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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 Fish egg, 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.
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 . 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.
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.
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
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 Fish egg 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.
Female Murray cod, upon first reaching sexual maturity, have egg counts of no more than 10,000. Very large female Murray cod can have egg counts as high as 80,000–90,000, although a recent, very large 33-kg specimen yielded an egg count of 110,000 viable eggs. Egg counts in female Murray cod of all sizes are relatively low compared to many fish species.
Murray cod spawn in spring, cued by rising water temperatures and increasing Photoperiodism (daylight length). Initially, fish biologists working with Murray cod considered spring floods and temperatures of to be necessary and that spring flooding is critical for successful recruitment (i.e. survival to juvenile stages) of young cod by providing an influx of pelagic zooplankton and early life-stage invertebrate off the flood plain into the main river channel for first feeding, but more recent research has shown Murray cod breed annually, with or without spring floods, and at temperatures as low as .
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. Decades of observations by recreational and commercial fishermen suggest such spring spawning movements are common across the Murray cod's geographical range. Spawning is initiated by pairing up and courtship rituals. During the courtship ritual a spawning site is selected and cleaned — hard surfaces such as rocks in upland rivers and impoundments, and logs and occasionally clay banks in lowland rivers, at a depth of , are selected. The female lays the large adhesive eggs as a mat on the spawning surface, which the male fertilises. The female then leaves the spawning site. The male remains to guard the eggs during incubation, which takes six to 10 days (depending on water temperature), and to guard the hatched larvae for a further week or so until they disperse. Larvae disperse from the nest site by drifting in river currents at night, and continue this behaviour around four to seven days.
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;
These findings do not mean that river regulation and water extraction have not had adverse effects on fish stocks. Rather, river regulation has been a major factor in the decline of Murray cod and other native fish. 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 generally rely on periodic spring flooding.
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.
A thorough reading of historical newspaper articles and historical government reports reveals that the history of wild Murray cod between the mid–1800s and the mid–1900s was one of citizen agitation, government inaction, and ongoing stock decline. 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. Fishing regulations were either not amended, or amended and largely unenforced and completely ignored. Heavy commercial, recreational and illegal fishing pressure continued. 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. All of these drivers of decline left this iconic Australian fish in a perilous situation. There are now concerns for the long-term survival of wild Murray cod populations.Victorian Department of Sustainability and Environment, 2004, Codwatch Newsletter Number 25, [4], Retrieved 28 September 2007
A study published in Biological Conservation in March 2023 listed 23 species which the authors considered to no longer meet the criteria as threatened species under the EPBC Act, including the Murray cod. The team, led by John Woinarski of Charles Darwin University looked at all species listed as threatened under the act in 2000 and 2022. The Murray cod was the only fish on the list, and the reason for their assessment was given as "Actual recovery over the period 2000–2022, from long period of decline".
Although angler effects are sometimes disregarded in the overall picture today, recent population studies have shown that while all year classes are well represented up to the minimum legal angling size (now 60 centimetres in most states), above that size, numbers of fish are dramatically reduced almost to the point of non-existence in many waters. Some emphasis has been made of the results of two small surveys which suggested a majority of Murray cod are released by anglers. However, there are valid questions as to the representativeness of these surveys: these surveys do not explain the dramatic disappearance of large numbers of young Murray cod at exactly the minimum size limit, and most importantly, any emphasis on these surveys miss the fundamental point — as a large, long-lived species with relatively low fecundity and delayed sexual maturity wild Murray cod populations are extremely vulnerable to overfishing, even with only modest angler-kill. A tightening of fishing regulations for wild Murray cod, as referred to above, and a switch by fishermen to a largely catch and release approach for wild Murray cod would alleviate this problem. 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. (i.e. only Murray cod between 55 and 75 cm may be taken; those above and below this size range or "slot" must be released.) This measure should have positive effects for the Murray cod population by protecting and increasing the proportion of large breeding Murray cod.
