The Murray cod (Maccullochella peelii) is a large Australian predatory freshwater fish of the genus Maccullochella in the family Percichthyidae.Dianne J. Bray & Vanessa J. Thompson (2011) Murray Cod, Maccullochella peelii. Fishes of Australia. Retrieved 29 August 2014 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.
The scientific name of Murray cod derives from an early Australian fish researcher Allan Riverstone McCulloch and the river from which the explorer Major Mitchell first scientifically described the species, the Peel River. This was for a number of years changed to M. peelii peelii to differentiate Murray cod from Mary River cod, which were designated as a subspecies of Murray cod. However, as of 2010, Mary River cod have been raised to full species status (M. mariensis), thus Murray cod have reverted simply to M. peelii.
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.
Description
The Murray cod is a large grouper-like fish with a deep, elongated body 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. Soft dorsal, anal, and caudal (tail) fins are all large and rounded, and are dusky grey or black with distinct white edges. The large, rounded pectoral fins are usually similar in colour to the flanks. The pelvic fins are large, angular, and set forward of the pectoral fins. The leading white-coloured rays on the pelvic fins split into two trailing white filaments, while the pelvic fins themselves are usually a translucent white or cream, tending toward opacity in large fish.
Murray cod are white to cream on their ventral (belly) surfaces. Their backs and flanks are usually yellowish-green to green, overlain with heavy darker green, but occasionally brown or black, mottling. The effect is a marbled appearance sometimes reminiscent of a leopard's markings. Colouration is related to water clarity; colouration is intense in fish from clear water habitats. Small to medium-sized Murray cod from clear-water habitats often have striking and very distinct colouration. Very large fish tend towards a speckled grey-green colouration.
Size 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. Large breeding fish are rare in most wild populations today due to overfishing.
Related species
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.
Murray cod, like a number of other Murray-Darling native fish species, have also managed to cross the Great Dividing Range at least once through natural river capture events, leading to several species and subspecies of coastal cod. The best known are eastern freshwater cod of the Clarence River system in northern New South Wales, and Mary River cod of the Mary River system in south eastern Queensland, both of which are endangered, but survive today. Coastal cod were also found in the Richmond River system in northern New South Wales and the Brisbane River system in southern Queensland, but are now extinct.
Taxonomy 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…".Mitchell, Thomas Livingstone. Three Expeditions into the Interior of Eastern Australia … 2 ed., vol 1. Cuvier had placed the largemouth bass and blue nose cod into a single genus, Gristes (Le Règne Animal…, rev. ed., t. 2 (1829), p. 145 ), but the North American black basses and Australian trout-cods are now considered unrelated.
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). Then, as trout cod declined into near extinction over the 1900s, the distinction between the two species was further eroded and finally questioned. In the 1970s, early genetic techniques confirmed that trout cod were a separate species and further showed that the original "Murray cod" specimen was in fact a trout cod. Following the rules of scientific classification, the name M. macquariensis remained with the original specimen, now known to be the trout cod, and a new name, M. peelii, for the Peel River where the new holotype was captured, was coined for the Murray cod. Subsequently, two further cod were identified as separate species, the eastern freshwater cod (M. ikei) and the Mary River cod (M. mariensis).
Range The Murray cod is named after the Murray River, part of the Murray-Darling basin in eastern Australia, Australia's largest and most important river system, draining around 14% of the continent. The Murray cod's natural range encompasses virtually the whole Murray-Darling basin, particularly the lowland areas, and extending well into upland areas — to about elevation in the southern half of the basin and to about in the northern half of the basin.
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. 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.
Age Murray cod are very long-lived, which is characteristic of many freshwater native fish in Australia. 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. Murray cod are the most long-lived freshwater native fish in Australia. The oldest Murray cod aged yet was 48 years of age, and the even larger specimens of years past leave little doubt that the species can reach considerably greater ages, of 70 years or more.
Diet The Murray cod is the apex aquatic predator in the rivers of the Murray-Darling basin, and will eat almost anything smaller than itself, including finned fishes such as smaller Murray cod, golden perch, silver perch, bony bream, eel-tailed catfish, western carp gudgeon, and Australian smelt and introduced fish such as carp, goldfish, and redfin (English perch), as well as crustaceans such as yabbies, freshwater shrimp, and Murray crayfish. Fish are eaten when abundant by mature Murray cod in lowland river and impoundment habitats, but crustaceans (shrimp, yabbies, crayfish) are likely the preferred diet under 'natural' river conditions, and mussels were an important food source, at least in the past. but crustaceans tend to dominate the diet under natural conditions, and freshwater mussels were commonly eaten in the past. Murray cod have also been known to eat ducks, cormorants, freshwater turtles, water dragons, snakes, mice, and frogs. The observations of the recreational fishermen fishing for Murray cod with surface lures at night reveal that the popular description of Murray cod as a demersal ambush predator is only partially correct. 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.
Reproduction
Murray cod reach sexual maturity between four and six years of age, generally five years. Sexual maturity in Murray cod is dependent on age. Therefore, roughly 70% of wild river Murray cod, with their slower growth rate, have reached sexual maturity by 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 in length. 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.
Large female Murray cod in the 15– to 35-kg (35– to 80-lb) range are the most important breeders because they produce the most eggs and for other reasons; large females in most fish species are also important because they produce larger larvae with larger yolk sacs, and are also more experienced breeders that display optimal breeding behaviours. Such large females may also have valuable, successful genes to pass on. All of these factors mean the spawnings of large female fish have far higher larval survival rates and make far greater reproductive contributions than the spawnings of small female fish. Not surprisingly, there is no truth to claim made by some recreational fishers that "large Murray cod don't breed".