, Jackson 1981). Brown trout (Salmo trutta) has been implicated in the decline of several species including Swan Galaxias (Galaxias fontanus) (Fulton 1978) and mountain galaxias (Galaxias olidus) (Tilzey 1976). This decline is usually due to predation by brown trout. See Fletcher (1986) for a review of the impacts of trout.
The effect of common carp (Cyprinus carpio) has not been conclusively demonstrated. Carp utilise a variety of food resources required by some native fish species and larger invertebrates (which are preyed upon by several native fish species). Modification of habitat due to feeding and spawning behaviour of carp may also occur, although this is not clearly documented. Carp have been strongly implicated in the reduction of shallow rooted and soft leaved species of aquatic vegetation (Fletcher et al. 1985). In some areas Murray cod and golden perch may feed on carp.
Gambusia (Gambusia holbrooki), also known as mosquitofish, is a small, aggressive, predatory species widespread in mainland Australia. It has been actively introduced by governments and military institutions for the control of mosquitoes. This species has been implicated in the extinction of small fishes in Asia and Africa and in the reduction in abundance or range of 25 species worldwide (Arthington and Lloyd 1989). In Australia Gambusia has been implicated in the decline of species of Ambassis, Chlamydogobius, Craterocephalus, Galaxias, Melanotaenia, Mogurnda, Pseudomugil, Retropinna and Scaturiginichthys. Causal factors include utilisation of similar food resources, utilisation of the same habitat, predation on eggs or fry and aggressive interactions (fin nipping).
Exotic cichlids have been introduced to waters of Queensland, Victoria and Western Australia. Interaction between cichlids and native fishes are poorly known mainly due to the scarcity of scientific studies. Bludhorn, Arthington and Mather (1990) demonstrated limited overlap in the diet of adult tilapia (Oreochromis mossambicus), spangled perch (Leiopotherapon unicolor) and eel-tailed catfish (Tandanus tandanus) in a Queensland impoundment. They suggested that resource partitioning may not occur among juveniles of these species.
Occurrence of cichlids in Victoria is restricted to the Hazelwood Pondage, a waterway artificially heated by the effluent cooling water of a power generating station. Due to low water temperatures downstream, it is unlikely that the species will spread from this location (Cadwallader et al. 1980).
It is significant that 10 of the 24 Species Recovery Outlines presented in this Action Plan nominate exotic fishes as the major threat. Five list brown trout as the threat, and five list gambusia.
Introduced native species
Introduction of native species to areas outside their natural distribution, can have a detrimental impact. For example, the climbing galaxias (Galaxias brevipinnis), not previously recorded in Lake Pedder prior to its flooding, may be implicated in the decline of the Pedder Galaxias (Galaxias pedderensis) (see Species Recovery Outlines).
Barlow et al. (1987) documented the role of introduced native species in the extinction in the wild of rainbowfish from Lake Eacham, North Queensland.
The effects of introduced native species on the environment and ecosystems are similar and perhaps more severe than those of introduced exotic species. These include: introduction of disease organisms; disturbance of ecosystems; and loss of biogeographic information. In addition introduction may result in loss of genetic diversity when separate stocks of the same species are mixed. Harris and Battaglene (1989) provide more details on the effects of native species translocation.
The ecological effects of introduced exotic and native fishes were the subject of an Australian Society for Fish Biology Workshop (see Pollard 1990).
Disease and parasites
Diseases or parasites have frequently been spread throughout the world by the translocation of fishes. Pathogens not endemic to a particular region are frequently more dangerous to the new hosts (the endemic fishes) than the original carrier. Even the translocation of native species within their natural range poses the risk of spreading taxon specific pathogens (Langdon 1989b). Supposedly taxon specific (species specific) diseases have been shown to be capable of infecting other non-related species. Redfin perch (Perca fluviatilis) carries a pathogenic virus (epizootic haematopoietic necrosis) which has been shown to be highly pathogenic for silver perch, mountain galaxias, Macquarie perch, and to a lesser extent, Murray cod (Langdon 1989b). Many other fishes are likely to be susceptible. Several additional pathogens have been introduced to native species from exotic species. In the Tambo River (Victoria), some Australian Grayling (Prototroctes maraena) have been found to be infected with anchor worm (Learnea cyprinacea) which depletes fish populations overseas (Hall 1988). Anchor worm was probably introduced with salmonids or redfin perch and is known to infect other native species including river blackfish and Murray cod (Langdon 1988).
For detailed information on the ecological and genetic impacts of introduced and translocated species, see Arthington (1991).
Worldwide, overfishing has been most critical for larger fish species, and has been implicated in the extinction of several species. For example, the decline and disappearance of the blue walleye (Stizostedion vitreum glaucum) from the North American Great Lakes was probably directly related to over exploitation by a largely unregulated commercial fishery (Campbell 1987). Commercial and recreational fishing has probably been instrumental in the decline of some species in
Australia. A commercial fishery existed for Murray cod from the mid 1800s to the 1930s (Rowland 1989). A small scale fishery continues in New South Wales, Victoria (despite the species being listed as endangered under that state's Flora and Fauna Guarantee Act 1988) and until recently in South Australia, where there is now a moratorium on recreational and commercial capture. Significant Murray cod recreational fisheries developed from the 1950s (Rowland 1989).
Exploitation of wild stocks of smaller fishes for the aquarium trade could become significant. Rarer species and those which are difficult or unlikely to breed in captivity are often targeted by illegal collectors.
A species affected by other threatening processes may be vulnerable to fishing pressure. Illegal fishing could be a potential threat to threatened species.
Many aspects of the biology and ecology of native fishes are poorly known. In addition nearly all of the above threatening processes are inadequately understood. Options for the conservation and management of threatened and non-threatened species are inhibited by lack of knowledge. Appropriate research is seldom funded and there are few long term monitoring programs to allow proper assessment of the effects of threatening processes.