The Decline of the Murray Cod: What Happened and What Is Being Done

Murray cod were once so abundant in the rivers of south-eastern Australia that early colonial accounts describe them being taken in numbers that are difficult to credit from the current state of the fishery. Commercial netting operations in the 19th and early 20th centuries removed fish by the tonne from the Murray system. Settlers ate them as a staple protein. The rivers that are now managed carefully for recreational catch-and-release fishing once produced Murray cod the way some rivers produce salmon runs — in volumes that seemed essentially inexhaustible.

They were not inexhaustible. The Murray cod is now listed as a vulnerable species under federal law. Its population in most parts of its range is a fraction of what it was at European contact, and in some tributaries that once held large numbers, it is functionally absent. Understanding how this happened — the overlapping causes, the compounding factors, the specific decisions that accelerated the decline — is necessary context for evaluating what recovery efforts are actually up against.

The Historical Baseline

Before any assessment of what has been lost, it's worth establishing what the pre-European baseline looked like, because the current condition of even the most productive Murray cod waters is not a reference point — it is already a degraded state against which further losses are measured.

Murray cod range naturally through the Murray-Darling river system and several coastal rivers of New South Wales. Their historical abundance across this range has been estimated from the accounts of explorers, early settlers, and commercial fishers whose records, adjusted for the exaggeration inherent in fishing stories, still point to populations that bear no resemblance to what exists now. Fish of 100 kg and above were regularly reported; modern fish over 30 kg are considered exceptional. The size distribution of the population has contracted dramatically, reflecting decades of selective removal of the largest individuals.

The commercial fishery that operated on the Murray from the 1850s into the 20th century is the starting point of the decline. Paddle steamers carried refrigerated cargo and steam-powered commercial netting operations removed fish in quantities that would be inconceivable under any contemporary regulatory framework. The Murray cod fishery was, by any objective assessment, mined rather than harvested — treated as an extractable resource with no management provision for sustainability.

The Barriers

Superimposed on the commercial overfishing was the construction of the weirs and locks that transformed the Murray River from a free-flowing, seasonally flooding system into the regulated, impounded waterway it is today.

Murray cod are migratory fish. Adults move significant distances during spawning migrations, and juveniles disperse extensively following flood events. The construction of 15 major locks and weirs on the Murray mainstem — and hundreds of additional barriers on tributaries throughout the basin — created a series of impoundments that blocked movement both upstream and downstream, fragmented populations that had historically been connected, and changed the hydraulic and thermal characteristics of the river in ways that affected spawning behaviour.

The thermal regime of the impounded Murray is particularly significant. Deep reservoirs stratify in summer, releasing cold, oxygenated water from depth that is several degrees below the natural summer temperature in the rivers below. Murray cod spawn in spring when water temperatures rise through the 15 to 20-degree range, cued by this warming as a spawning signal. Below certain dams, this warming signal is disrupted — cold water releases keep temperatures artificially depressed into the period when cod would normally begin spawning, delaying or suppressing reproduction. This "cold water pollution" is one of the most significant but least publicly understood factors affecting cod reproduction in regulated sections of the river.

Carp

Common carp arrived in the Murray-Darling system in the 1960s following an escape from aquaculture facilities near Mildura, and spread through the basin with extraordinary speed. By the 1980s they were established throughout the lowland Murray and its major tributaries. By the 1990s they constituted the dominant fish biomass in most of the lowland system.

The ecological impact of carp on Murray cod recovery operates through multiple pathways. The most direct is habitat degradation: carp disturb substrate in their feeding, resuspend sediment, destroy aquatic vegetation, and increase turbidity to levels that affect the ability of cod to hunt and of juveniles to find cover. The shallow, productive wetlands and floodplain environments that are most important for cod recruitment — where young-of-year cod find the invertebrate prey and vegetative cover they need to survive their first year — are systematically degraded by carp that reach these environments during flood events.

The less direct impact is through food web disruption. Carp compete with cod juveniles for invertebrate prey and alter the composition of the invertebrate community in ways that reduce the food resources available to native fish. In highly carp-impacted wetlands, native fish recruitment essentially ceases.

