The Murray cod (Maccullochella peelii peelii) is the largest freshwater fish found in Australia. Its natural distribution extends throughout the Murray-Darling Basin, ranging west of the divide from southeast Queensland, through NSW, into Victoria and South Australia. The Murray cod is a predatory fish with a large mouth and olive green mottled body.
The Murray cod inhabits the low lying rivers of the Murray-Darling system, preferring slow flowing waters and deep holes. It is a top order predator, and is highly territorial, vigorously defending its home against intruders. Cod grow large in the wild, with the biggest specimen ever caught weighing 113.5kg, at an estimated age of 80 years.
Murray cod spawn in spring, laying their eggs on a submerged log or other structure. The eggs hatch 6-13 days later, with juveniles feeding freely within 3-4 weeks. Each adult female will lay up to 40,000 eggs, with only a small proportion surviving to maturity. Unfortunately, fishing pressure along with habitat destruction and other factors has caused cod numbers in the wild to decline drastically.
The grow-out production of Murray cod in NSW is in its infancy, however several commercial hatcheries have operated for a number of years producing cod fingerlings for stocking into rivers, farm dams and impoundments. Stockings have been used as a method of increasing wild populations while enhancing recreational fishing.
In the past, most grow-out culture of Murray cod has been undertaken extensively in farm dams, as it was originally thought that the cod was unsuitable for intensive culture due to its aggressive and territorial nature. Trials by some NSW farmers have shown however that Murray cod do well in intensive recirculation tank systems. The stocking of fish at high densities reduces the opportunity for the cod to establish territories, and the aggressiveness towards other tank members is reduced. It is expected that this form of culture will expand rapidly over the next few years. For the production figures see the NSW DPI aquaculture production reports. Recent amendments to NSW DPI policy now allows the culture of Murray cod on the eastern drainage but only where it can be demonstrated that there is no chance of the fish escaping into natural waterways.
The intensive culture of Murray cod at this time is reported mainly suited to intensive recirculation tank systems with some work on hybrid systems involving on-growing of larger cod in pond systems. Culture can also occur extensively in farm dams and ponds, at a stocking rate of around 200 fish/ha. This activity has low production and variable growth rates compared to intensive culture and it is expected that commercial production from extensive culture will be limited.
Fingerlings for intensive production can be purchased from a number of hatcheries once they are weaned onto commercial pellet diets. Once quarantined and settled, fish are ready for stocking into tanks. Stocking rates in tank systems vary, depending on the capacity of the system and the intensity of the operation. In well-developed and highly advanced systems, stocking rates of up to 60kg/m3 may be obtained, however most systems average around 30-40kg/m3.
Murray cod require high protein diets, usually in extruded pellet form available in various classes depending on the size of the fish. Food conversion ratios (FCRs) for Murray cod have been reported from industry as low as 0.8:1 (kg of food: kg of growth), yet farmers should expect FCR's closer to 1.5-2:1. The use of intensive systems also means that they can be kept at optimum growing temperatures all year round, allowing the farmers to potentially achieve a market size of 500-600g in around 12-18 months.
The nature of intensive recirculation farming means that smaller volumes of water are required, particularly when compared to pond based culture. However, the high protein diets used and the sometimes-torrid feeding behaviour of cod means that water quality can suffer. Tanks should be siphoned and biofilters back-washed regularly to avoid malfunction. Back up systems are essential.
Murray cod are highly territorial and aggressive fish. While the high stocking densities often used in intensive recirculation systems do reduce overall aggression, there will always be size variation in stock, with some fish growing more quickly. To avoid a 'pecking order' developing and cannibalism occurring, it is recommended that regular size grading be undertaken. By keeping fish of the same size together, the farmer is able to ensure that all fish are being fed properly and growing at uniform rates.
Site selection for tank culture has a number of advantages over pond based culture in terms of lower land and water requirements as it allows access to more areas due to reduced soil and topography limitations. Using temperature-controlled systems also eliminates the need to consider ambient climate/temperature as criteria when selecting a site.
