Murrumbidgee River, Tharwa. Photo credit: Simon Lowes

Threats to Native Fish

Why  fish populations have declined in the Murray–Darling Basin

Various factors have contributed to the decline in native fish numbers over the period of European settlement, and the impacts of climate change are posing an additional significant risk to several species currently and into the future. The major threats (historic and current) to native fish in the Basin are shown below.

Click the icons to learn more.

Text Link

Climate change and extreme events

The climate is changing: this is not a ‘future’ issue but a current threat. The incidence and severity of extreme events such as drought, bushfire and flooding in recent years has revealed a raft of new or heightened threats to freshwater fish in the Basin. The Millennium Drought of 1997–2010 was a taste of what dry years might be like under climate change, and the succession of recent floods along eastern Australia are a stark reminder of what wet years will look like. The severe 2017-19 drought across much of the Northern Basin resulted in many streams drying or habitat becoming unsuitable for fish, with over 60 fish death reports and many rescues of threatened fish occurring. The 2019-20 megafires in eastern Australia burned more than 12.6 million hectares, and subsequent rainfall washed silt, ash and partly burned organic material into streams resulting in mass mortality of many fish species. Sub-lethal impacts of warmer aquatic conditions such as disrupted breeding cues, decoupling of evolutionary water temperature and day length relationships (i.e., water is warmer, earlier) and other things like host-parasite relationships and altered fitness are also of concern. 

Native fish species normally restricted to the Northern Basin are now colonising the Southern Basin, so the dynamics of fish populations and communities will also change with a changing climate. Some fish species, habitats, and locations seem to be more rapidly and severely impacted by this changing climate. Alien fish distributions are also changing as warmwater species push further upstream into previously colder headwaters. 

Impacts of extreme events on MDB fish

Millenium Drought

The Millennium Drought which extended from 1997 to 2010 resulted in the lowest inflows to reservoirs in the MDB in 1,000 years (hence the name). Many rivers ceased to flow, some dried to refuge pools, and for others water managers stopped releasing flows from impoundments to downstream reaches (e.g., the Lachlan River downstream of Lake Cargelligo). Low flows and drying streams results in extreme stress on freshwater fish as water temperature rises, dissolved oxygen levels fall, and parasites and alien fish species proliferate. Some small fish can survive low dissolved oxygen by utilizing the oxygen-rich surface boundary layer where they utilise a strategy called aquatic surface respiration. Large-bodied native fish such as Murray cod or Golden perch cannot effectively use this technique and perish. There was a series of fish kills during the Millennium Drought, with the largest in February 2004 when an extensive kill of Murray cod occurred on the Lower Darling-Baaka River between Menindee and Pooncarie. The deaths occurred over 160 kilometres of river and involved predominantly large Murray cod. It was estimated that around 3,000 Murray cod were killed and that it would take around 50 years for the replacement of the large Murray cod in the Darling-Baaka River. Research published in 2021 showed that even seven years after the Millennium Drought broke, runoff had not recovered in 37 per cent of catchments, and the number of recovered catchments was not increasing. This indicates that severe disturbances such as the Millennium Drought can cause very long-term, or even irreversible shifts in hydrology, with obvious consequent impacts on aquatic biota.

Many threatened fish species were pushed to the edge of localized extinction and many emergency rescues were conducted with fish maintained in hatcheries until conditions improved. One species (Yarra pygmy perch) only found in Lake Alexandrina in the Basin was actually driven to local extinction as the declining lake water levels isolated their fringing reedbed refuge habitat exposing them to predators such as Redfin perch. While some Yarra pygmy perch were rescued and bred in captivity, the subsequent releases back into the lake were unsuccessful, and the species is now extinct in the wild in the MDB. Some pygmy perch are still maintained as backup populations and can hopefully form the basis for a future successful reintroduction. Amazingly, there were no other Basin extinctions, but many local fish populations have never fully recovered.

