Murray Cod
|
Click to enlarge |
|
Click to enlarge |
Scientific Name
Maccullochella peelii
Other Common Names
Cod, codfish, goodoo, greenfish, Murray perch (incorrectly, in fishmongers)
See also other cod species
Size
Will reach at least 60 cm and 3 to 4 Kg in the small streams.
Murray Cod in larger waterways usually reach 90 to 100 cm and 15 to 20 Kg, if not taken by anglers. Occasionally recorded from 120 to 130 cm and 35 to 45 Kg.
The largest Murray cod ever officially recorded was 183 cm, 113 Kg (6 ft, 250 lbs), although there have been unsubstantiated claims for larger fish.
Unfortunately, excessive recreational fishing pressure has lead to a serious lack of large, mature brood fish in wild populations with an almost total absence of year classes above 50 cm (the minimum legal angling size) throughout its range.
Description
A large, elongate, deep bodied fish with small eyes and a short, rounded, depressed snout with a distinctly concave profile. Lower jaw protruding, or jaws equal in some (usually smaller) specimens. Low spiny dorsal fin and large, rounded soft dorsal, caudal (tail) and anal fins.
A light green to dark green colour on the back and sides that is overlain with heavy mottlings in dark green or black. Undersides are a creamy white. Soft dorsal, caudal and anal fins greyish to black in colour with striking white edges. Colouration of Murray cod is dramatic and often strikingly vivid and beautiful in small to medium sized specimens from clear water. Colouration of Murray cod in very turbid water however, tends to be washed out. Some very large Murray cod have a speckled green-grey appearance.
Larger Murray cod increase in size more by weight than length — large specimens are broad across the head and shoulders and are heavily built.
Conservation Status
Vulnerable, Listed under the Victorian Flora & Fauna Guarantee Act
Threatened, Species of National Significance, Listed under the Commonwealth Environmental Protection & Biodiversity Conservation (EPBC) Act
Distribution
|
Murray cod |
Widespread throughout the Murray-Darling system originally being found in virtually all waterways of that system, including some surprisingly small streams. The main exceptions were tiny tributaries and the alpine headwaters of some southern rivers within the basin. Murray cod were extremely abundant in the past and the species was in fact the most common of the larger native fish in the Murray-Darling system at the time of European settlement.
Today Murray cod are massively reduced in numbers. Wild stocks are now estimated to be less than ten percent of the population present at the time of European settlement. Unfortunately, Cod have become locally extinct in many small tributaries in which they once abounded, particularly in upland reaches of the southern and central Murray Darling Basin, and the fish is rare in the majority of the rest of its original range.
Murray cod have been stocked into many water reservoirs throughout the Eastern states and are a popular fish for farm dams in warmer areas. Some re-stocking of river populations has occurred but the most important tool in restoring cod populations to something like their former glory is the appropriate management of the river systems. This is a nettle yet to be fully grasped by government due to the inherent high costs involved.
Habitat
Murray cod are remarkable by any ecologist's standards in their adaptability and the diversity of habitats they occupy. Murray cod habitat varies greatly, from quite small clear, rocky, upland streams with riffle and pool structure on the upper western slopes of the Great Dividing Range to large, meandering, slow-flowing, often silty rivers in the alluvial lowland reaches of the Murray-Darling Basin.
It should be noted that Murray cod are not just inhabitants of the lowland reaches of the Murray Darling Basin (MDB), as is commonly believed. Murray cod had, and in some cases still do have, a significant presence in the upland reaches of the MDB. At the time of European settlement Murray cod appear to have had an altitudinal limit of around 700 metres in the southern half of the MDB, and around 1000 metres in the northern half.
Murray cod prefer deep holes with cover in the form of large rocks, fallen trees, stumps, clay banks and overhanging vegetation.
Very territorial.
Diet
(Top Predator) Murray cod have a varied diet of other fish, spiny freshwater crayfish, Yabbies, shrimp, freshwater mussels, frogs, water fowl, small mammals, tortoises and other reptiles. Virtually anything within its realm that moves and is small enough to fit in its cavernous mouth is considered fair game!
