WikiPortallus: Murray cod.html

The Murray cod (Maccullochella peelii peelii) is a large Australian predatory freshwater fish of the Maccullochella genus and the Percichthyidae family. Although the species is a called cod in the vernacular, it is not related to the northern hemisphere marine cod (Gadus) species. The Murray cod 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 exclusively freshwater fish in Australia, and one of the largest 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 and the river from which the explorer Major Mitchell first scientifically described the species, the Peel River. This was later changed to M. peelii peelii to differentiate Murray cod from Mary River cod, which are currently designated as a subspecies of Murray cod.

Murray cod populations have declined severely since European colonisation of Australia due to a number of causes including 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 mainly eat other fish. 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

Murray cod are a large grouper-like fish with deep, elongated bodies¹ that are round in cross section. They have 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.²³⁴

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.³ Soft dorsal, anal and caudal (tail) fins are all large and rounded, and are dusky grey or black with distinct white edges.¹⁴ The large, rounded pectoral fins are usually similar in colour to the 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,³ 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.³⁴ Their back and flanks are usually yellowish-green to green, overlain with heavy darker green, but occasionally brown or black, mottling.²³⁴ The effect is a marbled appearance sometimes reminiscent of a leopard's markings. Colouration is related to water clarity;⁴ 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.⁵

Size

Murray cod are large fish, with adult fish regularly reaching 80–100 centimetres (32–39 in) in length in all but the very smallest waterways. Murray cod are capable of growing well over 1 metre (3 ft) in length and the largest on record was over 1.8 metres (6 ft) and approximately 113 kilograms (250 lb) in weight.³⁶⁷ Large breeding fish are rare in most wild populations today due to overfishing.⁸⁹¹⁰¹¹

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.⁵

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.⁴¹²

Taxonomy

Originally only one species of cod was recognised, the Murray cod, which was then named Maccullochella macquariensis, after an early Australian fish researcher with the surname McCulloch¹³ and for the Macquarie River in New South Wales where the holotype was captured.¹³ In the 1970s the trout cod was recognised as a separate species and re-examination showed that this 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 River¹³ where the new holotype was captured, 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 latter 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.⁴

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 14% of the continent.¹¹⁴ The Murray cod's natural range encompasses virtually the whole Murray-Darling Basin,²³ particularly the lowland areas, and extending well into upland areas — to about 700 m (2,297 ft) elevation in the southern half of the basin and to about 1,000 m (3,281 ft) in the northern half of the basin.⁴

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.¹³⁶⁷ 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 possibly, in some cases, introduced trout stockings, which causes competition between juvenile Murray cod and introduced trout species.⁷

Age

Murray cod are very long-lived, which is characteristic of many freshwater native fish in Australia.⁴ 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.⁴ Murray cod are the most long-lived freshwater native fish in Australia.¹ The oldest Murray cod aged yet was 48 years of age,¹⁵ and the even larger specimens of years past leave little doubt that the species can reach considerably greater ages, of 70 years or more.⁷

Diet

The Murray cod is an apex predator that will eat almost anything smaller than itself, including finned fishes such as introduced carp, goldfish, redfin, native golden perch, bony 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 compose the majority of the diet of mature Murray cod in lowland river and impoundment habitats, and that Murray cod are apex predators in these habitats.¹⁶ Murray cod have also been known to eat ducks, cormorants, freshwater tortoises, 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.¹⁷

Reproduction

Murray cod reach sexual maturity between four and six years of age.⁷¹⁸¹⁹ Sexual maturity in Murray cod is dependent on age.¹⁸¹⁹ Therefore, roughly 70% of wild river Murray cod, with their slower growth rate, have reached sexual maturity by 50 centimetres (20 in) 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 60 centimetres (2 ft) in length.⁷¹⁸¹⁹ This data strongly indicates that the 50 centimetres (20 in) size limit for Murray cod is inadequate and should be increased substantially to allow greater chance of reproduction before capture. In recognition of this it is noted that New South Wales has recently increased the minimum size to 55 centimetres (22 in) and will move to 60 centimetres (2 ft) at the end of 2008. ²⁰²¹²² Victorian Fisheries are expected to follow suit soon.