Blackwater in 2010–12

The Millennium Drought broke with a bang in 2010 and 2011 with widespread high rainfall causing flooding in many rivers. In the lowlands, floodplains that had been dry for many years were flooded, inundating the accumulated leaf and other organic debris with the floodwater turning black as the floodplain organic matter acted like a giant teabag. As the flood receded, this black, nutrient-rich water (with very low dissolved oxygen owing the decomposing plant material), drained back to the rivers causing extensive fish kills. Approximately 1,800 km of the Murray River main channel were affected by hypoxia (low oxygen levels) for up to six months, along with tributaries such as the Edwards (Kolety)-Wakool.

Kills of large Murray cod were common, and Murray River crayfish walked out of the water to escape the toxic soup, with crayfish population levels declining by about 80 per cent in affected areas. Murray cod and crayfish populations have been extremely slow to recover, particularly crayfish which have very limited recolonization potential, with monitoring demonstrating little recovery after 10 years, and maybe 50 years will be required to reach pre-blackwater levels. A series of translocations of more than 1,000 Murray crayfish has occurred into areas impacted by the blackwater over several years that has helped kick-start localised recovery at some former habitats. For Murray cod and Golden perch, research indicates that it was natural immigration and recruitment that was most responsible for fish recovery and that post-blackwater stocking only played a limited role.

Unfortunately, another large scale blackwater event occurred in 2016-17 causing significant Murray cod fish deaths around the mid-Murray. At the time of writing, extensive flooding in the MDB between October 2022 and January 2023 was again being followed by large-scale hypoxic blackwater events. There have been documented observations of Murray crayfish walking out of the water in a number of locations along the River Murray and its tributaries, and several fish death events have been reported throughout these months. Estimates of the number of fish affected range from the hundreds to thousands. However, the true number of fish affected, and the long-term impacts and severity of these events on the native fish populations of the River Murray, and the MDB as a whole, is still unknown.

Many threatened fish species were pushed to the edge of localized extinction and many emer- gency rescues were conducted with fish maintained in hatcheries until conditions improved. One species (Yarra pygmy perch) only found in Lake Alexandrina in the Basin was actually driven to local extinction as the declining lake water levels isolated their fringing reedbed refuge habitat exposing them to predators such as Redfin perch. While some Yarra pygmy perch were rescued and bred in captivity, the subsequent releases back into the lake were unsuccessful, and the species is now extinct in the wild in the MDB. Some pygmy perch are still maintained as backup populations and can hopefully form the basis for a future successful reintroduction. Amazingly, there were no other Basin extinctions, but many local fish populations have never fully recovered.

Menindee Fish Kills

A series of high-profile fish kills occurred on the lower Darling-Baaka River around Menindee in late 2018-early 2019. Four separate fish kills with a combined mortality of millions of individual fish occurred between mid-December 2018 and early February 2019.

Tens of thousands of fish died along a 30 kilometre stretch of the lower Darling-Baaka River on 15 December 2018 with a second, larger event (hundreds of thousands of fish) occurring on 6 January 2019. A third event followed on 28 January 2019, killing millions of fish, with a fourth event beginning 4 February 2019. From June 2019 to March 2020, further broader-scale fish kills occurred throughout 600 kilometre of the Darling–Baaka River as isolated refuge pools contracted and water quality deteriorated. It is estimated that millions of Bony herring, thousands of Golden perch, hundreds of Murray cod and hundreds of carp were killed in the early fish kills of 2018-2019. Examination of a subsample of carcases indicates that Murray Cod up to 26+, Golden perch up to 16+, and Silver perch up to 10+ years old were killed. The numbers of smaller fish or decapod (yabbies, freshwater prawns, shrimps) species or individuals killed is unknown, as scavenging by birds and other predators likely removed the majority of such individuals.

Dead Bony herring in the lower Darling Baaka, January 2019.

The cause of the fish kills was related to the extremely hot, dry conditions of preceding months and years, the low water levels in the river, excess upstream diversion of water for irrigation, river and water management and the concentration of fish in isolated pools, including the Menindee weir pool. The Darling–Baaka River ceased to flow in January 2019 and flows did not resume until March 2020. The lack of flow and stratification of the weir pool meant that there was a substantial volume of deoxygenated water in the lower depths of pools. When a series of sudden weather changes (cold fronts with high winds) caused the pools to rapidly destratify (mix) the low oxygen levels along with the inability of fish to escape downstream or upstream resulted in fish kills.