Reproduction and Biology
Murray cod are extremely long-lived. Specimens regularly reach ages of 30-35 years. The oldest specimen yet recorded is 48 years of age, but they almost certainly reach far greater ages, most likely as much as 100 years. Extreme longevity is a survival strategy for many native fish and particularly Murray cod. This enables them to outlast prolonged periods of drought, so as to capitalise on exceptional conditions for spawning and recruitment when they do occur. SE Australia's current (as at May 2007) decade-long drought is a case in point of the sort of prolonged dry period which has figured in Australia's climate for millenia — on average a drought of this magnitude occurs about once every century or so — and is just the sort of event for which Murray cod have had to develop a survival strategy.
Murray cod reach sexual maturity at 4 to 6 years of age (most fish 5 years) and 2 to 3 kg in weight. The species has relatively low fecundity (fertility) compared to many other freshwater fish. Egg counts range from <10,000 eggs for a barely mature female to approximately 90,000 for females around the 22 kg mark. It is likely that large female Murray cod that are in the 15-25 kg range and "in their prime" are perhaps the most important breeders because they produce the most eggs and for other reasons; research is now showing large females in most fish species are also important because they produce larger larvae with larger yolk sacs and are also more experienced breeders that display optimal breeding behaviours (e.g. Trippel, 1995; Marteinsdottir & Steinarsson, 1998; Marteinsdottir & Begg, 2002). Both of these factors mean the spawnings of large female fish have far higher larval survival rates and make far greater reproductive contributions than the spawnings of small female fish. Such large females may also have valuable, successful genes to pass on.
Breeding of Murray cod is a complicated subject, not yet fully understood, although great strides have been made in that understanding in recent years. Based on preliminary research from the 1960s, it is popularly believed that Murray cod need spring floods to breed, will not spawn in reservoirs and dams, spawn on the actual floodplain in times of flood, do not migrate and, to make matters worse, the lowland Murray-Darling rivers in low flow conditions lack the necessary food for Murray cod larvae. Research has demonstrated that none of these beliefs are correct.
In reality: Murray cod will breed with or without spring-floods, including in low flow conditions in lightly regulated rivers; they will spawn in reservoirs and farm dams (although survival of larvae is very poor, for reasons not known); cod do not spawn on the floodplain; and, Murray cod actually undertake quite significant migrations. Further, lowland rivers in low flow conditions abound in benthic (river-bed-based) zooplankton that Murray cod and other native fish larvae can and do feed on.
It should be noted that the now out dated spring-flood only theory, with its emphasis on floodplain inundation, only ever applied to lowland river habitats, and not the many upland river habitats in which Murray cod were and sometimes still are found, since upland rivers generally do not have floodplains, a fact that seems to have been overlooked!
The picture has now changed when it comes to the breeding biology of Murray cod. Previously Murray cod were considered by many to be odd fish with odd larvae that do not survive unless they hatch into extremely food-rich, spring-flood conditions. Today this fish is recognised as an adaptable and flexible one with large, well-nourished, strongly swimming and flexible larvae. These larvae are evolved to survive and recruit under a variety of natural river flow conditions and habitats, including during times of low flow as well as in upland habitats. In lowland habitats, while Murray cod have recruited most strongly in the past under spring-flood conditions, and those conditions may well be the most ideal for Murray cod recruitment, it is clear that successful recruitment in low flow conditions is possible and does actually occur.
A number of caveats apply:
However, given that we now know Murray cod can recruit to some extent in low flow conditions, it is still an excellent question as to is why Murray cod do not recruit more successfully in low conditions in more lightly regulated rivers in the Murray-Darling Basin. Other reasons for this poor recruitment in these rivers must be found. Likely culprits include severe and continuing over fishing of spawning adults, desnagging and other habitat loss and massive competition between Murray cod larvae and (literally) billions of exotic carp larvae (carp being an illegal and enormously damaging introduction to Australia and the Murray-Darling River system).
