Murray cod
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[Photo] Species: Murray Cod (Maccullochella peelii peelii). Family: Percichthyidae. Melbourne Aquarium. Photo by http://en.wikipedia.org/wiki/User:Fir0002 | Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License". |
The Murray cod (Maccullochella peelii peelii) is a large and striking predatory freshwater fish of the Maccullochella genus and the Percichthyidae family. It is an important and charismatic part of Australia's vertebrate wildlife and is found in the Murray-Darling river system in Australia. The Murray cod is the largest wholly freshwater fish in Australia, and one of the largest wholly freshwater fish in the world. Other common names for Murray cod include cod, greenfish and goodoo.
The scientific name of Murray cod derives from an early Australian fish researcher with the surname McCulloch (=Maccullochella) and the river from which the explorer Major Mitchell first scientifically described the species, the Peel River (=peeli). This was later changed to M. peelii peelii to differentiate Murray cod from Mary River cod (see Related Species) which are currently designated as a sub-species of Murray cod.
Description
Murray cod are a large grouper-like fish with a deep, elongated body that is round in cross section. They have a broad, scooped head, and a large mouth lined with pads of very small needle-like teeth. In Murray cod the jaws are equal or the lower jaw protrudes slightly.
In Murray cod the spiny dorsal fin 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 flanks. The pelvic fins are large and 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”) surface. Their back and flanks are usually yellowish-green to green, overlain with heavy darker green (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 have striking and extremely beautiful colouration. Very large fish tend towards a speckled grey-green colouration.
Related Species
Murray cod continue a pattern present in Murray-Darling native fish genera of speciation into lowland and specialist upland species, with Murray cod being the primarily lowland species and the endangered trout cod being the specialist upland species. The pattern is slightly blurred in the cod species because, being such adaptable, versatile and successful fish, Murray cod push significant distances into upland habitats, while trout cod stray (or did stray) 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 clear.
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 sub-species 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
Originally only one species of cod was recognised ??? Murray cod ??? then named Maccullochella macquariensis. In the 1970s the trout cod was recognised as a separate species and re-examination 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, was coined for the Murray cod. Subsequently two further cod were identified, a separate species, the eastern freshwater cod (M. ikei), and a sub-species, the Mary River cod (M. peelii mariensis), with the later causing Murray cod to be re-named M. peelii peelii). The latest research suggests that Mary River cod may actually be a sub-species of eastern freshwater cod which would give them the designation as M. ikei mariensis.
Range
The Murray cod is named after the Murray River. The Murray River is part of the Murray-Darling Basin in eastern Australia, Australia's largest and most important river system, draining approximately 13% of the continent. The Murray cod's natural range encompasses virtually the whole Murray-Darling Basin, particularly the lowland areas, but extending well into upland areas; to ~700m elevation in the southern half of the basin and to ~1000m in the northern half of the basin.
Consequently, Murray cod inhabit a remarkably wide variety of habitats, from cool, clear 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 (Cadwallader and Backhouse, 1983; Rowland, 1989; McDowall, 1996; Rowland, 2005). Unfortunately, Murray cod have died out of 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 and mining and possibly, in some cases, introduced trout stockings and competition between juvenile Murray cod and introduced trout species (Rowland, 2005).
Murray cod played a very important role in the mythology of many Aboriginal tribes in the Murray-Darling Basin, and for some tribes, particular those living along the Murray River, Murray cod were the icon species. 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 and the beating of its tail enlarges the river and creates 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. All 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 (Rowland, 1989, 2005).
Size
Murray cod are large fish with adult fish regularly reaching 80???100 cm in length in all but the very smallest waterways. Murray cod are capable of growing well over 1 m in length and the largest on record was over 1.8 m in length and approximately 113 kg in weight (Rowland, 1989; McDowall, 1996; Rowland, 2005). Large breeding fish are rare in most wild populations today due to excessive angler take and inadequate fishing regulations.
Age
Murray cod continue the trend of native fish of southeast Australia being incredibly long-lived. 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 Ni??a years and may only occur every one or two decades. Murray cod are in fact the most long-lived native fish of all in southeast Australia. The oldest Murray cod aged yet was 48 years of age (Anderson et al., 1992), and the even larger specimens of yester-year leave us in no doubt the species reaches considerably greater ages of 70 or more years (Rowland, 2005).
Diet
The Murray cod is an apex predator that eats almost anything smaller than itself including finned fishes such as introduced carp, goldfish, redfin, native golden perch, boney bream, eel-tailed catfish, western carp gudgeon and Australian smelt, and crustaceans such as yabbies, shrimp and Murray River crayfish. A recent study has confirmed that fish make up the majority of the diet of Murray cod in lowland river and impoundment habitats, and that Murray cod are indeed apex predators in these habitats (Ebner, 2006). Murray cod have also been known to eat ducks, cormorants, freshwater tortoises, water dragons, snakes, mice and frogs. The observations of the many recreational fishermen now fishing for Murray cod with surface lures at night reveal that the characterisation of Murray cod as a demersal ambush predator is only partially correct. While this behaviour is typical during the day, Murray cod at night are active pelagic predators, venturing into shallow waters and frequently taking prey from the surface.