Management responses to the fish kills included removing approximately four tonnes of dead fish from around Menindee after the third fish kill; mapping of refuge habitats in the river to inform subsequent drought actions; trials of pump-driven mechanical aeration in early 2019 to determine efficacy at aerating long stretches of connected river channel, and then subsequent deployment of 10 large mechanical aeration devices at key refugia locations along the river during the summer of 2019/20; fish community monitoring to assess the impact of the 2018/19 Menindee fish kills and subsequent changes in the fish community condition throughout the Lower Darling-Baaka River from 2019 to 2021; more than 1,600 stranded or at-risk native fish (916 Murray Cod, 616 Golden Perch and 86 Silver Perch) were rescued by NSW DPI Fisheries staff and approved volunteer groups from drying pools downstream of Menindee.

Monitoring has shown that 18 months after the fish kills, the abundance of many species is still lower than would be expected. The time required for full recovery is unknown, but as noted above for the Millennium Drought, recovery of Murray cod may take decades.

More recently, another major fish kill occurred in March 2023 in the Darling-Baaka River at Menindee. The kill was larger than those of 2018-19, resulting in millions of dead fish, but was caused by a different combination of environmental factors at the recession of major flooding in the Darling-Baaka and not a lack of water or flow as was the case in 2018-2019. The 2023 kill is thought to be a result of the boom in the numbers of juvenile Carp and Bony herring (spawned during the 2022 floods) returning to the river, along with high nutrient concentrations from the receding floodwaters.

Drought across the northern Basin in 2017–19

The 2017-19 drought in the northern Basin was the driest 36-month period on record (Jan 2017- Dec 2019) when averaged over the Murray–Darling Basin and New South Wales, with all three April−September periods ranked in the 10 driest on record for the Murray–Darling Basin and for New South Wales. Many streams dried or again, shrank to refugial pools.

For example, in September 2019 WaterNSW warned that without significant rainfall the towns of Dubbo, Cobar, Nyngan and Narromine would lose water supply with the Macquarie River forecast to run dry by November. The towns of Forbes, Cowra and Parkes were also likely to lose water supply by March 2020 as the Lachlan River was projected to run dry by March 2020. Almost 90 per cent of the Lower Balonne’s rivers were dry by summer 2019–2020. Over 60 fish death events were reported to the MDBA in 2019 due mainly to drying refuge waterholes and poor water quality.

More fish kills (even more severe than the Menindee fish kills described above) were expected over the 2019-20 summer due to the prospect of little rain, high temperatures and record low river inflows. Significant concerns were held for the catchments in the northern Basin, including the Border Rivers, Gwydir, Namoi, Macquarie and Barwon-Darling, as well as the Lower Darling-Baaka and Lachlan catchments. Importantly the Commonwealth Environmental Water Office and NSW Governments were able to provide 69GL of Water for the Environment over 3 northern Basin flow releases for native fish in 2018, 2019 & 2020 aiding to keep fish alive in refuge waterholes. The NSW Government launched the largest-ever fish rescue and restocking program to ensure that in the worst-case scenario some individuals would be available from which to breed for reintroduction programs.

As in the Millennium Drought of a decade earlier, many rescues of threatened fish were undertaken in the Basin with rescues including Southern pygmy perch, Southern purple-spotted gudgeon, Murray cod, Olive perchlet, Silver perch and Freshwater catfish as well as non-threatened species such as Golden perch. Rescues were undertaken in the Gwydir, Border Rivers, Macquarie, Lachlan, and Upper Murray catchments. Before the drought could break, part of the Basin and large swathes of the east coast were engulfed by the Black Summer bushfires of 2019-20 (see below).

Black Summer bushfires 2019–20

At the end of the 2017–19 drought, Eastern Australia experienced the Black Summer bushfires with fires commencing in July 2019 and extended until March 2020 with fires gradually being extinguished by widespread rains from February 2020 onwards. The fires burnt a staggering 12.6 million hectares across eastern and southern Australia, including large areas of the MDB.

