[4][6] The species exhibits a high degree of parental care for their eggs, which are spawned in the spring and are generally laid in hollow logs or on other hard surfaces.

[10] Soft dorsal, anal, and caudal (tail) fins are all large and rounded, and are dusky grey or black with distinct white edges.

Ignoring glaring differences in size at sexual maturity, and via some rather unscientific reasoning, some prominent fisheries scientists (e.g. Whitley) insisted on recognising only one species of cod—the Murray cod (then named Maccullochella macquariensis, after an early Australian fish researcher with the surname McCulloch[20] and the Macquarie River in New South Wales where the holotype was captured[20]).

[7] Consequently, Murray cod inhabit a remarkably wide variety of habitats, from cool, clear, fast-flowing streams with riffle-and-pool structure and rocky substrates in upland areas to large, slow flowing, meandering rivers in the extensive alluvial lowland reaches of the Murray-Darling basin.

[8][10] [12][13] Murray cod have died out in many of their upland habitats, particularly in the southern Murray-Darling basin, due to a combination of overfishing, siltation, dams and weirs blocking migration, pollution from arsenic-based sheep-dips, mining, and in some cases, introduced trout stockings, which causes competition between juvenile Murray cod and introduced trout species.

[7] Longevity is a survival strategy in variable Australian environment to ensure that most adults participate in at least one exceptional spawning and recruitment event, which are often linked to unusually wet La Niña years and may only occur every one or two decades.

While this behaviour is typical during the day, at night, Murray cod are active pelagic predators, venturing into shallow waters and frequently taking prey from the surface.

[28][29] Therefore, roughly 70% of wild river Murray cod, with their slower growth rate, have reached sexual maturity by 50 cm (20 in) in length.

Wild Murray cod in impoundments like Lake Mulwala, with their faster growth rates, do not reach sexual maturity until they are well over 60 cm (2.0 ft) in length.

[13][28][29] These data strongly indicate the 50-cm (20-in) size limit for Murray cod is inadequate and should be increased substantially to allow for a greater chance of reproduction before capture.

[10][13][28][29][34] Additionally, recent research has shown abundant epibenthic/epiphytic (bottom dwelling/edge clinging) prey in unflooded lowland rivers,[34][35] traits in Murray cod larvae that should allow survival in a variety of challenging conditions,[36] and a significant proportion of Murray cod larvae feeding successfully in unflooded rivers.

[38] Decades of observations by recreational and commercial fishermen suggest such spring spawning movements are common across the Murray cod's geographical range.

[13][28][29][34][40][41][42][43] During this dispersal process, larvae simultaneously absorb the remainder of their yolk sac and begin to feed on small, early life-stage macroinvertebrates and epibenthic/epiphytic (bottom dwelling/edge clinging) microinvertebrates.

[9] Thermal pollution is also a major problem, evidence indicates strong Murray cod recruitment events (which may be important for sustaining Murray cod populations over the long term) can result from spring flooding, and the health of Australian lowland river ecosystems generally rely on periodic spring flooding.

The long-term viability of wild Murray cod, other native fish species and river ecosystems, in the face of this fact, are of great concern.

For decades, riverside residents, commercial fishermen, recreational fishermen, local fisheries inspectors, fish retailers, and others agitated in newspapers and other fora about the declining Murray cod stocks, to be met in turn either with government denials, or conversely, with various ineffective inquiries into Murray cod stocks and fisheries, and various ineffective control measures.

Debate about excessive fishing pressure, number of fishermen, number of nets, net mesh size, bag limits, minimum size limits and take of small cod, closed seasons and the taking of spawning cod full of eggs during spring, and other sundry issues, continued without resolution.

The end result was a Murray cod population, initially abundant, continually fished down until in the early to mid 20th century a number of other factors such as river regulation (listed below) emerged to drive the species even further into decline.

[16] This and catch data and computer modelling exercises[16] on wild Murray cod stocks indicate measures such as raising the size limit to 70 centimetres and reducing the bag and possession limits from 2 and 4 fish respectively to 1 fish are urgently needed to maintain the long-term viability of wild Murray cod populations.

[16] Recognising these issues, in late 2014 the New South Wales and Victorian fishery departments amended their regulations so that a slot limit of 55 to 75 cm now applies in these states.

[7] At this point in time a closed season is in place for the spring spawning period, during which anglers are not allowed to target Murray cod, even on a catch and release basis.

River regulation for irrigation has reversed these natural flow patterns, with negative effects on the breeding and recruitment of Murray cod.

[7] The rare floods that do break free of the dams and weirs of the Murray-Darling system have their magnitude and duration deliberately curtailed by river regulators.

Increasing research indicates this management practice is very harmful and drastically reduces the general ecosystem benefits and breeding and recruitment opportunities for Murray cod and other Murray-Darling native fish species these now rare floods can provide.

Dams, weirs and other instream barriers block the migration of adult and juvenile Murray cod and prevent recolonisation of habitats and maintenance of isolated populations.

[59] Hundreds of thousands, perhaps more than a million, submerged timber "snags", mainly River Red Gum, have been removed from lowland reaches of the Murray-Darling basin over the past 150 years.

State government fisheries departments support Murray cod populations by stocking with hatchery-bred fish, especially in man-made lakes.

[49] Important issues affecting restoration of cod populations, such as the need for spring floods and excessive angler take, are slowly being acknowledged [49] but are yet to be definitely addressed.

In their search for water to survive in, the Murray cod flocked into the mouths of the few small creeks feeding the lake and died there by the thousands.

It is increasingly farmed, in large dams holding water used for irrigation of farmland, where the presence of effluent produced by the fish is not a problem [70]