Murray Cod
Encyclopedia
- This article is about the Australian freshwater fish. For other uses, see Cod (disambiguation) and Murray (disambiguation)
The Murray cod (Maccullochella peelii) is a large Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
n predatory freshwater fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
of the Maccullochella
Maccullochella
Maccullochella is a genus of large Australian predatory freshwater fish within the Percichthyidae family. The Maccullochella genus was named after an early Australian fish researcher with the surname McCulloch....
genus and the Percichthyidae family. Although the species is a called cod in the vernacular
Vernacular
A vernacular is the native language or native dialect of a specific population, as opposed to a language of wider communication that is not native to the population, such as a national language or lingua franca.- Etymology :The term is not a recent one...
, 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
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
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
Peel River (New South Wales)
The Peel River is a river in New South Wales, Australia. It is part of the Murray-Darling Basin.The Peel rises on the northern slopes of the Liverpool Range south of the village of Nundle. It flows generally north and west through the foothills of the Great Dividing Range, through Woolomin and...
. This was for a number of years changed to M. peelii peelii to differentiate Murray cod from Mary River cod
Mary River cod
Mary River cod are a Maccullochella cod found in the coastal Mary River system of southern Queensland, Australia...
, which were designated as a subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
of Murray cod. However, as of 2010 Mary River cod have been raised to full species status (M. mariensis) and 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
Murray-Darling Basin
The Murray-Darling basin is a large geographical area in the interior of southeastern Australia, whose name is derived from its two major rivers, the Murray River and the Darling River. It drains one-seventh of the Australian land mass, and is currently by far the most significant agricultural...
, 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
Spawn (biology)
Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...
in the spring and are generally laid in hollow logs or on other hard surfaces. Murray cod are a popular angling target and aquaculture
Aquaculture
Aquaculture, also known as aquafarming, is the farming of aquatic organisms such as fish, crustaceans, molluscs and aquatic plants. Aquaculture involves cultivating freshwater and saltwater populations under controlled conditions, and can be contrasted with commercial fishing, which is the...
species. Often available through the aquarium trade, they are also a popular aquarium species in Australia.
Description
Grouper
Groupers are fish of any of a number of genera in the subfamily Epinephelinae of the family Serranidae, in the order Perciformes.Not all serranids are called groupers; the family also includes the sea basses. The common name grouper is usually given to fish in one of two large genera: Epinephelus...
-like fish with deep, elongated bodies 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.
The spiny dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
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 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, 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 habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
s. 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 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. Large breeding fish are rare in most wild populations today due to overfishing.Related species
Trout Cod
The trout cod or bluenose cod, Maccullochella macquariensis, is a large predatory freshwater fish of the Maccullochella genus and the Percichthyidae family which was originally found in the south-east corner of the Murray-Darling river system in Australia...
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
Trout Cod
The trout cod or bluenose cod, Maccullochella macquariensis, is a large predatory freshwater fish of the Maccullochella genus and the Percichthyidae family which was originally found in the south-east corner of the Murray-Darling river system in Australia...
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
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
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
Great Dividing Range
The Great Dividing Range, or the Eastern Highlands, is Australia's most substantial mountain range and the third longest in the world. The range stretches more than 3,500 km from Dauan Island off the northeastern tip of Queensland, running the entire length of the eastern coastline through...
at least once through natural river capture
River capture
Stream capture, river capture, or stream piracy is a geomorphological phenomenon occurring when a stream or river drainage system or watershed is diverted from its own bed, and flows instead down the bed of a neighbouring stream...
events, leading to several species and sub-species of coastal cod. The best known are eastern freshwater cod
Eastern freshwater cod
Eastern freshwater cod, Maccullochella ikei, also known as eastern cod, are a large and striking predatory freshwater fish of the Maccullochella genus and the Percichthyidae family, that occur in the coastal Clarence River system of north-eastern New South Wales...
of the Clarence River
Clarence River (New South Wales)
The Clarence River is situated in northeastern New South Wales, Australia. The river originates on the watershed that marks the Queensland border. After flowing south and northeast for 394 km it then empties into the Pacific Ocean at Iluka/Yamba. On its journey it passes through the towns of...
system in northern New South Wales
New South Wales
New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...