While fish can mostly survive the actual fire, heavy rainfall on severely burned catchments results in large quantities of ash, partially-burned leaves and twigs and sediment entering waterways. This results in significant drops in water quality (particularly dissolved oxygen) and smothering of fish gills, river substrates and food resources. The loss of riparian vegetation removes stream shade, resulting in increased water temperatures. Sediment loads can be enormous, with previously deep refuge pools filled with sand, gravel and silt, and fish spawning/feeding sites smothered. Such impacts are often long-lasting, with stream morphology (i.e. depth, and pool/riffle/run sequences) remaining altered for decades (long after vegetation cover has recovered).

Across Australia numerous fish kills were recorded as a result of the fires, and a series of fish, crayfish and mussel rescues again had to be conducted to ensure several species or populations did not go extinct. Some small populations of fish were lost (e.g. Macquarie perch in Mannus Creek) and some species in the MDB came perilously close to extinction (e.g. Stocky galaxias). The bushfires highlighted the very limited management facilities, knowledge and resources available to rescue, maintain or captively breed many fish species, with many small or threatened species having never been maintained or captively bred previously.

Back to all threats

Flow regulation

The regulation of almost all of the Basin's rivers to provide water for irrigation and other purposes has greatly affected native fish populations. Damming, storage, extraction, interception and regulated flow procedures:

  • severely reduce available water;
  • alter and sometimes reverse the normal seasonal flow patterns;
  • result in periods of unnaturally constant flow;
  • reduce the occurrence of small to medium sized floods and flow sequences;
  • reduce the size, rate of rise and duration of flood events;
  • reduce the opportunity, duration and extent of longitudinal and lateral connectivity and fish movement;
  • reduce the occurrence of low flows due to unseasonal irrigation releases;
  • convert open-flowing riverine environments to a sequence of pools because of the construction of weirs;
  • cause parts of wetlands and floodplains to be continually saturated (waterlogged) rather than experiencing a wetting and drying period; and
  • cause parts of wetlands and floodplains to experience extended durations of no flooding resulting in organic litter buildup increasing the risk of blackwater etc.

These conditions affect water quantity available, water quality and change the flow-related triggers for fish movement, recolonisation and spawning. They also reduce habitat diversity and the flows necessary for fish breeding, recruitment and occupancy. As a result, native fish species diversity and abundance has decreased in rivers. Substantial numbers of larval and small fish are removed from waterways with water extracted or diverted by pumps and irrigation canals. These fish usually end up in unsuitable habitats and are effectively lost from the river. Pumps and undershot weirs also cause significant injury and mortality to fish that pass through them. Farm dams in uplands and slopes areas intercept water, preventing it from reaching streams. The reduction and loss of small and medium flows results in the drying of refuge waterholes during drought.

Back to all threats

Habitat degradation

There are many different types of aquatic systems in the Basin, such as floodplains, upland and lowland rivers, wetlands, billabongs and lakes. Each of these systems contains many habitats such as weed beds, undercut banks, rocks, logs and bank vegetation, and it is these habitats that provide essential shelter, resting areas, spawning sites and food sources for fish.  

These habitats and the fish populations they support have been greatly affected by the removal of snags and riverside vegetation, realignment or reconstruction of riverbanks, invasion of alien plants such as willows and other pest aquatic plants, and greater input of sediment from eroding streams. Over-abundant feral animals such as horses in the high country, and pigs, goats and rabbits along with poor stock management more broadly are causing significant habitat degradation to streams and riparian habitats. All of these human-related processes degrade important fish habitats: increased sediment smothers spawning sites, infills pools and drought refuge waterholes and reduces food abundance; removal of snags and undercut banks means loss of shade and shelter from predators and the river current; loss of bank vegetation results in less shade, lowered nutrient input for fish-food production and warming of the water. Even if all the alien fish species could be magically removed, and normal flow patterns reinstated, many habitats would not recover to their pre-European condition. If native fish populations are to recover, instream and riparian habitats must be actively protected and rehabilitated.