On a more general note, research over the last several decades, has revealed the general spawning biology of Murray cod to be as follows:
Female Murray cod carry out the majority of egg development over winter (June, July, August), prior to their spring spawning. Anglers are advised that research indicates capture and release of Murray cod brood fish in winter causes resorption of the eggs and spawning failure (Lake, 1967; Rowland, 1988, 2005). As a result, NFA discourages targeting this species during winter.
Evidence suggests that Murray cod spawn in spring every year, whether there is a flood or not. Rising water temperatures and increasing photo-period (or length of daylight) are the cues for breeding. If a spring flood coincides with this general ripening period, however, it often will trigger breeding. 20°C is usually quoted as the temperature threshold for breeding, but this appears to be flexible, and there are strong indications of Murray cod breeding at lower temperatures in Victoria.
Spawning is initiated by pairing up and courtship rituals. During the courtship ritual a spawning site is selected and cleaned — hard surfaces such as rocks in upland rivers, and logs and occasionally clay banks in lowland rivers, at a depth of 2 to 3 m, are selected. The female lays the large adhesive eggs as a mat on the spawning surface, which the male fertilises. The female then leaves the spawning site. The male remains to guard the eggs during incubation, which takes 6-10 days (depending on water temperature), and to guard the hatched larvae for a further week or so until they leave the nest site (dispersal). Larvae leave the nest site by drifting in river currents at night, and continue this behaviour for around 4-7 days. During this dispersal process, larvae simultaneously absorb the rest of their yolk sac and begin to feed on pelagic ("surface layer") zooplankton, small, early life-stage macroinvertebrates and epibenthic/epiphytic (bottom dwelling/edge clinging) microinvertebrates.
Murray cod breed in reservoirs, as well as in earthen dams in captivity if provided with suitable spawning sites. 200 litre (44 gal) drums with the ends removed are suitable structures in a farm dam. Fisheries and commercial breeders often use more sophisticated devices to facilitate the removal of eggs, but the general idea is to simulate a large submerged hollow log. In commercial and fisheries department situations, spawning structures are examined for eggs and when present they are removed to a hatchery for incubation. If left in the structure, as would usually be the case in a farm dam, a much lower survival rate is to be expected, although this is not really a problem in that case.
For a long time fish researchers believed Murray cod were sedentary and non-migratory (albeit ignoring the anecdotal evidence of anglers, many of whom had observed strong upstream spawning migrations of Murray cod). The question was settled with a Murray cod radio tracking project in the early 1990s. Large Murray cod were fitted with transmitting radio tags and tracked by foot, boat and aeroplane, revealing that Murray cod make strong upstream spawning migrations in late winter and spring. Migratory tendencies and distances travelled varied considerably between individual Cod, but many fish travelled 40 or 50 kilometres upstream and some travelled up to 120 kilometres upstream. Floods were important in stimulating these migrations — fish tended not to migrate if there were no floods. Interestingly, most Murray cod returned downstream to the exact same snag after spawning migrations. This remarkable homing behaviour is almost unknown in freshwater fish and emphasises the importance of snags to Murray cod.
These radio-tracking studies and other research demonstrates that Murray cod do not spawn on the actual floodplain during spring-flood conditions, as was previously conjectured, but rather, they spawn on the flooded margin of the main river channel. It appears that under these conditions, eggs are spawned on and subsequently hatch in the heavy timber at the edge of the channel and, after hatching, the larvae lurk at the edge of the channel, intercepting zooplankton and other prey as the food-rich floodplain waters spill over the edge of the channel. The overall picture is of a fish that spends the vast majority of its life cycle within the main river channel. Indeed contrary to earlier beliefs, Murray cod could be considered to be a main channel specialist at all life stages.
History
Murray cod, although arising from a marine ancestor like most freshwater fish, are nonetheless an ancient species. Fossils of fish anatomically identical to modern Murray cod have been unearthed in NSW from strata dating to 26 millions years ago. However, it is possible the species is as old as the Murray-Darling Basin itself about 50 to 60 million years.
