Aquaculture can be divided into two main forms of culture; intensive and extensive. Much of the interest over the years in aquaculture has been primarily focused on the intensive culture of fish.
Intensive culture involves intervention in the rearing process through supplemental feeding, water aeration and exchange and in some cases environmental manipulation.
Traditionally, intensive culture has much higher production rates, shorter grow-out periods, greater overheads and production costs and potentially greater returns per unit area.
However, trials have highlighted a number of possible aquaculture candidates generally unsuitable to intensive culture but suitable for extensive culture. Extensive culture does not rely on excessive intervention in the growing process. The stock is left to grow on its own, utilising natural food sources. Low stocking densities remove the need for water exchange and supplementary aeration. No nutrients (e.g feed) are added to the system during the grow-out process.
The extensive rearing process generally begins with the initial release of seedstock into the dams or production waters. The seedstock are either fingerlings (juvenile fish) or juvenile stages of crayfish species. The grow-out of stock relies on the natural productivity of the aquatic system; the stock feed on existing food without supplemental addition by the farmers. In some cases yabby farmers will cover the bottom of ponds with a layer of hay, or grow a fodder crop such as Lucerne prior to filling. Natural breakdown of plant material after filling provides nutrients to the pond system, thereby starting the natural food chain.
There will usually be some size variation in stock after the expected grow-out period, so the farmer should harvest the larger specimens and allow the smaller stock to grow-out further. Common practice is to replace harvested stock with new fingerlings or juveniles, although some species will breed naturally and maintain self-sustaining populations. In this circumstance, it is imperative that appropriate genetic variation exists in original stock to ensure that inbreeding does not occur.
Although extensive culture experiences much lower production rates than intensive culture, there are a number of advantages:
Much of the impetus and research in NSW has been focused on intensive culture, however extensive culture currently accounts for the largest aquaculture industry in NSW; the oyster industry. The extensive culture of mussels also represents a growing industry and trials are underway for the culture of pearl oysters, clams, scallops and sea ranching of abalone - all forms of extensive culture.
Extensive production of NSW native finfish and freshwater crayfish is minor, with yabbies being the most common extensively farmed species.
There are 2 classes of aquaculture permit listed under the Fisheries Management (Aquaculture) Regulation 2007 that regulate land based extensive culture; the Class C permit and the Class E permit. Class C Aquaculture permits allow extensive culture on a single site (private land), while Class E permits allow extensive culture on two or more privately owned locations.
A number of native finfish species have been identified as suitable for extensive culture. The following looks at some freshwater species that may have potential for extensive farming on a larger scale.
This is predatory fish naturally found throughout the Murray-Darling Basin (MDB). The fish is thickset, with a brownish olive coloured dorsal surface, changing to light yellow cream on the flanks and ventral side. The golden perch is carnivorous and feeds on small fish, insects and yabbies. It has been known to grow to 10kg, however is usually caught at between 700g - 3kg. This species is one of four freshwater species targeted by most inland recreational fishers.
Golden perch have a firm white flesh and are of good flavour however off-flavour can still be a problem and the fish should be purged in clean water prior to sale.
Golden perch are often reluctant to accept artificial diets, which restricts their intensive culture prospects. However, fingerlings are readily available from a number of hatcheries and this species shows good potential for extensive culture, provided that an adequate supply of food is available in the dams. Golden perch are able to be extensively cultured at stocking densities of up to 250 fish/ha, and are able to reach edible size after 2-3 years. Some farmers are trialling the intensive culture of golden perch in ponds and tanks.
Eel tailed catfish range naturally throughout the rivers of the Murray-Darling basin. They are similar in appearance to most catfish species, with the exception that their caudal is not forked and is similar to an eel tail. The catfish is brown-grey in colour, often with a mottled appearance. This species has been known to reach over 7kg, however is usually caught much smaller (1-3kg). Eel tailed catfish are often caught by recreational fishers.
Despite their unusual appearance, eel tailed catfish are of excellent eating quality and are often preferred over other freshwater native fish such as golden perch. The flesh is white and well flavoured. Eel tailed catfish are rarely seen at seafood retailers. However, it should be noted that the American catfish farming industry provides the basis of a billion-dollar industry, with production in excess of 250,000 tonnes annually. Most of this product is processed for fish fingers and processed fillets.
Eel tailed catfish have not been trialed in intensive conditions, so its potential for intensive aquaculture is relatively unknown. Fingerlings are irregularly available and provided adequate natural food is made available, this species can be stocked at a rate of about 250 fish/ha, and should be marketable within 2-3 years.
Yabbies are a species of freshwater crayfish that have a distribution extending throughout the range of the Murray Darling Basin. Their colour ranges from a pale cream colour to almost black, with bluish claws. This species can grow to 150mm in length. Yabbies are targeted by inland commercial fishers. They are used for consumption and fishing bait.
Yabbies are excellent eating, with the edible meat contained within the tail and claws. Significant amounts of product is sold live through the Sydney Fish Market as well as to local restaurants in regional areas and to a number of other retailers throughout Australia. There is also an established export market, particularly to Europe and Asia where they are considered a delicacy. It is recognised that considerable potential for expansion exists to export this species overseas.
Yabbies can be cultured in intensive situations, with commercial diets readily available. There is a small intensive yabby-farming sector in NSW. However, large amounts of capital are often required to construct facilities required for intensive culture however yabbies can be grown extensively in farm and irrigation dams with little additional infrastructure required (with the exception of purging/packing facilities). Due to the yabby's ability to breed profusely in dams, placing 200-400 adults per 1ha water storage will usually lead to a rapid population increase. Ongoing selective harvesting must be implemented to manage the population. The omnivorous diet allows yabbies to utilise most food sources available. It is expected that extensive farming of yabbies will continue to expand, as new and larger markets open up.
For further information, call NSW DPI Port Stephens Fisheries Centre on 0249 82 1232
You can obtain copies of licensing and policy requirements for extensive farming from here. Further information on extensive farming can also be obtained from the following references;
Allen, GR (1989). Freshwater Fishes of Australia. T.F.H Publications: New Jersey, USA.
Mackinnon, M (1995). Fish for Farm Dams. Queensland Department of Primary Industries, Brisbane.
McCormack, G and Jackson P (1991). The Farm Fish Book - Proceedings of the Seminar on Stocking Fish in Farm Dams for Recreation and Farm Table Use. Queensland Department of Primary Industries, Brisbane.
NSW Fisheries (1994). Freshwater Crayfish Advisory Pack. NSW Fisheries, Sydney.
NSW Fisheries (1995). 'Fish in Farm Dams'. Fishfact. NSW Fisheries, Sydney