Back to all threats

Lowered water quality

Many factors have contributed to the decline of water quality in the Murray–Darling Basin. Significant changes to water quality include increasing levels of salinity, agricultural chemicals, and point source discharges from industries and sewage treatment works. Whilst many adult fish are tolerant of increased salinity, eggs or larvae have been shown to be highly susceptible, resulting in greatly lowered survival of these early life stages. A more recent threat is the addition of endocrine-disrupting chemicals to waterways. These chemicals either disrupt normal hormone function, or mimic hormones to give an unnatural response. One group of endocrine disruptors is the environmental oestrogens, which can mimic the female hormone, oestrogen. Major sources of environmental oestrogens are pesticides, detergents and prescription drugs such as antibiotics and the contraceptive pill. These chemicals flow into waterways via runoff from agriculture or discharge of treated sewage effluent. Overseas research indicates that exposure to environmental estrogens can alter sex ratios or feminise aquatic species, particularly fish. Potentially, this has a severe impact on the ability of the species to reproduce successfully.  

Many native fish species use increasing water temperature as a cue to commence migrations and spawning. The quality of water released from dams may be a problem if drawn from lower levels in the reservoir where it is usually much colder than the surface waters. The release of cold water (cold-water pollution) during the breeding season can inhibit migration or spawning of native fish, and if releases of cold water are regular and persistent, native fish may not breed at all. Similarly cold-water pollution below dams impacts productivity, limiting the numbers of adult fish and slowing the growth of juvenile fish, which exposes them to higher predation risk for a greater period of their life. Cold-water pollution can persist for hundreds of kilometres downstream of dams.

Back to all threats

Barriers to fish movement

Many fish use different parts of a river system at different stages in their life cycle. The presence of dams, weirs and other barriers on rivers or floodplains stops fish moving from one part of a stream to another. Such movements may be necessary for feeding, breeding or other reasons. For example, Macquarie perch can live and feed in still waters such as lakes and reservoirs, but must move into flowing waters to breed. If access to flowing waters is not available, the population will die out. Barriers also prevent recolonisation of streams by fish after local ‘catastrophes’ or depletion. Barriers such as regulators in lowland forests have been shown to restrict movement of fish between floodplains and rivers, and structures such as levee banks also isolate floodplains from their rivers. Even small barriers such as piped culverts or road crossings fragment a species range into small, isolated segments that are then more vulnerable to local, chance extinction events.  

More than 5,000 barriers are documented across the Basin. Fishways of different kinds exist on approximately 150 of the barriers, but older fishways can be poorly designed and offer only limited fish passage. The impacts of structures on downstream movement of fish have also been recognized as an issue in the Basin. There is evidence that adult fish actively avoid going over weirs and that weir pools and dams may not provide optimal habitat for larval fish, which drift downstream.

There are >5,000 barriers to fish passage and ~150 fishways in the Basin

Recent research has shown that mortality of fish larvae is higher for some weir types, such as undershot weirs, than others. More study is required of the requirements of fish for downstream movement. At the boundary between freshwater and the estuarine waters of the Coorong are a series of barrages that were constructed in the 1930s to prevent saltwater intrusion into the Lower Lakes as well as to stabilise river levels for irrigation and navigation. Prior to the installation of the Barrages, the salinity of the Lower Lakes would have fluctuated, with saltwater intrusions in dry, hot years. However, the lakes were predominantly freshwater. The Barrages were constructed in response to increasing upstream development and agricultural water use reducing river flows in the early 20th century, which increased the risk to the Lower Lakes of saltwater intrusion.

These barrages also prevent free movement of diadromous fish that require access to marine habitats for part of their life cycle. For example, the Short-headed lamprey and Pouched lamprey spend most of their adult life in the marine environment, moving into freshwaters to breed, with juveniles remaining in freshwaters for some years before returning to the sea. Similarly, Congolli make downstream adult spawning migrations, and upstream young-of-year migrations. During the Millennium Drought the closure of the tidal barrages blocked the downstream migration of females to marine spawning habitats with an associated >90 per cent reduction in the abundance of young-of-year upstream migrants.