Prior to European colonisation aboriginal people were able to exploit the species as a major food source using relatively simple technologies aided by the fish's great abundance. They were impressed by the Murray cod, for in addition to being a major food source, it was the largest, most abundant and most beautiful of the native fish species. These people had and still do have enormous respect and reverence for the Murray cod. The importance of Murray cod to aboriginal people of the Murray-Darling basin is reflected by the fact that many groups living along the Murray River made the Murray cod a central animal in their mythology, including their creation stories. Many Murray River groups believed that the wide reaches and bends of the Murray River were created by a giant Murray cod, swimming down the formerly narrow trickle to the sea, while being pursued by a dream-time hero.
In modern Australia, the Murray cod is nation's largest and best-known freshwater fish. The stuff of legends, the Murray cod is represented in practically every pub in South Eastern Australia by photos and mounted examples. Stories abound of titanic struggles with the almost super natural fish. Ply an old codger with a few beers and you are likely to hear how when he was a boy one of the local water holes was inhabited by some gigantic Cod. The story usually goes along the lines that some farmer lost a large bait to the fish and kept coming back with heavier and heavier tackle until eventually he turned up with a whole kangaroo as bait on a meat hook connected to the steel cable of the winch on his tractor. The fish in the story then fights the farmer and his tractor to a standstill, usually resulting in the demise of said tractor. You are then invited to inspect the wreck of the machine under an ancient red gum tree near the pub and shown the stretch of water which was straightened by the fish in the struggle!
On a more serious note, Murray cod were originally extremely common and supported a substantial commercial fishery in the nineteenth century and in the early decades of the twentieth. Records from this fishery indicate that Murray cod were numerically the dominant native fish in the Murray-Darling system until the 1950s.
Decline
The decline in Murray cod numbers is due to multiple factors. Contrary to popular belief, over fishing has played a massive role in the decline of Murray cod. Murray cod were grotesquely over fished by commercial fishermen in the latter half of the nineteenth century. For instance, in year 1883, more than 147 tons of Murray cod were sent to market from just one port (Moama). It is staggering to think that that real figure, incorporating unreported catches, was probably at least double that. It boggles the imagination to think what the total catch for all ports may have been.
During this time, recreational fishermen also massively overfished the species. Old photographs abound of recreational anglers stringing up literally dozens of massive Murray cod at a time. The fishery must have seemed inexhaustible. Sadly today we know that it was not. Yet, excessive angler kill continues to cause profound problems for these long-lived fish.
By the very early 1900s, alarm was expressed at the decline of Murray cod and a Royal Commission was held on the decline of Murray cod. Unfortunately, no decisive action resulted.
Finally, once over fishing had severely damaged Murray cod stocks, an even more potent agent of decline was to take over — river regulation. The many dams and weirs, and the regulation of river flows that they allow, mean that rivers do not follow natural flow patterns anymore. In particular, these dams and weirs curtail or halt completely the spring floods in lowland rivers that create the zooplankton-rich conditions which are one of the conditions under which the strongest recruitment occurs. With no spring floods, Murray cod are unable to recruit in large numbers in many lowland rivers and opportunities for migration and colonisation are also lost. All the while river ecosystems lose productivity boosts and events that maintain habitat. Meanwhile, heavy river regulation and/or habitat loss, competition with exotic carp larvae and other factors preclude or diminish recruitment in low flow conditions in many lowland rivers.
Dams and weirs have other impacts as well:
Another major cause of decline is river-desnagging. Hundreds of thousands of snags have been removed from Murray-Darling system. Almost unbelievably this still occurs, though thankfully the practice is now frowned upon and is even illegal in some areas. The damage this desnagging has done to native fish, especially Murray cod, is incalculable. Snags are critical habitat for Murray cod at all sizes, vital spawning sites, and in the silty alluvial lowland rivers of the Murray-Darling system, one of the few hard substrates available and thus critical sites for algae, bacteria, fungi and aquatic invertebrates. In other words, snags are critical sites for food production and ecosystem function in our lowland rivers. Some researchers have described large red gum snags, hundreds or even thousands of years old (redgum are virtually impervious to rot), as the coral reefs of our lowland rivers.
