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The largest Australian predatory freshwater fish in the family Percichthyidae

The Murray cod (Maccullochella peelii) is a large Australian predatory freshwater fish of the genus Maccullochella in the family Percichthyidae. Although the species is called a cod in the vernacular, it is not related to the Northern Hemisphere marine cod (Gadus) species. The Murray cod is an important part of Australia's vertebrate wildlife—as an apex predator in the Murray Darling River system—and also significant in Australia's human culture. The Murray cod is the largest exclusively freshwater fish in Australia, and one of the largest in the world. Other common names for Murray cod include cod, greenfish, goodoo, Mary River cod, Murray perch, ponde, pondi and Queensland freshwater cod. Murray cod populations have declined severely since European colonisation of Australia due to a number of causes, including severe overfishing, river regulation, and habitat degradation and are now a listed threatened species. However, they once inhabited almost the entire Murray Darling basin, Australia's largest river system, in very great numbers. A long lived fish, adult Murray cod are carnivorous and eat crustaceans (shrimp, yabbies, crays), fish and freshwater mussels. 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. Murray cod are a popular angling target and aquaculture species. Often available through the aquarium trade, they are also a popular aquarium species in Australia. that is round in cross section. It has a broad, scooped head, and a large mouth lined with pads of very small, needle like teeth. The jaws of the Murray cod are equal, or the lower jaw protrudes slightly. The spiny dorsal fin of Murray cod is moderate to low in height and is partially separated by a notch from the high, rounded soft dorsal fin.

Murray cod are large fish, with adult fish regularly reaching in length. Murray cod are capable of growing well over in length and the largest on record was over and about in weight.

Murray cod continue a pattern present in Murray Darling native fish genera of speciation into lowland and specialist upland species: Murray cod are the primarily lowland species and the endangered trout cod are the specialist upland species. The pattern is slightly blurred in the cod species because, being adaptable and successful fish, Murray cod push significant distances into upland habitats, while the now endangered trout cod stray (or did stray, before their decline) well down the upland/lowland transition zone, which can be extensive in Murray Darling Rivers. Nevertheless, the basic pattern of speciation into a primarily lowland species and a specialist upland species is present.

In Mitchell's original description, he classified the fish as "Family, Percidae; Genus, Acerina; Subgenus, Gristes, Cuv. or Growler; Species, Gristes peelii mihi, or Cod perch", observing "This fish may be identical with the fish described by MM. Cuvier and Valenciennes Volume 3 page 45 under the name of Gristes macquariensis: but it differs from their description ". In the 1800s and early 1900s, commercial fishermen, recreational fishermen, riverside residents, and some fisheries scientists (e. g. Anderson, Stead, Langtry) distinctly recognised two species of cod in the southern Murray Darling basin, Murray cod and trout cod or "blue nose cod". Taxonomically however, confusion abounded. 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 and the Macquarie River in New South Wales where the holotype was captured The Murray cod's natural range encompasses virtually the whole Murray Darling basin,
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. Murray Cod have also been introduced into other drainage basins, such as the Cooper Basin in Queensland.

Murray cod are very long lived, which is characteristic of many freshwater native fish in Australia. and the even larger specimens of years past leave little doubt that the species can reach considerably greater ages, of 70 years or more. but crustaceans tend to dominate the diet under natural conditions, and freshwater mussels were commonly eaten in the past.