Back to all threats

Alien species

At least 12 alien fish species from overseas and at least three translocated native species are present in the Basin, some considered pests and others providing valuable recreational fisheries. The abundance and attributes of certain alien fish continue to cause damage to habitats and populations of native species. Transfers of water between river catchments within the MDB for domestic water supply, irrigation or hydro-power purposes are also likely to transfer fish (and their associated diseases and parasites). The risk and impacts of further introductions, in particular of Tilapia, into the Basin—especially from aquariums and nearby rivers outside the Basin—also need to be considered.  

There is growing evidence of the detrimental impact of Carp on native fish species and the aquatic environment. In the seven decades since their illegal release into Lake Hawthorn, near Mildura, Carp are now estimated to account for 70–90 per cent of fish biomass in some rivers of the Murray–Darling Basin. However, Carp are not the only problem species. Redfin perch, trout and Eastern gambusia all have significant impacts on native fish through competition, predation or introduction of diseases and parasites. The recent identification of multiple new species of Galaxias both within and outside the Basin has highlighted the devasting historical impacts of trout on these species.  

The focus of alien fish management should be on preventing future introductions and minimising the impacts of existing alien fish on native species rather than on complete eradication of numbers. Eradication is usually not feasible unless attempted in the very early stages of invasion. The best approach to reducing such impacts is an integrated one that combines a range of techniques. For example, an alien fish control program could include rehabilitating the wetting and drying cycles for floodplain wetlands, commercial exploitation, preventing access to spawning areas, and the installation of screens to prevent spread into uninfested areas. There is unlikely to be one ‘silver bullet’ that can solve any single alien species problem. Control of alien species needs to be part of an overall river rehabilitation process.

Back to all threats

Diseases and parasites

The introduction of alien species has also brought several pathogens that affect native fish. Both exotic and endemic disease outbreaks have potentially devastating effects on native fish populations. Our knowledge of fish diseases and parasites is far from complete.

Redfin perch are the main host for Epizootic Haematopoietic Necrosis Virus (EHNV). This virus, unique to Australia, was first isolated in 1985 on Redfin perch. The virus also affects trout species, which can act as vectors to spread the disease. Experimental work has demonstrated that a number of native fish species (including Macquarie perch, Silver perch and Mountain galaxias) are extremely susceptible to the disease, but many native fish species have not been tested. Once EHNV has been recorded in a water body it is considered impossible to eradicate.

Carp or Redfin perch is suspected of introducing the parasitic Anchor worm (Lernaea cyprinacea). This parasite has been recorded on introduced trout species, Carp and Goldfish in the Basin, as well as many native fish species, including Murray cod, Golden perch, Silver perch, Macquarie perch, River blackfish, Freshwater catfish, Southern pygmy perch and Mountain galaxias. Similarly, Carp or Eastern gambusia are considered the source of the Asian fish tapeworm Schyzocotyle acheilognathi which has been recorded from carp gudgeons in the Canberra region. This was the first Australian record of this parasite, which is known to cause high levels of mortality in juvenile fish overseas and may have similar effects on local native species. It has also been recorded in several fish species in Western Australia. The nematode Eustrongylides is now also suspected of being introduced and has been recorded in several native and alien fish species in the Basin.

Large image: Previous attachment locations (red lesions) of Anchor worm (Lernaea cyprinacea) on the body of a Rainbow trout. Photo: M. Lintermans

Top-left image: A Lernaea attached on the fin of a Golden perch. Photo: E. Beaton

Top-right image: A whole Lernaea (removed from a fish), showing the 'anchor' head processes on the left and paired egg sacs on the right. Photo: E. Beaton

Back to all threats


Long before European settlement, native fish species provided an important source of food for Aboriginal people. Large quantities were sustainably harvested and some of the traditional fish traps used are still in use along the Darling-Baaka/Barwon River. Large quantities were also harvested by European settlers in the early to mid-19th century, but there was a rapid decline in the commercial catch through the latter part of the last century and the early years of this century. In 2003, commercial fishing for native species ceased in the Basin’s rivers, although there is still a commercial fishery in the Lower Lakes near the Murray mouth.  