Reproduction
Murray cod reach sexual maturity between 4 and 6 years of age (Rowland, 1998a, 1998b, 2005). Sexual maturity in Murray cod is dependent on age (Rowland, 1998a, 1998b, 2005). Therefore roughly 70% of wild river Murray cod, with their slower growth rate, have reached sexual maturity by 50 cm in length. Wild Murray cod in impoundments like Lake Mulwala, with their faster growth rates, do not reach sexual maturity until well over 60 cm in length (Rowland, 1998a, 1998b, 2005). Overall, this data strongly indicates that the 50 cm size limit for Murray cod is inadequate and should be increased substantially to allow greater chance of reproduction before capture.
Large female Murray cod that are in the 15???30 kg range and "in their prime" are the most important breeders because they produce the most eggs and for other reasons; research is now showing 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 (e.g. Trippel, 1995; Marteinsdottir & Steinarsson, 1998; Marteinsdottir & Begg, 2002). Both 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. Such large females may also have valuable, successful genes to pass on.
Female Murray cod, upon first reaching sexual maturity, will have egg counts of around <10,000. Very large female Murray cod can have egg counts as high as 80,000???90,000 (Rowland, 1989a).
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 photoperiod (daylight length). Initially spring floods and temperatures of 20???21° C were considered necessary, but research has shown Murray cod breed annually, with or without spring floods, and at temperatures as low as 15° C (Rowland, 1998a, 1998b; McDowall, 1996; Rowland, 2005; Humphries, 2005).
Latest research has also shown that Murray cod are main channel specialists at all life stages. Earlier ideas that Murray cod spawn on floodplains, or that Murray cod 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 (Koehn, in prep.).
Spawning is preceded by significant upstream migrations if high spring flows or floods allow. Radio-tracked Murray cod in the Murray River have migrated up to 120 kilometres upstream to spawn, before returning to the exact same snag they departed from, an unusual homing behaviour in a freshwater fish (Koehn, in prep.). Decades of observations by recreational and commercial fishermen suggest such spring spawning migrations 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 logs and occasionally clay banks in lowland rivers, at a depth of 2 to 3 m, 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 6???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 for aound 4???7 days (Lake, 1967; Cadwallader, 1977; Rowland, 1983, 1998a, 1998b, 2005; Humphries, 2005; Koehn and Harrington, 2005). During this dispersal process larvae simultaneously absorb the remainder of their yolk sac and begin to feed on pelagic zooplankton, small, early life-stage macroinvertebrates and epibenthic/epiphytic (bottom dwelling/edge clinging) microinvertebrates.
While it is now recognised Murray cod are annual spawners, and breed with or without spring floods, until recently spring flooding has been considered critical for successful recruitment (i.e. survival to juvenile stages) of young cod by providing an influx of pelagic zooplankton and early life-stage macroinverebrates off the flood plain into the main river channel for first feeding.
More recent research has shown abundant epibenthic/epiphytic (bottom dwelling/edge clinging) prey in non-flooded lowland rivers (King, 2004), traits in Murray cod larvae that should allow survival in a variety of challenging conditions (Humphries, 2005), and a significant proportion of Murray cod larvae feeding successfully in non-flooded rivers (King, 2004; Kaminskas and Humphries, in prep.).
This information shows that the relationship between river flows and Murray cod recruitment are more complex than first thought, and that 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:
non-river-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 (also see Tonkin et al., 2006); and
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.
These findings do not deliver river regulation and water extraction from blame however ??? river regulation has been a major factor in the decline of Murray cod and other native fish. Thermal pollution is a major problem (see below), there is evidence that strong Murray cod recruitment events (which may be important for sustaining Murray cod populations over the long term) can result from spring flooding, and copious evidence that the health of Australian lowland river ecosystems generally rely on periodic spring flooding. Excessive water extraction meanwhile has caused numerous fish kills of late.
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.
Conservation
Once the most common large native fish in the Murray-Darling Basin, the first serious declines in Murray cod were caused by gross overfishing; in the latter half of the 1800s and the early 1900s Murray cod were caught in unimaginable numbers by both commercial and recreational fishermen (Rowland, 1989, 2005). Further serious declines in the 20th century have been driven by:
River regulation for irrigation. 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, not surprisingly, adapted to these natural flow patterns. River regulation 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.
Reductions in flood duration. 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 irresponsible 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 all-too-rare floods can provide.
Excessive water extraction. It is estimated somewhere in the vicinity of 80% of the Murray-Darling Basin's flows are now extracted for irrigation.
De-snagging. 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.
Overfishing. Inadequate fishing regulations and continued gross overfishing by recreational fishermen are an extremely serious threat to Murray cod. The current size limit of 50 cm in most states (Queensland has a 60 cm size limit) is seriously inadequate now that scientific studies have documented average size at sexual maturity in Murray cod. This and catch data and computer modelling exercises (Nicol et al., 2005) on wild Murray cod stocks indicate measures such as raising the size limit to 70 cm 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. At this point there are no immediate moves to implement these reforms.
