Murray Cod
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| Maccullochella peelii peelii (Mitchell, 1838) |
The Murray Cod (Maccullochella peelii peelii) is the largest freshwater fish in Australia. Despite its name, it is not a member of the cod family, but belongs to one of the families of Perciformes.
Other common names for Murray Cod include Cod, Greenfish and Goodoo.
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Classification and Related Species
The scientific name of Murray cod derives from an early Australian fish researcher with the surname McCulloch (=Maccullochella) and the river where Major Mitchell first recorded the species from, the Peel River (=peeli). This was later changed to M.peelii peelii to differentiate Murray Cod from Mary River Cod, which are currently designated as a sub-species of Murray Cod.
There are a number of closely related species and sub-species to Murray Cod, the taxonomy of which has been somewhat confused. Originally only one species 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. peeli, 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.
Murray Cod continue a pattern present in Murray-Darling native fish genera of speciation in lowland and specialist upland species, with Murray Cod being the primarily lowland species and Trout Cod being the specialist upland species. The pattern is slightly blurred in the Cod species because, being such adaptable, versatile and succesful fish, Murray Cod pushed significant distances into upland habitats, while Trout Cod strayed well down into the upland/lowland transition zone, which can be extensive in Murray-Darling rivers. Neverless, the basic pattern of speciation into a primarily lowland species and a specialist upland species is clear.
Description
Murray Cod possess a cream to brilliant white ventral surface with a generally olive green colour over the head and body marked with dark grey to greenish blotches giving a mottled or marbled appearance. 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.
Size
Murray Cod are large fish with adult fish regularly reaching 80-100cm in length in all but the very smallest waterways. Murray Cod grow to well over 1 metre in length and the largest on record was over 1.8 metres in length and approximately 113kg in weight. Large breeding fish are rare in most wild populations today due to excessive angler take and inadequate fishing regulations.
Diet
The Murray Cod is a predator that eats almost anything smaller than itself including crustaceans such as yabbies, shrimp and spiny freshwater crayfish, freshwater mussels and finned fishes such as introduced carp, goldfish, redfin, native Eel-tailed Catfish, Golden Perch, Western Carp Gudgeon, Australian Smelt and even other cod. It has also been known to eat ducks, cormorants, freshwater tortoises, water dragons, snakes, mice and frogs.
Reproduction
Murray Cod spawn at the edges of stream channels, especially in lowland areas in time of the natural spring floods. The eggs are adhesive and are laid on hard surfaces such as logs, rocks and occasionally clay banks at a depth of 1 to 3 metres. The male guards the eggs during incubation, which takes about 6-10 days, and guard the newly- hatched larvae for a further week or so until they disperse. Once the newly hatched larvae have dispered and absorbed the majority of the yolk sac they begin to feed on planktonic organisms being washed into the channels by the receeding flood waters.
Flooding is critical to successful recruitment of young cod which depend on the influx of food off the flood plain. However, due to the regulation of most of the rivers in the Murray-Darling river system, mainly for irrigation purposes, and despite cod breeding most years recruitment only appears to occur now in years of exception spring flooding.
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 Niña 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, and the even larger specimens of yester-year leave us in no doubt the species reaches considerably greater ages of 70 or more years.
Range
The Murray Cod is named after the main river in which it is found, the Murray River. The Murray is part of the Murray-Darling Basin in eastern Australia, Australia's largest and most important river system, draining approximately 13% of the continent. Murray Cod are widespread throughout the basin, particularly in lowland areas, but extend well into upland areas; to ~700m elevation in the southern half of the basin and to ~1000m in the northern half of the basin.
Consquently, Murray Cod inhabit a wide variety of habitats, from cool clear streams with a rocky substrate in upland areas to large slow flowing, meandering rivers in the extensive alluvial lowland reaches of the Murray-Darling. Unfortunately, Murray Cod have died out of many of their upland habitats due to a combination of overfishing, siltation, dams and weirs blocking migration, and possibly, in some cases, competition between juvenile Murray Cod and introduced Trout species.
Many Aboriginal myths describe the creation of the river system (or indeed all river systems of Australia) by a gigantic Murray Cod ploughing through the earth to escape from a hunter, or hunters. In some stories the cod is caught and chopped up to create all the fishes of the river. Its head is kept intact, and it is decreed no Murray Cod may outgrow that size.
Conservation
Once the most common large fish in the Murray-Darling Basin, Murray Cod in previous decades (particularly in the late 19th and early 20th centuries) were caught in enormous numbers by both recreational anglers and professional fishermen. In addition, river "improvements" such as removing submerged timber mainly River Red Gum (so called de-snagging) and regulating water flows for river navigation and irrigation with the consequent reduction in water temperatures especially in spring and summer (thermal pollution) combined with increased silt loads, and the effects of the removal of riparian vegetation have all had an adverse influence on this iconic Australian fish. 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 areas) above that size numbers of fish are dramatically reduced almost to point of non-existence in many waters.
A point that is not grasped by most is that as a large, long-lived species with relatively low fecundity and delayed sexual maturity wild Murray Cod populations are extremely vulnerable to overfishing.
State Government Fisheries departments suport 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.
External links
References
Anon. 2004. Advice to the Minister for the Environment and Heritage from the Threatened Species Scientific Committee (TSSC) on Amendments to the list of Threatened Species [pertaining to Murray Cod] under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). Department of Environment and Heritage (DEH). Online at: http://www.deh.gov.au/biodiversity/threatened/species/m-peelii-peelii.html
Cadwallader, P.L. (ed.) 1977. J.O.Langtry's 1949-50 Murray River Investigations. Fisheries and Wildlife Paper. Ministry for Conservation, Victoria.
Kearney, R.E. and Kildea, M.A. 2001. The status of Murray cod in the Murray-Darling Basin.
Department of Environment and Heritage (DEH).
Online at:
http://www.deh.gov.au/water/basins/murray-cod/index.html
Rowland, S.J. 1989. Aspects of the history and fishery of the Murray cod, Macullochella peeli (Mitchell) (Percichthyidae). Proceedings of the Linnean Society of New South Wales 111(3): 201-213.