Yet another significant problem, particular for upland rivers or smaller lowland rivers holding Murray cod, has been siltation of the stream bed through clearing of native riparian (river-bank) vegetation and the effects of introduced large, hard-hooved animals such as cattle which trample the river banks.
Cattle (whichever variety) very quickly obliterate riparian vegetation and crush river banks, destroying the structure of the river. This leads to rapid, massive siltation of deep holes which are habitat for native fish and a complete loss of stream bed diversity which is required by the entire aquatic ecosystem. This is the death knell for Murray cod in these rivers, as well as for the ecosystems concerned. This long established farming practice is unfortunately still widespread and is often unrecognised for the problem it represents. The broad shallow sandy-bottomed streams produced as a result are very picturesque to the casual observer. Consequently it is very difficult to explain to land owners the importance of keeping stock away from streams. Some leadership is needed at all levels of government to get cattle away from all river banks, and to have all rivers replanted with their native riparian vegetation.
Speaking of vegetation, willows are also a problem — to Murray cod, to native fish in general and to rivers and their ecosystems. Compared to indigenous trees willows are incredibly greedy with water, and can suck a small river dry. They cause deoxygenation with mass autumn leaf dumps, wipe out many species of native aquatic invertebrates (fish food) reliant on better water quality and continuous leaf fall from native vegetation for food and have other negative effects. Willows are not an asset, and the sooner they are replaced with vegetation native to streams the better.
Murray cod have been stocked into many water reservoirs throughout the Eastern states and are a popular fish for farm dams in warmer areas. However, there are serious inbreeding problems with some hatchery breeding of Murray cod. Inbreeding can produce unhealthy fish and loss of the genetic variation required to be a healthy adaptable predator in the wild, both as individuals and at the population level. Due to logistics, the fish hatcheries produce will always have lower genetic diversity compared to wild bred fish. Thus, hatchery breeding and stocking of Murray cod is not the magic answer and will not ensure their long term survival. (Stocking of hatchery bred Murray cod in rivers can actually cause damage, as remnant wild Cod populations with their diverse gene pool are swamped by hordes of hatchery Cod with a narrow gene pool — as little as two parents in a worst case scenario.)
The only way to ensure the long term survival of Murray cod is to have healthy breeding populations in the wild which requires healthy rivers and more natural flow regimes. This in turn will require some drastic changes to the way we currently manage river catchments in Australia, an issue which politicians seem reluctant to tackle head-on or to provide effective leadership for.
|
Trout cod |
Trout cod Maccullochella macquariensis are a second species of Cod found in the Murray-Darling system, now critically endangered. They appear to be a Cod that has speciated into a specialist inhabitant of the cooler upper reaches of rivers, but their range does overlap with that of Murray cod.
Trout cod have been severely adversely effected by the introduction of trout into upland habitats throughout their range.
Murray cod have also managed to cross the Great Dividing Range into coastal river systems at least once during their long existence on the Australian continent, via ancient river capture events. Due to isolation from cod of the Murray-Darling Basin, these coastal Cod have diverged into separate species. At the time of European settlement, 4 coastal rivers in northern NSW and southern QLD (the Clarence, Richmond, Brisbane and Mary Rivers) held naturally occurring populations of Cod.
The Richmond River and Brisbane River Cod unfortunately became extinct in the 1950s. Decades of habitat destruction and gross over fishing by European settlers were followed by whole-of-catchment scale bushfires and massive ash fish kills in the late 1930s, causing a terminal decline. The Cod of the Clarence River and Mary River survive, although both are endangered.
|
Eastern freshwater cod |
Clarence River cod are named Eastern freshwater cod Maccullochella ikei being a separate species in the same genus as Murray cod. Based on mitochondrial DNA divergence rates, it is estimated that Cod crossed into the Clarence system somewhere between 0.8 and 1.7 million years ago, with the older date considered more likely.
|
Mary River cod |
Mary river cod Maccullochella mariensis were formerly classified as a sub-species of Murray cod (Macculochella peelii mariensis). However, recent genetic analysis indicates they are actually species in their own right. also, for a time it was considered that they may have been a sub species of Eastern freshwater cod with the scientific name Maccullochella ikei mariensis.
