Murray Cod

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. …

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 700 m (2,300 ft) elevation in the southern half of the basin and to about 1,000 m (3,300 ft) 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.

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 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.

Murray cod were originally the most common large native fish in the Murray-Darling basin. Contrary to some fishery department literature, the first serious declines in Murray cod were caused by overfishing. In the latter half of the 1800s and the early 1900s, Murray cod were caught in large numbers by both commercial and recreational fishermen. For example, one commercial fishing operation commenced on the Murray River near Echuca in 1855, targeting Murray cod over hundreds of kilometres of river, and yet within eight years, grave concerns over the sustainability of this operation, and complaints about the near-absence of Murray cod in their heavily fished grounds, were being raised in the main state newspaper, The Argus. 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. Similarly, in 1883, more than 147,000 kg of Murray cod were sent to Melbourne from just one river town (Moama). By the 1920s Murray cod had been overfished to the point where large-scale commercial fishing operations were no longer feasible. …