, and Mary River cod
Mary River cod
Mary River cod are a Maccullochella cod found in the coastal Mary River system of southern Queensland, Australia...
of the Mary River
Mary River (Queensland)
The Mary River is a river system in South East Queensland, Australia. The river rises at Booroobin in the Sunshine Coast hinterland, west of Landsborough...
system in south eastern Queensland
Queensland
Queensland is a state of Australia, occupying the north-eastern section of the mainland continent. It is bordered by the Northern Territory, South Australia and New South Wales to the west, south-west and south respectively. To the east, Queensland is bordered by the Coral Sea and Pacific Ocean...
, both of which are endangered but survive today. Coastal cod were also found in the Richmond River
Richmond River
The Richmond River is a river in the north-eastern corner of New South Wales, Australia. It runs for approximately 170 km from the foothills of the Border Ranges past the towns of Kyogle, Casino, Coraki, Woodburn, where it turns northward and empties into the Pacific Ocean at Ballina. It has a...
system in northern New South Wales and the Brisbane River
Brisbane River
The Brisbane River is the longest river in south east Queensland, Australia, and flows through the city of Brisbane, before emptying into Moreton Bay. John Oxley was the first European to explore the river who named it after the Governor of New South Wales, Thomas Brisbane in 1823...
system in southern Queensland, but are now extinct.
Taxonomy
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 BasinMurray-Darling Basin
The Murray-Darling basin is a large geographical area in the interior of southeastern Australia, whose name is derived from its two major rivers, the Murray River and the Darling River. It drains one-seventh of the Australian land mass, and is currently by far the most significant agricultural...
, Murray cod and trout cod
Trout Cod
The trout cod or bluenose cod, Maccullochella macquariensis, is a large predatory freshwater fish of the Maccullochella genus and the Percichthyidae family which was originally found in the south-east corner of the Murray-Darling river system in Australia...
or "blue nose cod". Taxonomically however, confusion abounded. Only one species of cod was taxonomically recognised, 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
New South Wales
New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...
where the holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
was captured) and as trout cod declined into near extinction over the 1900s the distinction between the two species was eroded and finally questioned. In the 1970s early genetic techniques confirmed forever that trout cod
Trout Cod
The trout cod or bluenose cod, Maccullochella macquariensis, is a large predatory freshwater fish of the Maccullochella genus and the Percichthyidae family which was originally found in the south-east corner of the Murray-Darling river system in Australia...
were a separate species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
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
Eastern freshwater cod
Eastern freshwater cod, Maccullochella ikei, also known as eastern cod, are a large and striking predatory freshwater fish of the Maccullochella genus and the Percichthyidae family, that occur in the coastal Clarence River system of north-eastern New South Wales...
(M. ikei) and the Mary River cod
Mary River cod
Mary River cod are a Maccullochella cod found in the coastal Mary River system of southern Queensland, Australia...
(M. mariensis).
Range
The Murray cod is named after the Murray RiverMurray River
The Murray River is Australia's longest river. At in length, the Murray rises in the Australian Alps, draining the western side of Australia's highest mountains and, for most of its length, meanders across Australia's inland plains, forming the border between New South Wales and Victoria as it...
. The Murray River is part of the Murray-Darling Basin
Murray-Darling Basin
The Murray-Darling basin is a large geographical area in the interior of southeastern Australia, whose name is derived from its two major rivers, the Murray River and the Darling River. It drains one-seventh of the Australian land mass, and is currently by far the most significant agricultural...
in eastern Australia, Australia's largest and most important river system, draining approximately 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 (2,297 ft) elevation in the southern half of the basin and to about 1000 m (3,281 ft) in the northern half of the basin.