Large female Murray cod that are in the 15–35 kilogram (33-66 lb) range are the most important breeders because they produce the most eggs and for other reasons;⁷ 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.¹¹²³²⁴²⁵ Such large females may also have valuable, successful genes to pass on.¹¹ Al 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.¹¹²³²⁴²⁵

Female Murray cod, upon first reaching sexual maturity, will have egg counts of no more than around 10,000. Very large female Murray cod can have egg counts as high as 80,000–90,000⁶, although a recent, very large 33 kg specimen yielded an egg count of 110,000 viable eggs.¹¹ Egg counts in female Murray cod of all sizes are relatively low compared to many fish species.¹¹²⁶

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 20–21° Celsius (68-70°F) to be necessary²⁶ 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 macroinvertebrates 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 15° Celsius (59°F).³⁷¹⁸¹⁹²⁷ Additionally, recent research has shown abundant epibenthic/epiphytic (bottom dwelling/edge clinging) prey in non-flooded lowland rivers,²⁷²⁸ traits in Murray cod larvae that should allow survival in a variety of challenging conditions, ²⁹ and a significant proportion of Murray cod larvae feeding successfully in non-flooded rivers.²⁹³⁰

Latest research has also shown that Murray cod in fact live their entire life cycle within the main channel of the stream. 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.³¹

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 (75 mi) upstream to spawn, before returning to the exact same snag they departed from, an unusual homing behaviour in a freshwater fish.³¹ Decades of observations by recreational and commercial fishermen suggest such spring spawning migrations are common across the Murray cod's geographical range.⁴ 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 2-3 metres (6-10 ft), 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 ten 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 around four to seven days.⁷¹⁸¹⁹²⁷³²³³³⁴³⁵ 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.²⁹³⁰

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;³⁶ 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.⁸

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.² Thermal pollution is also 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.⁸ 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.⁸

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.⁶⁷ Further serious declines in the 20th century have been driven by a number of factors, which are listed below.

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.³⁷³⁸

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 centimetres in most states), above that size, numbers of fish are dramatically reduced almost to the point of non-existence in many waters.¹⁰³⁷ 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.¹⁰³⁷ 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.¹⁰

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.⁷³²³⁹ 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.⁴ 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.³⁷ Important issues affecting restoration of cod populations, such as the need for spring floods and excessive angler take, although yet to be addressed,³⁷ will be dealt with in the National Recovery Plan when it is completed.⁴⁰

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.²³³⁷⁴¹

It is estimated that flows at the river mouth by 1995 had declined to only 27% of natural outflows.⁴² The probability of the bottom end of the Murray experiencing drought like flows had increased from 5% under natural conditions to 60% by 1995.⁴²

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.⁴

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.²³³⁷⁴¹

Physical barriers to fish movement

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.⁴ Additionally, recent study has proven approximately 50% of Murray cod larvae are killed when they pass through undershot weirs.⁴³

Habitat modification

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.⁴⁴ 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.³⁷⁴⁴

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.³ This is exacerbated by removal of riparian (riverbank) vegetation which causes siltation and degrades river ecosystems in many ways.⁴

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 centimetres in most states (Queensland has a 60 centimetre (2 ft) size limit) is seriously inadequate now that scientific studies have documented average size at sexual maturity in Murray cod.¹⁰ This and catch data and computer modelling exercises¹⁰ 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. At this point there are no immediate moves to implement these reforms.

Introduced carp

Evidence is mounting that there is serious competition for food between larval/early juvenile introduced carp and larval/early juvenile native fish.³⁶ Introduced carp dominate the fish faunas 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.³⁶

Introduced pathogens

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 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. Ebner¹⁶ reports a young adult Murray cod killed by severe Lernaea infestation.

