Murray Cod
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{{Taxobox | color = pink | name = Murray Cod | image = Murray_Cod.png | regnum = Animalia | phylum = Chordata | classis = Actinopterygii | ordo = Perciformes | familia = Percichthyidae | genus = Maccullochella | species = M. peelii | subspecies = M. p. peelii | trinomial = Maccullochella peelii peelii | trinomial_authority = (Mitchell, 1838) }}
The Murray Cod (Maccullochella peelii peelii) is a large and striking predatory freshwater fish of the Maccullochella genus and the Percichthyidae family and is found in the Murray-Darling river system in Australia. The Murray Cod is the largest wholly freshwater fish in Australia, and one of the largest wholly freshwater fish in the world. Other common names for Murray Cod include Cod, Greenfish and Goodoo.
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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 successful fish, Murray Cod push significant distances into upland habitats, while Trout Cod stray (or did stray) well 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 are white to cream on their ventral (belly) surface; their back and sides are yellowish-green to green, overlain with heavy darker green mottling. The effect is a marbled appearance sometimes reminiscent of a leopard's markings. Soft dorsal, anal and caudal (tail) fins are a dusky grey or black with distinct white edges. Colouration is related to water clarity; colouration is intense in fish from clear water habitats. Small to medium sized Murray cod from clear water habitats have striking and extremely beautiful colouration. Very large fish tend towards a speckled grey-green colouration.
Size
Murray Cod are large fish with adult fish regularly reaching 80-100cm in length in all but the very smallest waterways. Murray Cod are capable of growing 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.
Age
Murray Cod continue the trend of native fish of southeast Australia being incredibly long-lived. Longevity is a survival strategy in variable Australian environment to ensure that most adults participate in at least one exceptional spawning and recruitment event, which are often linked to unusually wet La 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.
Large Murray Cod are commonly 20 - 35 years of age.
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 reach sexual maturity between 4 and 6 years of age. Sexual maturity in Murray Cod is dependant on age. Therefore roughly 70% of wild river Murray Cod, with their slower growth rate, have reached sexual maturity by 50cm in length. Wild Murray Cod in impoundments like Lake Mulwala, with their faster growth rates, do not reach sexual maturity until well over 60cm in length.
Large female Murray Cod are the most important breeders because they produce the highest egg counts 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 higher survival rates.
Female Murray Cod, upon first reaching sexual maturity, will have egg counts of around 10,000. Very large female Murray Cod can have egg counts as high as 80,000 - 90,000.
Egg counts in female Murray Cod of all sizes are relatively low compared to most other fish species.
Murray Cod spawn in spring, cued by rising water temperatures and increasing photoperiod (daylight length). Initially spring floods and temperatures of 20-21° C were considered necessary, but research has shown Murray cod breed anually, with or without spring floods, and at temperatures as low as 16° C.
Spawning is preceded by significant migration (if flows allow), pairing up and courtship rituals. During the courtship ritual a spawning site is selected and cleaned - hard surfaces such rocks in upland rivers, and logs and occasionally clay banks in lowland rivers, at a depth of 2 to 3 metres, are selected. The female lays the large adhesive eggs as a mat on the spawning surface, which the male fertilises. The female then leaves the spawning site. The male remains to guard the eggs during incubation, which takes 6-10 days (depending on water temperature), and to guard the hatched larvae for a further week or so until they disperse. Once the newly hatched larvae have dispered and absorbed the majority of the yolk sac they begin to feed on zooplankton, small, early life-stage macroinvertebrates and epibenthic/epiphytic (bottom dwelling/edge clinging) microinvertebrates.