Another issue is that Murray cod caught and released in winter, while developing their eggs, or in spring prior to spawning, resorb their eggs and do not spawn. This may be a minor issue compared to some of the other threats facing Murray cod, nevertheless, concerned fishermen try to avoid catching wild Murray cod at these times. 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.
It is estimated that flows at the river mouth by 1995 had declined to only 27% of natural outflows., retrieved 28 September 2007 The probability of the bottom end of the Murray experiencing drought-like flows had increased from 5% under natural conditions to 60% by 1995.
Thermal pollution is the artificial reduction in water temperatures, especially in summer and autumn, caused when frigid water is released from the bottom of reservoirs for irrigation demands. Such temperature suppression typically extends several hundred kilometres downstream. Thermal pollution inhibits both the breeding of Murray cod and the survival of Murray cod larvae, and in extreme cases inhibits even the survival of adult Murray cod.
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.
Blackwater events are often described as "natural" events—while there are some historical records of relatively severe events in smaller, more ephemeral systems (e.g. lower Lachlan, upper Darling), there is no record of severe events in the Murray River and its largest southern tributaries before water extraction and river regulation. In the Murray and large southern tributaries, very severe large-scale blackwater events are a relatively new but recurring phenomenon and appear to be an effect of river regulation curtailing the winter/spring flood events that formerly swept leaf litter away annually, exacerbated by long-term declines in rainfall and recurring prolonged drought events.
Flood events in 2010 and 2012 following the prolonged Millennium Drought (1997–2009) induced very severe blackwater events; while formal studies of these events were limited due to the relatively rapid response times required and logistical difficulties,
Vegetation clearing and cattle trampling river banks create severe siltation, which fill in pools, degrade river ecosystems and make rivers and streams uninhabitable for Murray cod. This is exacerbated by removal of riparian (riverbank) vegetation which causes siltation and degrades river ecosystems in many ways.
Murray cod continue to play important roles in the culture of First Nations Peoples along the Murray and Darling rivers and the Murray-Darling Basin overall.
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 Fish meal 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 off-stream dams holding water to be used for irrigation of farmland, where the presence of effluent produced by the fish is not a problem.
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.
(1996). 9780730104629, Reed Books. ISBN 9780730104629
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.
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. [2] 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 [3] ), but the North American Black bass and Australian trout-cods are now considered unrelated.
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.
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 European perch, as well as such as common yabby, Atyidae, 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.
Reproduction
Murray cod reach sexual maturity between four and six years of age, generally five years.
Conservation
History
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. Recreational fishermen took similarly excessive hauls during this era, using rods and reels, handlines, setlines, drum nets, gill nets, and even explosives, with hauls often either wasted or illegally sold. Perhaps this extreme overfishing and its impacts of wild Murray cod stocks is best summarised by a short article in the Register News (a South Australian newspaper) in 1929:
In the 29 years 26,214,502 lbs (nearly 11,703 tons) 11,915,683 of Murray cod has been eaten by the people of Melbourne. The Superintendent of Markets (Mr G. B. Minns) included these figures in a statement he made today pointing out that the supply was declining. In 1918, the peak year, was received at the market, but since 1921, when was sent to Melbourne, supply has decreased. Last year 1928 it was only .
Status
Since 3 July 2003 and , the Murray cod is listed as a vulnerable species under the EPBC Act ( Environment Protection and Biodiversity Conservation Act 1999). It is listed as a species of Least Concern on the IUCN Red List of Threatened Species, but under state legislation in both South Australia and Victoria, it is an endangered species.
Threats
Overfishing
While extremely severe commercial and recreational overfishing in the 1800s and the early 1900s caused the first strong declines of Murray cod, overfishing by recreational fishermen, aided by inadequate fishing regulations, continues today and remains an extremely serious threat to Murray cod. The current size limit of 60 centimetres in most states is inadequate now that scientific studies have documented average size at sexual maturity in Murray cod. This and catch data and computer modelling exercises 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. As of November 2014, the NSW Department of Fisheries has introduced a maximum size limit of 75 cm for Murray Cod to provide protection for large breeding fish, as well as a new minimum size limit of 55 cm.