This finding has interesting implications. It suggests, for example, that Murray cod only crossed the Great Dividing Range once (into the Clarence River system) and then leap-frogged north into the next 3 river systems via further river capture events albeit dodging the Tweed River. It suggests that all four coastal Cod populations are/were actually sub-species of Eastern freshwater cod Maccullochella ikei.
Another possibility is that the fish did not miss the Tweed river system on their way up the coast. Rather, there may have been cod in that system in the pre-European past. Subsequently then, the fish became extinct before European settlement perhaps as a result of whole-of-catchment scale bushfires and subsequent massive ash fish kills similar to an event which almost claimed Eastern freshwater cod about 2000 years ago, as revealed by genetic analysis. Unfortunately, the likelihood of ever finding fossil evidence for this is vanishingly small. However, we do have the evidence of the event (as mentioned earlier) which did in fact claim the Richmond and Brisbane River fish in the early twentieth century after their very dramatic decline caused by the early settlers, which suggests that the "cod in the Tweed" scenario is possible.
The Richmond River has now been restocked with Cod from the Clarence River system, while the Brisbane River has now been restocked with Cod from the Mary River system. Grave inbreeding problems have recently surfaced with the breeding and stocking of Cod of the Clarence River system. It is questionable as to whether the genetic guidelines required to be followed in this program have indeed been followed. Breeding and stocking of Cod for the Clarence River has now been suspended for the time being.
The coastal Cod are remarkable fish. Their coastal river habitats can be radically different to Murray cod habitats. For example, former Cod holding tributaries of the Richmond and Brisbane River are rainforest streams with cobble bottoms, tannin-stained water and rainforest trees and ferns — the Cod of these streams were veritable rainforest Cod. It is hard to imagine a more different environment to that which exists through most of the Murray-Darling. As a matter of interest, restocked coastal Cod are doing well in these streams now. It is testament to the resilience and adaptability of the Cod that it has been able to survive in such radically different habitats.
Angling
Generally regarded as Australia's premier freshwater angling species, the powerful Murray cod can be a difficult customer to deal with. Most cod specialists use relatively heavy tackle. Line breaking strains in the 40 to 50 lb or even higher are common amongst those aiming for the larger fish. The new super braided lines have proven useful, especially when trolling in heavy cover, which is where you find the fish.
Cod respond well to large baits and lures. Many cod specialists fish only large trolled or cast deep diving lures with a wide action at dead slow speed. Surface lures work well at night and large flies have been tried with success. Bardi grubs, Yabbies, shrimps and scrub worms all catch cod. Cod have even been caught on such diverse baits as rabbits and hard boiled eggs! If using fly, you need heavy gear, as cod can pull like a steam train. Even small cod will take very large baits and lures, and give you a run for your money to boot.
Fish close to snags and other structure — if you're not getting snagged up, you're not fishing in the right place!
Size and bag limits and closed seasons apply in all States where Murray cod occur, so check your local regulations.
Murray cod are excellent eating in the smaller legal sizes, up to around 6-8 Kg. Can be filleted or steaked and are great on the barbeque. Larger fish can be very oily and are best still swimming in the river <grin>. If fishing for the table NFA recommends that you fish in impoundments that have been stocked by fisheries, rather than in river systems supporting wild populations.
In fact, NFA strongly encourages you to practice catch and release when fishing for Murray cod. This especially applies with large fish which are important breeding stock for the species and which are less palatable in any case.
Recent research has shown that all cod species caught and released during the period prior to the spawning season (ie during winter) re-absorb their eggs and thus do not participate in the breeding season. For this reason NFA very strongly recommends that cod not be targetted during winter, especially so for naturally recruited populations. Please avoid fishing for cod in rivers and most importantly Lake Mulwala, during this time. It is also most important to avoid fishing for Eastern Cod (which are totally protected anyway) or in Eastern Cod habitats at this time. If you feel you really must fish for cod during winter, please target only Murray cod in impoundments supported by stocking and without natural recruitment (this does not include Lake Mulwala).