Substrate (marine biology)
Stream substrate is the material that rests at the bottom of a stream. There are several classification guides. One is:*Mud – silt and clay.*Sand – Particles between 0.06 and 2 mm in diameter.*Granule – Between 2 and 4 mm in diameter....
in upland areas to large, slow flowing, meandering rivers in the extensive alluvial lowland reaches of the Murray-Darling Basin.
Overfishing
Overfishing occurs when fishing activities reduce fish stocks below an acceptable level. This can occur in any body of water from a pond to the oceans....
, siltation
Siltation
Siltation is the pollution of water by fine particulate terrestrial clastic material, with a particle size dominated by silt or clay. It refers both to the increased concentration of suspended sediments, and to the increased accumulation of fine sediments on bottoms where they are undesirable...
, dams and weir
Weir
A weir is a small overflow dam used to alter the flow characteristics of a river or stream. In most cases weirs take the form of a barrier across the river that causes water to pool behind the structure , but allows water to flow over the top...
s blocking migration, pollution from arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
-based sheep-dips, mining
Mining
Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...
and in some cases, introduced trout
Trout
Trout is the name for a number of species of freshwater and saltwater fish belonging to the Salmoninae subfamily of the family Salmonidae. Salmon belong to the same family as trout. Most salmon species spend almost all their lives in salt water...
stockings, which causes competition between juvenile Murray cod and introduced trout
Trout
Trout is the name for a number of species of freshwater and saltwater fish belonging to the Salmoninae subfamily of the family Salmonidae. Salmon belong to the same family as trout. Most salmon species spend almost all their lives in salt water...
species.
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 spawningSpawn (biology)
Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...
and recruitment event, which are often linked to unusually wet La Niña
La Niña
La Niña is a coupled ocean-atmosphere phenomenon that is the counterpart of El Niño as part of the broader El Niño-Southern Oscillation climate pattern. During a period of La Niña, the sea surface temperature across the equatorial Eastern Central Pacific Ocean will be lower than normal by 3–5 °C...
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 an apex predatorApex predator
Apex predators are predators that have no predators of their own, residing at the top of their food chain. Zoologists define predation as the killing and consumption of another organism...
that will eat almost anything smaller than itself, including finned fishes such as introduced carp
Carp
Carp are various species of oily freshwater fish of the family Cyprinidae, a very large group of fish native to Europe and Asia. The cypriniformes are traditionally grouped with the Characiformes, Siluriformes and Gymnotiformes to create the superorder Ostariophysi, since these groups have certain...
, goldfish
Goldfish
The goldfish is a freshwater fish in the family Cyprinidae of order Cypriniformes. It was one of the earliest fish to be domesticated, and is one of the most commonly kept aquarium fish....
, redfin
European perch
The European perch, Perca fluviatilis, is a predatory species of perch found in Europe and Asia. In some areas it is known as the redfin perch or English perch, and it is often known simply as perch. The species is a popular quarry for anglers and has been widely introduced beyond its native area,...
, native golden perch
Golden perch
The golden perch, Macquaria ambigua, is an Australian native freshwater fish, primarily of the Murray-Darling river system. It is not a true perch of the genus Perca....
, bony bream
Bony bream
Bony bream Nematalosa erebi are a widespread and common, small to medium sized Australian freshwater fish often found in large shoals throughout much of northern and central Australia, and the Murray-Darling basin.-Description:...
, eel-tailed catfish
Eel-tailed Catfish
The eel-tailed catfish, Tandanus tandanus, is a species of catfish of the family Plotosidae. This fish is also known as dewfish, freshwater catfish, jewfish, and tandan.-Distribution:...
, western carp gudgeon
Western Carp Gudgeon
The western carp gudgeon, Hypseleotris klunzingeri, is a carp gudgeon of the Eleotridae family. Carp gudgeons are small perciform fish found in the Australian Murray-Darling river system, mainly in lowland environments, but some have been observed in upland environments...
and Australian smelt
Australian smelt
Australian smelt, Retropinna semoni, is a small, pelagic silvery freshwater fish found in large numbers in waters of the south eastern Australian mainland.-Distribution:Widely distributed through the south eastern part of the Australian mainland...