Murray cod reach sexual maturity between four and six years of age, generally five years. Sexual maturity in Murray cod is dependent on age. Such large females may also have valuable, successful genes to pass on. Murray cod spawn in spring, cued by rising water temperatures and increasing photoperiod (daylight length). Initially, fish biologists working with Murray cod considered spring floods and temperatures of to be necessary Additionally, recent research has shown abundant epibenthic/epiphytic (bottom dwelling/edge clinging) prey in unflooded lowland rivers, traits in Murray cod larvae that should allow survival in a variety of challenging conditions, and a significant proportion of Murray cod larvae feeding successfully in unflooded rivers. Latest research has also shown that Murray cod in fact live their entire lifecycle within the main channel of the stream. Earlier ideas that Murray cod spawn on floodplains, or the larvae feed on floodplains, are incorrect. Murray cod breed in the main river channel or, in times of spring flood, the inundated upper portion of the main channel and tributary channels, but not on floodplains. Murray cod larvae feed within the main river channel or, in times of spring flood, on the inundated upper portion of the main channel and the channel/floodplain boundary, but not on the floodplain. Spawning is sometimes preceded by upstream or downstream movements. Radio tracked Murray cod in the Murray River have moved up to upstream to spawn, before returning to exactly the same snag from where they departed, an unusual homing behaviour in a freshwater fish. 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. The relationship between river flows and Murray cod recruitment are more complex than first thought, and in less regulated rivers, Murray cod may be able to recruit under a range of conditions including stable low flows. (Significant recruitment of Murray cod in low flow conditions in less regulated lowland rivers has now been proven.) This information also suggests that nonriver regulation related causes of degradation are playing a larger role in the survival and recruitment of Murray cod larvae than first thought; competition from extremely large numbers of invasive carp larvae are negatively affecting the survival and recruitment of Murray cod larvae to a much greater degree than first thought; and that decades of overfishing is playing a far larger role in the current state of Murray cod stocks, through depletion of spawning adults, than first thought. Also, due to the regulation of most of the rivers in the Murray Darling River system, mainly for irrigation purposes, only exceptional spring floods manage to "break free". 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. Yet fishing effort continued to increase in the region, so in the late 1880s and early 1890s, between 40,000 and 150,000 kg of mostly Murray cod (between 7,500 and 27,000 fish, at an average weight of 5.5 kg) were caught near Echuca.</blockquote>

Twenty years later, the aquatic ecologist J. O. Langtry criticised the heavy fishing pressure, in the form of both uncontrolled small scale commercial fishing and rampant illegal fishing, which he found in all reaches of the Murray River he investigated 1949–1950.

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.

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. The removal of such a vast number of snags has had devastating impacts on Murray cod and river ecosystems. Snags are critical habitats and spawning sites for Murray cod. Snags are also critical for the functioning of lowland river ecosystems — as one of the few hard substrates in lowland river channels composed of fine silts snags are crucial sites for biofilm growth, macroinvertebrate grazing and general in stream productivity. Chilodonella is a single celled, parasitic protozoa that infects the skin of Murray cod and has caused a number of serious kills of wild Murray cod. Lernaea puncture wounds are often secondarily infected by bacteria. Severe Lernaea infestations probably causes the death of many more adult Murray cod than commonly recognised. Other concerns such as the stocking of Murray cod in areas where trout cod (M. macquariensis) are recovering encourages hybridisation and needs consideration for future restocking programs.

Murray cod play a very important role in the mythology of many Aboriginal tribes in the Murray Darling basin, The myths of these tribes describe the creation of the Murray River by a gigantic Murray cod fleeing down a small creek to escape from a renowned hunter. In these myths, the fleeing Murray cod enlarges the river and the beating of its tail create the bends in it. The cod is eventually speared near the terminus of the Murray River, chopped into pieces, and the pieces thrown back into the river. The pieces become all the other fish species of the river. The cod's head is kept intact, told to "keep being Murray cod", and also thrown back into the river. It has long been known that Murray cod could be translocated to impounded water. In the 1850s, landholder Terence Aubrey Murray stocked a large and beautiful billabong—Murray's Lagoon just north of Lake George—with Murray cod fished out of the Molonglo River at Murray's other property of Yarralumla. At some time the billabong overflowed and introduced Murray cod into the slightly brackish lake itself. They bred rapidly, and, from the 1850s to the 1890s, Lake George abounded with them. Due to the lengthy Federation Drought, by 1902, the lake dried out completely. 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. Farming Murray cod uses fishmeal and fish oil as food, but the species has a better ‘wild fish in to farmed fish out’ ratio than other farmed species such as rainbow trout and Atlantic salmon. It commands a premium price compared to those species. 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