Thermal pollution. 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. 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.
Barriers to migration. Dams and weirs block the migration of adult and juvenile Murray cod and prevent recolonisation of habitats and maitenance of isolated populations.
Weirs killing Murray cod larvae and juveniles. A recent study has proven approximately 50% of Murray cod larvae are killed when they pass through undershot weirs (Baumgartner et al., 2006).
Siltation. 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;
Removal of riparian vegetation. Removal of riparian ("riverbank") vegetation causes siltation and degrades river ecosystems in many ways.
Competition for food from introduced carp at the larval/early juvenile stages. Suspicions are mounting that there is serious competition for food between larval/early juvenile introduced carp and larval/early juvenile native fish. Introduced carp grotesquely dominate the fish faunas of lowland Murray-Darling rivers; the sheer amount of biomass carp now take up, and the sheer numbers of larvae carp produce, must be having serious negative effects on river ecosystems and native fish (also see Tonkin et al., 2006).
Introduced disease organisms. Murray cod have soft skin and fine scales that leave them vulnerable to infection from exotic disease organisms. The following exotic disease organisms all seriously affect wild Murray cod; all have been introduced by reckless imports of exotic fish. Chilodenella 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 that frequently infects Murray cod eggs and the skin of Murray cod that have been roughly handled through poor catch and release technique. Wild Murray cod populations across their range suffer extremely severe infestations of Lernaea or "anchor worm", a parasitic copepod that burrows into the skin of Murray cod. Severe Lernaea infestations probably causes the death of many more adult Murray cod than commonly recognised. Ebner (2006) reports a young adult Murray cod killed by severe Lernaea infestation.
All of these drivers of decline have left this iconic Australian fish in a perilous situation. There are now grave concerns for the long term survival of wild Murray cod populations.
Although angler effects are sometimes disregarded in the overall picture, recent population studies have shown that while all year classes are well represented up to the minimum legal angling size (50 cm in most states) above that size numbers of fish are dramatically reduced almost to point of non-existence in many waters. Some light has been made 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.
A little recognised 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 (Lake, 1967; Rowland, 1988, 2005). 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.
State Government Fisheries departments support Murray cod populations by stocking with hatchery bred fish, especially in man made lakes, but the most important issues effecting restoration of cod populations such as the need for spring floods and excessive angler take are yet to be addressed.
In Aquariums
Hatchery bred Murray cod are now readily available in aquarium stores in southern Australia. Murray cod are intelligent fish and make immensively attractive, rewarding and responsive aquarium fish. It is widely agreed by everyone who has kept a Murray cod in an aquarium that they are fish with "real personalities".
Watching one of these predatory native fish ambush a live shrimp or fish can be an awe-inspiring display.
Whilst slow growing overall, Murray cod will grow quickly in their first 3???4 years of life, so large tanks are advisable ??? a minimum of 4 ft in length, and preferably 5 or 6 ft. Murray cod like having structure they can retreat to, so structure such as sunken timber pieces will be appreciated.
Murray cod experience very cold water temperatures in the wild in winter, so they do not need water heaters. A temperature of 24° C however will enable fast (optimum) growth, which given the large size Murray cod reach, is generally not desirable. Some experienced aquarists use water heaters and a temperature of 24° C to grow Murray cod quickly to a size (15???20 cm) where they can be conveniently fed on feeder fish, fish pieces and other foods, whereupon they gradually turn off and remove the water heater to slow growth rates back to normal levels. Conversely, Murray cod are susceptible to extremely high water temperatures ??? temperatures above 30° C are not recommended, and temperatures significantly above 30° C will cause death. In heatwave conditions in the summer of 2004/2005 a number of aquarists in Australia suffered unfortunate losses as house and tank temperatures soared far above usual levels. Soft drink bottles filled with water and frozen can be used as an emergency measure to reduce tank temperatures.
Unlike many freshwater fish, Murray cod are highly tolerant of salt due to the raised salinity levels they sometimes experience in the lowland reaches of the Murray-Darling Basin during extreme drought. Indeed, common sea salt is a useful tool for keeping Murray cod in tanks, a salinity of 2 grams per litre improves health and keeps most pathogens ("diseases") at bay. Ensure, however, that the sea salt used is free of additives such as iodine and free flow agent.
Caution, and ideally, quarantine procedures, should be used to ensure that feeder fish do not introduce diseases to the tank. With soft skin, Murray cod can be susceptible to white-spot or "ich" and fungal skin infections. At a minimum, disinfect all live food for 1 hour in a 10 gram per litre salt solution before feeding.
Water pH should be kept neutral or slightly alkaline, a pH between 7.0 and 8.0 is perfectly acceptable, 7.0???7.5 is ideal. A small amount of shell grit will provide a buffer against violent pH swings.
A common complaint voiced about small Murray cod in aquariums is that they "hide all day" and "don't show themselves". This is usually only a problem for the first few months of life. Most Murray cod in aquariums, upon reaching a certain size, suddenly gain confidence and then become inquisitive, responsive and rewarding aquarium fish.
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