, and crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s such as yabbies
Common yabby
The common yabby, Cherax destructor, is an Australian freshwater crustacean in the Parastacidae family. It is listed as a vulnerable species of crayfish by the World Conservation Union , though the validity of this listing is questionable; wild yabby populations remain strong, and have expanded...
, shrimp
Shrimp
Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...
and Murray River crayfish
Murray crayfish
The Murray crayfish, Euastacus armatus, is a species of freshwater crayfish endemic to Australia that belongs to the family Parastacidae. The Murray crayfish has the largest geographic range of any of the Euastacus crayfish in Australia, being found in the Murray and Murrumbidgee Rivers as well as...
. 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. Murray cod have also been known to eat duck
Duck
Duck is the common name for a large number of species in the Anatidae family of birds, which also includes swans and geese. The ducks are divided among several subfamilies in the Anatidae family; they do not represent a monophyletic group but a form taxon, since swans and geese are not considered...
s, cormorant
Cormorant
The bird family Phalacrocoracidae is represented by some 40 species of cormorants and shags. Several different classifications of the family have been proposed recently, and the number of genera is disputed.- Names :...
s, freshwater tortoise
Tortoise
Tortoises are a family of land-dwelling reptiles of the order of turtles . Like their marine cousins, the sea turtles, tortoises are shielded from predators by a shell. The top part of the shell is the carapace, the underside is the plastron, and the two are connected by the bridge. The tortoise...
s, water dragons, snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...
s, mice
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...
and frog
Frog
Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...
s. 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
Demersal zone
The demersal zone is the part of the sea or ocean comprising the water column that is near to the seabed and the benthos. The demersal zone is just above the benthic zone and forms a layer of the larger profundal zone....
ambush predator
Ambush predator
Ambush predators or sit-and-wait predators are carnivorous animals that capture prey by stealth or cunning, not by speed or necessarily by strength. These organisms usually hide motionless and wait for prey to come within striking distance. They are often camouflaged, and may be solitary...
is only partially correct. While this behaviour is typical during the day, at night Murray cod are active pelagic
Pelagic zone
Any water in a sea or lake that is not close to the bottom or near to the shore can be said to be in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means "open sea". The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes...
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. 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 50 centimetres (20 in) in length. Wild Murray cod in impoundments like Lake MulwalaLake Mulwala
Lake Mulwala was created in 1939 by building a dam across the Murray River between Yarrawonga in Victoria and Mulwala in New South Wales to provide irrigation water for the surrounding district...
, with their faster growth rates, do not reach sexual maturity until they are well over 60 centimetres (2 ft) in length. This data strongly indicates that the 60 centimetres (20 in) 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 15–35 kilogram (35–80 lb) range 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. 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.
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 80,000–90,000, 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
Spawn (biology)
Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...
in spring, cued by rising water temperatures and increasing photoperiod
Photoperiodism
Photoperiodism is the physiological reaction of organisms to the length of day or night. It occurs in plants and animals.Photoperiodism can also be defined as the developmental responses of plants to the relative lengths of the light and dark periods...
(daylight length). Initially, fish biologists working with Murray cod considered spring floods and temperatures of 20–21° Celsius (68-70°F) 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 macroinvertebrates
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
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). Additionally, recent research has shown abundant epibenthic/epiphytic (bottom dwelling/edge clinging) prey in non-flooded lowland rivers, traits in Murray cod larvae that should allow survival in a variety of challenging conditions, and a significant proportion of Murray cod larvae feeding successfully in non-flooded rivers.
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.
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 exactly the same snag they departed from, an unusual homing behaviour in a freshwater fish. 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 2–3 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. During this dispersal process larvae simultaneously absorb the remainder of their yolk sac and begin to feed on pelagic zooplankton
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
, small, early life-stage macroinvertebrates
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
and epibenthic/epiphytic (bottom dwelling/edge clinging) microinvertebrates.