In the aquarium
Smaller examples make excellent and very impressive aquarium specimens. Juvenile hatchery bred Murray cod are readily available through the aquarium trade. Murray cod should be kept on their own as they are very territorial and aggressive to other fish, including their own species. Best fed on a mixed diet of live yabbies, shrimp (if available) and fish, although cod are easily trained to accept dead food. For anglers, a diet of skinned fillets of carp are an easy to obtain and cheap food. A standard 3 foot aquarium is satisfactory for a small cod, but make sure the space you use will accept a much larger tank. Popular tank sizes (in feet) for cod are 4x2x2 up to 6x2x2 or even 6x3x2, a six foot tank will last a fish for many years. Cod are very strong fish and the tank should be made from heavy glass and have a heavy cover as the fish can accidentally jump out if it makes a strike at an insect on the surface, or even a bubble!
Murray cod will do well in unheated aquarium indoors, but generally it is best to use a heater to put a floor under the temperature. When used this way the heater should be set to around 18°C at which temperature the fish will remain active and feeding all year round. Some enthusiasts will set the temperature up to around 24°C while the fish are young to encourage fast growth until the fish reaches a size of around 150-200 mm when it may conveniently be fed on live feeder fish. The heater is then gradually turned down to 18°C - if done in summer, this can be achieved without risk of chilling the fish since ambient temperatures are likely to be above 20°C anyway in most places.
Very small Murray cod, such as those newly purchased from an aquarium shop, sometimes appear to be very timid and will hide much of the time. Most fish will grow out of this, however you can assist the progress by providing plenty of cover for the fish, depending on fish size such things as small sections of PVC tube, broken terracotta flower pots and so on will help the fish become more confident. In particular many young Murray cod do not like to have a light coloured substrate and these fish will feel more secure if the substrate is darker, or if running in a bare tank, with a sheet of dark plastic under the tank.
Like for most Australian native fish, salt is a very useful agent for managing the health of captive Murray cod. A salinity of 0.5 to 1 gram per litre is a good general purpose tonic for maintenance and in times of illness or stress the salinity can be bumped up to 6-8 g/l. The salt acts to reduce stress and in particular osmotic stress in sick or injured fish, as well as helping control parasites such as white spot or Lernaea, which may be accidentally introduced with feeder fish. As an aid to reducing the likelihood of this sort of introduced infection, it is best if all live food is quarantined for at least a week, but if this is not possible, the food can be placed in a salt bath (a salt solution of around 10 g/l) for an hour or so prior to feeding it to the Murray cod. When using salt, it must not contain any additives like Iodine, copper salts (often used in swimming pool salt in tropical areas) or free flow agent (used in some cooking salt). Ideally use sea salt but if it is unavailable a good, inexpensive source is to buy swimming pool salt, just make a call to the manufacturer and ask if the particular brand available to you is suitable for aquaculture.
Water quality is important and pH should be maintained at neutral or slightly alkaline ideally at around 7.0-7.5, although a range of 7.0 to 8.0 is acceptable.:nbsp; Care should be taken to prevent the water becoming acidic and the use of granular Calcium Carbonate (do not use powder) or shell grit is recommended to act as a pH buffer. If running at an elevated temperature or if feeding heavily it is important to monitor ammonia levels and any trace of ammonia shown by a test kit should be treated as an emergency and a significant water change (at least 30%) should be undertaken immediately.
Murray cod make very rewarding aquarium fish and often become very attached to their owners. Many owners hand feed their fish and if kept in a location where there is a lot of activity they can become very interactive. A Murray cod of around 50 to 60 cm is certain to provoke comment from your visitors!
On the web
Australian Desert Fishes Pages — Murray Cod
South Australian Department of Primary Industries: Murray Cod
References