Effects of river regulation
The Murray River and southern tributaries originally displayed a pattern of high flows in winter, high flows and floods in spring, and low flows in summer and autumn. The breeding of Murray cod and other Murray-Darling native fish was adapted to these natural flow patterns. River regulation for irrigation has reversed these natural flow patterns, with negative effects on the breeding and recruitment of Murray cod. The Murray and most southern tributaries now experience high irrigation flows in summer and autumn and low flows in winter and spring. Small and medium floods including the once annual spring flood-pulse have been completely eliminated.
(1997). 9780730667971, Murray River Entitlements Committee. ISBN 9780730667971
Blackwater events
Blackwater events are emerging as a very serious threat to wild Murray cod stocks in lowland river reaches. Blackwater events occur when floodplains and ephemeral channels accumulate large quantities of leaf litter over a number of years and are then finally inundated in a flood event. The leaf litter releases large quantities of dissolved organic carbon, turning the water a characteristic black colour and inducing a temporary explosion in bacterial numbers and activity, which in turn consume dissolved oxygen, reducing them to levels harmful or fatal to fish. (Fish essentially asphyxiate.) Water temperature is a critical regulator of blackwater events as warmer water temperatures increase bacterial activity and markedly reduce the intrinsic oxygen carrying capacity of water; events that may be tolerable for fish in winter or early spring may be catastrophic in late spring or summer due to the increase in water temperature.
Physical barriers to fish movement
Dams, and other instream barriers block the migration of adult and juvenile Murray cod and prevent recolonisation of habitats and maintenance of isolated populations. Additionally, recent study has proven approximately 50% of Murray cod larvae are killed when they pass through undershot weirs.
Habitat degradation / siltation
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.
Introduced carp
There is serious competition for food between larval/early juvenile introduced carp and larval/early juvenile native fish. Introduced carp dominate the fish of lowland Murray-Darling rivers; the sheer amount of biomass carp now take up, and the large numbers of larvae carp produce, causes serious negative effects on river ecosystems and native fish. Carp are the main vector of the introduced Lernaea parasite ( Lernaea cyprinacea) and serious vectors of the introduced Asian fish tapeworm ( Bothriocephalus acheilognathi).
Introduced pathogens
Murray cod have soft skin and very fine scales that leave them particularly vulnerable to infection from exotic diseases and parasites. The following exotic diseases and parasites all seriously affect wild Murray cod; all have been introduced by imports of exotic fish. 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. Saprolegnia is a fungus-like oomycete or "water mould" that frequently infects Murray cod eggs and the skin of Murray cod that have been roughly handled through poor catch and release technique. (It is essential that Murray cod intended for release only touch cool wet surfaces and are not put down on any hard, dry, rough or hot surfaces, e.g. boat gunwales, boat floors, dry grass, dry rocks, gravel banks, dry towels or mats, etc. Hands should also be wetted before touching them. They must not be hung vertically by the mouth or gill covers.) Wild Murray cod populations across their range suffer extremely severe infestations of Lernaea or "anchor worm", a parasitic copepod vectored by introduced carp and that burrows into the skin of 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. Ebner reports a young adult Murray cod seemingly killed by severe Lernaea infestation.
Conservation measures
State government fisheries departments support Murray cod populations by stocking with hatchery-bred fish, especially in man-made lakes. Important issues affecting restoration of cod populations, such as the need for spring floods and excessive angler take, are slowly being acknowledged but are yet to be definitely addressed. 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.
Relationship with humans
Murray cod play a very important role in the mythology of many Aboriginal tribes in the Murray-Darling basin, and for some tribes, particularly those living along the Murray River, Murray cod were the icon species.David Unaipon (2001) Legendary Tales of the Australian Aborigines, p. 17, The Miegunyah Press, Melbourne. . The mythology 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.
Aquaculture
The Murray cod is renowned as a good-tasting fish for eating. In recent years, despite the decline in the wild population, farmed fish are being harvested.
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