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
Carp
Carp are various species of oily freshwater fish of the family Cyprinidae, a very large group of fish native to Europe and Asia. The cypriniformes are traditionally grouped with the Characiformes, Siluriformes and Gymnotiformes to create the superorder Ostariophysi, since these groups have certain...
larvae are negatively affecting the survival and recruitment of Murray cod larvae to a much greater degree than first thought; 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.
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 (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 ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
s generally rely on periodic spring flooding. Also, due to the regulation of most of the rivers in the Murray-Darling river system, mainly for irrigation
Irrigation
Irrigation may be defined as the science of artificial application of water to the land or soil. It is used to assist in the growing of agricultural crops, maintenance of landscapes, and revegetation of disturbed soils in dry areas and during periods of inadequate rainfall...
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
Murray cod were originally the most common large native fish in the Murray-Darling BasinMurray-Darling Basin
The Murray-Darling basin is a large geographical area in the interior of southeastern Australia, whose name is derived from its two major rivers, the Murray River and the Darling River. It drains one-seventh of the Australian land mass, and is currently by far the most significant agricultural...
. Contrary to some fishery department literature, the first serious declines in Murray cod were caused by overfishing — extremely severe overfishing. In the latter half of the 1800s and the early 1900s, Murray cod — very large, very long-lived fish — were caught in unimaginable numbers by both commercial and recreational fishermen. For example, one commercial fishing operation commenced on the Murray River
Murray River
The Murray River is Australia's longest river. At in length, the Murray rises in the Australian Alps, draining the western side of Australia's highest mountains and, for most of its length, meanders across Australia's inland plains, forming the border between New South Wales and Victoria as it...
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. And yet fishing effort continued to increase in the region so that 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 last] 29 years 26,214,502 lbs (nearly 11,703 tons) [11,915,683 kg] 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, 2,229,024 lbs [1,045,010 kg] was received at the market, but since 1921, when 1,101,520 lbs [501,145 kg] was sent to Melbourne, supply has decreased. Last year [1928] it was only 551,040 lbs [250,473 kg].
Incredibly, 20 years later the aquatic ecologist J.O. Langtry was still compelled to criticise the extremely heavy, lawless, fishing pressure, in the form of both uncontrolled small-scale commercial fishing and rampant illegal fishing, he was confronted with in all reaches of the Murray River he investigated in 1949–1950.
Indeed, a thorough reading of historical newspaper articles and historical government reports reveals that the whole 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 decade after decade, riverside residents, commercial fishermen, recreational fishermen, local fisheries inspectors, fish retailers and others agitated in newspapers and other fora about the evidently, continuously 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. For decade after decade, 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, raged on and on without proper resolution, with fishing regulations either not amended, or amended and largely unenforced and completely ignored, and with heavy commercial, recreational and illegal fishing pressure continuing unrelenting and unabated. The end result was a Murray cod population, initially unimaginably abundant, continually fished down and down and down without pause, 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 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 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
Catch and release
Catch and release is a practice within recreational fishing intended as a technique of conservation. After capture, the fish are unhooked and returned to the water before experiencing serious exhaustion or injury...
approach for wild Murray cod would alleviate this problem.
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.
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, although yet to be addressed, will be dealt with in the National Recovery Plan when it is completed.
Effects of river regulation
The Murray riverMurray River
The Murray River is Australia's longest river. At in length, the Murray rises in the Australian Alps, draining the western side of Australia's highest mountains and, for most of its length, meanders across Australia's inland plains, forming the border between New South Wales and Victoria as it...
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
Irrigation
Irrigation may be defined as the science of artificial application of water to the land or soil. It is used to assist in the growing of agricultural crops, maintenance of landscapes, and revegetation of disturbed soils in dry areas and during periods of inadequate rainfall...
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.
It is estimated that flows at the river mouth by 1995 had declined to only 27% of natural outflows. 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
Thermal pollution
Thermal pollution is the degradation of water quality by any process that changes ambient water temperature.A common cause of thermal pollution is the use of water as a coolant by power plants and industrial manufacturers...
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.
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
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
benefits and breeding and recruitment opportunities for Murray cod and other Murray-Darling native fish species these now rare floods can provide.