Flooding is critical to successful recruitment of young cod which appear to depend on the influx of zooplankton and early life-stage macroinverebrates off the flood plain for first feeding. However, due to the regulation of most of the rivers in the Murray-Darling river system, mainly for irrigation purposes, and despite cod breeding anually, recruitment only appears to occur now in years of exceptional spring floods that manage to "break free". The long term viability of wild Murray cod populations, in the face of this fact, are of great concern
Range
The Murray Cod is named after the Murray River. The Murray River is part of the Murray-Darling Basin in eastern Australia, Australia's largest and most important river system, draining approximately 13% of the continent. The Murray Cod's natural range encompasses virtually the whole Murray-Darling Basin, particularly the lowland areas, but extending well into upland areas; to ~700m elevation in the southern half of the basin and to ~1000m in the northern half of the basin.
Consquently, Murray Cod inhabit a 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. Unfortunately, Murray Cod have died out of many of their upland habitats, particularly in the southern Murray-Darling Basin, due to a combination of overfishing, siltation, dams and weirs blocking migration, pollution from arsenic based sheep-dips and mining and possibly, in some cases, 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, the first serious declines in Murray Cod were caused by gross overfishing; in the late 19th and early 20th centuries they were caught in unimaginable numbers by both recreational anglers and professional fishermen. Further serious declines in the 20th century have been driven by:
- so called "river improvements", particularly the removal of hundreds of thousands of submerged timber "snags", mainly River Red Gum, that are critical habitat, spawning and ecosystem function sites;
- regulating water flows for river navigation and irrigation, with a consequent reversal of natural flow patterns, virtual elimination of small and medium floods including the once annual spring flood-pulse, and artificial reductions in water temperatures especially in spring and summer (thermal pollution);
- increased silt loads through vegetation clearing and cattle trampling river banks;
- removal of riparian vegetation; and
- continued gross overfishing by recreational fishermen.
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 surival of wild Murray cod populations. Although angler effects are sometimes disregarded in the overall picture, recent population studies have shown that while all year classes are well represented up to the minimum legal angling size (50 cm in most areas) above that size numbers of fish are dramatically reduced almost to point of non-existence in many waters.
As a large, long-lived species with relatively low fecundity and delayed sexual maturity wild Murray Cod populations are particularly 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.
In Aquariums
Hatchery bred Murray Cod are now readily available in aquarium stores in southern Australia. Murray Cod are intelligent fish and make immensively attractive, rewarding and responsive aquarium fish. It is widely agreed by everyone that has kept a Murray Cod in an aquarium that they are fish with "real personalities".
Whilst slow growing overall, Murray Cod will grow quickly in their first 3-4 years of life, so large tanks are advisable - a minimum of 4 feet in length, and preferably 5 or 6 feet. Murray Cod like structure to retreat to, so structure such as sunken timber pieces will be appreciated.
Murray Cod experience very cold water temperatures in the wild in winter, so they do not need water heaters. A temperature of 24° C however will enable fast (optimum) growth, which may or may not be desirable. Some experienced aquarists use water heaters and a temperature of 24° C to grow Murray Cod quickly to a size (15-20 cm) where they can be conveniently fed on feeder fish, fish pieces and other foods, whereupon they gradually turn off and remove the water heater. Caution, and ideally, quarantine procedures, should be used to ensure that feeder fish do not introduce diseases to the tank. With soft skin, Murray Cod can be susceptible to white-spot or "ich" and fungal skin infections.
Unlike many freshwater fish, Murray Cod are highly tolerant of salt due to the raised salinity levels they sometimes experience in the lowland reaches of the Murray-Darling Basin during extreme drought. Indeed, common sea salt is a useful tool for keeping Murray Cod in tanks, a salinity of 2 grams per litre improves health and keeps most pathogens ("diseases") at bay. Ensure, however, that the sea salt used is free of additives such as iodine and free flow agent.
Water pH should be kept neutral or slightly alkaline, a pH range between 7.0 and 7.5 is ideal. A small amount of shell grit will provide a buffer against violent pH swings.
A common complaint voiced about small Murray Cod in aquariums is that they "hide all day" and "don't show themselves". This is usually only a problem for the first few months of life. Most Murray Cod in aquariums, upon reaching a certain size, suddenly gain confidence and then become inquisitive, responsive and rewarding aquarium fish.
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.