Historic commercial and recreational fishing have contributed to the decline of several freshwater native fish species, such as Trout cod, Macquarie perch, Murray cod and blackfish. Overfishing is not always the primary reason for the decline of a species, but it may prevent populations of threatened fish from recovering. Closed seasons, closed waters, bag, size and gear limits are designed to prevent overfishing, but illegal fishing is still a significant threat to some species. High fishing pressure can cause the depletion of breeding adults for recreational fish species. Overfishing is generally not involved in the decline of smaller species, although the harvesting of some small fish as bait and aquarium fish may lead to localised declines.

Commercial catches of Murray cod, Freshwater catfish and Silver perch in NSW between 1947 and 1996.

Back to all threats

Translocation and stocking

Fish stocking (the release of hatchery-bred fish) plays an important role in managing recreational fisheries. Stocking enables the development of fish populations in habitats that cannot naturally sustain such populations, for example, in artificial lakes without spawning streams, or in rivers upstream of barriers such as weirs or dams. The composition and evolution of native fish com- munities can be threatened by the liberation of native and alien fish outside their natural range or from hatcheries. Murray cod and Trout cod are now hybridising in the upper Murrumbidgee River following stocking of Murray cod outside their known range. To minimise this risk, appropriate guidelines and codes of practice have been developed. However, while some States do have guidelines in place, it is difficult to verify whether the guidelines are being followed.  

Natural populations of native fish are also at risk from the potential release of genetically restricted individuals from native fish aquaculture operations bred using limited numbers of brood stock. The release of these individuals has potential to reduce the genetic fitness and hence viability of fish populations. Aquaculture is a rapidly developing industry throughout the Murray–Darling Basin. Measures to protect native fish from adverse impacts need to keep pace with this development.

Back to all threats

Genetic issues

The importance of genetic integrity and genetic diversity of animal populations has attracted increasing attention in recent decades, and this is also the case for fishes in the Basin. Many threatened fish species now persist as small, isolated populations with no opportunity for inter- mixing. Almost all of these isolated populations are slowly losing their genetic diversity through chance events and inbreeding and this reduced diversity means they are less able to respond to new challenges like climate change. Genetic integrity is also challenged by hybridisation between closely related species and forms/genetic lineages within a species. Within currently described species there are different genetic lineages on their own evolutionary path. For example, the Southern pygmy perch and Flatheaded gudgeon have different genetic composition in coastal and MDB rivers and mixing the two has already been documented. Similarly, there is evidence that mixing of Golden perch from Lake Eyre and Fitzroy basins with MDB fish has occurred during stocking programs.

Hybridisation is a natural phenomenon in fishes but can be devastating when a native species is translocated outside its natural range into the range of a closely related species/form. For example, endangered Trout cod in the upper Murrumbidgee catchment are now hybridising with Murray cod stocked for recreational fishing. Similarly, the carp gudgeon complex contains some species that will readily hybridise with parent species potentially disappearing as the hybrids takeover. The newly described Bald carp gudgeon is at significant risk if other carp gudgeons are introduced to its range.  

The genetic composition and diversity of stocked fish can be significantly different to that of wild fish, as hatcheries often have relatively few and sometimes related broodstock. When hatcheries breed fish for domestic consumption as well as release to the wild, there may be competing objectives around genetic diversity (i.e. rapid growth and reduced aggression vs wild charac- teristics). Consequently the genetic composition of hatchery-bred fish may be different to wild fish, which can affect behaviour and survival of stocked fish. Managing the genetics of hatchery programs is complex but critical.

Back to all threats

What's being done to recover our native fish?

The Native Fish Recovery Strategy is a joint Australian Government initiative developed in partnership with Basin state governments, Aboriginal Nations and the wider community. It sets out a program of actions involving government, communities and industries across the Basin to ‘recover native fish for future generations’.

Learn More

Discover related content on the Finterest website, your home for stories about our Australian Freshwater Native Fish.

Since 2013, Finterest has been sharing great stories and information about the work being undertaken across Australia to bring back our native fish, particularly across the Murray-Darling Basin. It's a great source of inspiration and knowledge for anyone interested in Australian freshwater fish and native fish, and is updated with new stories regularly.
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.