Physical barriers to fish movement
Dams, weirWeir
A weir is a small overflow dam used to alter the flow characteristics of a river or stream. In most cases weirs take the form of a barrier across the river that causes water to pool behind the structure , but allows water to flow over the top...
s 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 GumRiver Red Gum
The River Red Gum is a tree of the genus Eucalyptus. It is one of around 800 in the genus. It is a plantation species in many parts of the world, but is native to Australia, where it is widespread, especially beside inland water courses...
, 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
Substrate (marine biology)
Stream substrate is the material that rests at the bottom of a stream. There are several classification guides. One is:*Mud – silt and clay.*Sand – Particles between 0.06 and 2 mm in diameter.*Granule – Between 2 and 4 mm in diameter....
in lowland river channels composed of fine silts snags are crucial sites for biofilm
Biofilm
A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...
growth, macroinvertebrate grazing and general in-stream productivity.
Vegetation clearing and cattle trampling river banks create severe siltation
Siltation
Siltation is the pollution of water by fine particulate terrestrial clastic material, with a particle size dominated by silt or clay. It refers both to the increased concentration of suspended sediments, and to the increased accumulation of fine sediments on bottoms where they are undesirable...
, 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.
Overfishing
While extremely severe commercial and recreational overfishing in the 1800s and the early 1900s caused the first strong declines of Murray cod, overfishingOverfishing
Overfishing occurs when fishing activities reduce fish stocks below an acceptable level. This can occur in any body of water from a pond to the oceans....
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. 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. Introduced carp dominate the fish faunaFauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...
s of lowland Murray-Darling rivers; the sheer amount of biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
carp now take up, and the large numbers of larvae carp produce, causes serious negative effects on river ecosystems and native fish.
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 protozoaProtozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
that infects the skin of Murray cod and has caused a number of serious kills of wild Murray cod. Saprolegnia
Saprolegnia
Saprolegnia is a genus of freshwater mould often called a "cotton mould" because of the characteristic white or grey fibrous patches it forms. Current taxonomy puts Saprolegnia as a genus of the heterokonts in the order Saprolegniales.-Habits:...
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
Catch and release
Catch and release is a practice within recreational fishing intended as a technique of conservation. After capture, the fish are unhooked and returned to the water before experiencing serious exhaustion or injury...
. (It is essential that Murray cod intended for release are not put down any hard, dry, rough or hot surfaces, e.g. boat gunwhales, boat floors, dry grass, dry rocks, gravel banks, dry towels or mats, etc.) Wild Murray cod populations across their range suffer extremely severe infestations of Lernaea or "anchor worm", a parasitic copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
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 reports a young adult Murray cod seemingly 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, and for some tribes, particular those living along the Murray River, Murray cod were the icon species. The mythMythology
The term mythology can refer either to the study of myths, or to a body or collection of myths. As examples, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece...
s 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.
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. Watching one of these predatory Australian native fish attack and eat a live shrimp or fish can be an awe-inspiring display.
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. 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). 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. Common sea salt
Sea salt
Sea salt, salt obtained by the evaporation of seawater, is used in cooking and cosmetics. It is historically called bay salt or solar salt...
is a useful tool for keeping Murray cod in tanks, a salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
of 0.5 to 1 gram per litre improves health and keeps most pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s at bay. Ensure, however, that the sea salt used is free of additives such as iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
, copper salts
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
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" ( a common term in aquarium use, short for Ichthyophthirius multifiliis) and fungal skin infections
Mycosis
A mycosis is a fungal infection of animals, including humans. Mycoses are common, and a variety of environmental and physiological conditions can contribute to the development of fungal diseases...
. At a minimum, disinfect all live food for an hour in a 10 gram per litre salt solution before feeding.
Water pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
should be kept neutral or slightly alkaline; a pH between 7.0 and 8.0 is acceptable, and 7.0–7.5 is ideal. A small amount of shell grit will provide a buffer against violent pH swings.
Small Murray cod in aquariums can be very timid, however, 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.