Relationship with humans

Murray cod play a very important role in the mythology of many Aboriginal tribes in the Murray-Darling Basin,² 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 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.⁶⁷

In aquariums

Hatchery bred Murray cod are now readily available in aquarium stores in southern Australia. Murray cod make immensely attractive, rewarding and responsive aquarium fish.⁴ Watching one of these predatory Australian native fish attack and eat a live shrimp or fish can be an awe-inspiring display.

While slow growing overall, Murray cod will grow quickly in their first three to four years of life, so large tanks are advisable — a minimum of four ft in length, and preferably five or six foot.⁴ Murray cod like having structure they can retreat to, so structure such as sunken timber pieces will be appreciated.²⁶

Murray cod experience very cold water temperatures in the wild in winter, so they do not need water heaters. A temperature of 24° Celsius (75°F) 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 centimetres, 6-8 in) 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 more manageable levels.⁴ Conversely, Murray cod are susceptible to extremely high water temperatures — temperatures above 30° Celsius (86°F) are not recommended, and temperatures significantly above 30° Celsius will cause death, the upper limit being listed as 33° Celsius (91°F).²⁶ Soft drink bottles filled with water and frozen can be used as an emergency measure to reduce tank temperatures.

Unlike many freshwater aquarium 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.⁴ Common sea salt is a useful tool for keeping Murray cod in tanks, a salinity of 0.5 to 1 gram per litre improves health and keeps most pathogens at bay.⁴ Ensure, however, that the sea salt used is free of additives such as iodine, copper salts and free flow agent.⁴

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" ( a common term in aquarium use, short for Ichthyophthirius multifiliis) and fungal skin infections.⁴ At a minimum, disinfect all live food for an hour in a 10 gram per litre salt solution before feeding.⁴

Water pH should be kept neutral or slightly alkaline; a pH between 7.0 and 8.0 is acceptable, and 7.0–7.5 is ideal.²⁶ A small amount of shell grit will provide a buffer against violent pH swings.⁴

Small Murray cod in aquariums can be very timid,⁴ however, this is usually only a problem for the first few months of life.⁴ Most Murray cod in aquariums, upon reaching a certain size, suddenly gain confidence and then become inquisitive, responsive and rewarding aquarium fish.⁴

References

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^ Cadwallader, P.L. (ed.) (1977), "J.O.Langtry's 1949–50 Murray River Investigations", Fisheries and Wildlife Paper. (Ministry for Conservation, Victoria)
^ Rowland, S.J. (1983). "Spawning of the Australian freshwater fish Murray cod Maccullochella peeli (Mitchell), in earthen ponds". Journal of Fish Biology 23: 525–534. doi:10.1111/j.1095-8649.1983.tb02932.x.
^ Koehn, J.D.; & Harrington, D.J. (2005). "Collection and distribution of early life stages of the Murray cod (Maccullochella peelii peelii) in a regulated river". Australian Journal of Zoology 53: 137–144. doi:10.1071/ZO04086.
^ ^a ^b ^c Tonkin, D.Z.; Humphries, P. & Pridmore, A.P. (2006). "Ontogeny of feeding in two native and one alien fish species from the Murray-Darling Basin, Australia". Environmental Biology of Fishes 76: 303–315. doi:10.1007/s10641-006-9034-3. http://www.springerlink.com/content/u71k0384p1202m32/. Retrieved on 4 October 2007.
^ ^a ^b ^c ^d ^e ^f ^g ^h Koehn, J.D. (2004) (PDF), Threats to Murray cod, Murray Darling Basin Commission, http://www.mdbc.gov.au/__data/page/641/4_John_D._Koehn.pdf
^ Victorian Department of Sustainability and Environment, 2004, Codwatch Newsletter Number 25, [1], Retrieved 28 September 2007
^ Rowland, S.J. (1988). "Hormone-induced spawning of the Australian freshwater fish Murray cod, Maccullochella peeli (Mitchell) (Percichthyidae)"

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