Altered Flows and Flood Events

Murray cod evolved in a river system defined by boom and bust hydrology — drought years with minimal flow, wet years with extensive flooding of the floodplain. The floodplain inundation events that occur during high-flow years are critical for cod reproduction and recruitment: spawning cod are stimulated by rising water, larvae and juveniles disperse onto the floodplain where food resources are enormously abundant, and a proportion of each year class survives to recruitment.

Water extraction for irrigation has reduced the frequency, duration, and extent of these flood events substantially. The Murray's flow regime today is a fraction of its natural variability — the extreme lows and extreme highs that characterised the system have been moderated by extraction and regulation. The ecological consequences for cod are significant: good recruitment years, which depended on flood events, now occur less frequently, meaning the population receives fewer inputs of young fish and is more dependent on the survival of adult fish than was historically the case.

What Is Being Done

Against this background, a series of interventions has been developed and is now being implemented with varying effectiveness.

Restocking programs are the most visible and politically popular intervention, and also the most complex to evaluate. State fisheries agencies have been stocking Murray cod into rivers and impoundments since the 1980s, producing fish in hatcheries and releasing fingerlings at various life stages. The results are genuinely mixed. In impoundments — storages like Lake Eildon, Lake Mulwala, and the Hume Reservoir — stocking has produced recreational fisheries from essentially zero, and the fish in these waters are overwhelmingly hatchery origin. In rivers, the situation is more complicated: hatchery fish have lower survival rates than wild fish, can compete with wild recruits, and may dilute the local genetic adaptations of wild populations if not managed carefully.

Fish passage infrastructure — fishways installed at barriers — addresses the migration barrier problem with increasing sophistication. Modern vertical slot fishways allow a broader range of species and sizes to pass than the old Denil and pool-and-weir designs, and a program of retrofitting existing barriers and requiring fishways on new structures has improved connectivity on some regulated reaches. The limitations are real: fishways work when they're operational, which requires maintenance and flow management, and they don't fully replicate the hydraulic conditions of a free-flowing reach.

Environmental water — water held for environmental purposes under the Basin Plan — is increasingly being used to deliver strategic flow events that stimulate cod spawning and recruitment. Environmental water releases timed to coincide with cod spawning season, creating artificial freshes that trigger the same spawning responses that natural floods historically provided, have produced documented recruitment responses in monitoring programs on the Murrumbidgee and lower Murray.

Cold water pollution mitigation is being addressed at some dams through the installation of selective withdrawal structures — infrastructure that draws from multiple depths in the reservoir and blends water to reduce the temperature suppression in downstream reaches. Lake Hume now has a multilevel offtake that has measurably improved the thermal regime in the Murray below Albury.

The Honest Prognosis

Murray cod is a resilient species in the sense that it has survived everything that has been done to it and remains present across most of its range. It is not recovering in the sense that population-level improvement across the system is being documented. The interventions underway are meaningful; they are not yet sufficient to reverse a trajectory established over 150 years of habitat degradation, flow alteration, and ecological disruption.

The fishing is still extraordinary. A large Murray cod in good habitat is one of the most remarkable freshwater predators in the world. The obligation that comes with catching one is to support the systems and policies that give the species a genuine recovery trajectory — not just to release it and consider the obligation discharged.

The Angler's Role

Recreational anglers are both part of the problem and part of the solution, and being honest about both sides of this is important.

The historical commercial fishery created the initial population collapse. Recreational fishing today operates under size and bag limits specifically designed to allow fish to reach reproductive maturity and to limit extraction from already-reduced populations. The current minimum size limits in most states — 55 centimetres — represent a compromise between the angling community's desire to keep fish and the biological requirement for fish to mature and spawn before harvest. A 55 cm cod is not a large fish. It is a fish that has recently become capable of reproducing for the first time. Taking large numbers of fish at this minimum size limits the number of large, highly fecund individuals in the population — exactly the cohort most important for recovery.

Some anglers practice voluntary catch-and-release across the size range. Others keep their full bag limit at minimum size. Both are legal. The conservation consequences are not equivalent.

Anglers also represent the constituency most directly invested in the health of the Murray cod population, and therefore the most naturally motivated political force for the policy changes that recovery requires: adequate environmental water allocation, carp management at scale, improved cold water mitigation, and restoration of habitat complexity through large woody debris programs. This constituency has historically been underutilised. The Murray cod fishery is a reason to change that.