Basic intensive recirculation systems consist of a number of tanks (usually 1000-1300L in size) housed within a vermin proof and preferably climate controlled room or shed. Tanks are independently or group filtered by large biofilters, which are used to strip nitrogenous waste and nutrients from the water. Recirculation systems can also incorporate a number of other filtration units including UV and ozonation systems to disinfect water and protein skimmers to remove protein based waste. After passing through the filters, the water is then recycled back to the tanks. Specialised technical advice should be sought to determine the best set-up for your needs.
There are certain site selection requirements that must be met for an operation to be able to run efficiently and meet strict licensing conditions. When looking for potential sites the following must be considered:
Murray cod are entirely a freshwater species and will not tolerate high salinity levels. They are also generally quite hardy, however may be susceptible to bacterial and fungal infections at high stocking densities. Therefore excellent water quality must be maintained.
As a general rule, stocking densities in tanks should not exceed 40kg/m3 unless the farmer is confident that limitations of the system will allow higher densities. Water should be clean, with a relatively neutral pH between 6-8, dissolved oxygen above 3mg/L and free ammonia levels of less than 0.1mg/L. Temperatures should be maintained at 24-25° C. Water sources for the operation should be free of pesticides and other contaminants.
It should be noted that the above parameters are ideal levels, although the Murray cod may tolerate conditions outside of this range for short periods of time. Prolonged exposure to unsuitable or undesirable conditions will stress the fish and leave them open to attack from a number of parasites, fungal and bacterial infections, and viruses. This may cause mortality, and infection may spread to other tanks causing expensive losses. Even if no infection occurs, growth rates may be inhibited.
Like any other fish, Murray cod can be susceptible to various pathogens and diseases. Disease outbreaks are often in response to stresses being placed on the stock, which in most cases could be avoided.
Prevention is the best cure and a combination of good husbandry and management techniques will ensure that stock remains relatively disease free. General husbandry should include quarantine of all new stock to the site, with regular salt baths of new batches at a concentration of 5-10ppt for at least 1-hour before placement into any of the grow-out tanks. Water quality should be maintained to relieve stress on the fish, and regular inspection of fish should be undertaken to monitor health. Upon the first signs of a disease outbreak a sample of fish should be inspected for obvious disease symptoms, and water quality should be checked. If a source of infection or disease cannot be identified, a qualified fish veterinarian should be consulted.
The live fish market in Australia is still developing, particularly for native freshwater species, however there is huge potential for expansion. This can already be demonstrated with other natives such as silver perch that have increased markedly in production and demand in recent years.
Murray cod has generated much interest in the live fish trade, due to its appearance and premium quality flesh and taste. Murray cod sold as a live product has been very well received, and is perceived by some chefs to be one of the best tasting freshwater fish in the world. This is reflected in the average price paid for live Murray cod; around $20/kg at the farm gate. (This varies depending on size of fish - larger fish (1-3 kg) usually attract higher prices).
While most of the product is sold domestically, there may be potential for export trade of cod when production is increased. Overseas traders have expressed interest in the product and Asian restaurants, indicating acceptance within the Asian community purchase most live product. Although it is expected that average prices for live product will drop as more producers come on-line, the almost legendary status of the cod, its excellent flavour and majestic appearance should ensure its place in the top range of the market.
For further information, call NSW DPI Port Stephens Fisheries Centre on 0249 82 1232. Copies of licensing and policy requirements for Murray cod are available from here.
Allen, GR (1989). Freshwater Fishes of Australia. T.F.H. Publications: New Jersey, USA
Forteath, N (1990). A Handbook on Recirculating Systems for Aquatic Organisms. Fishing Industry Training Board of Tasmania Inc, Australia
Hart, P and O'Sullivan, D (eds) (1993). Recirculation Systems: Design, Construction and Management. Aquaculture Sourcebook, Tasmania
Mosig, J (1999). 'Murray Cod Grower Finds Success in Tanks'. Austasia Aquaculture, 13(1):14-16.