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.
Advantages of extensive culture:
Although extensive culture experiences much lower production rates than intensive culture, there are a number of advantages:
- Can be undertaken in existing farm dams.
- Often rural properties have a number of dams with existing fish/crayfish stocks. Extensive culture allows this resource to be utilised without the need for constructing specially designed dams.
- Low overheads and production costs due to no feeding and aeration requirements, production costs are substantially reduced.
- The use of existing farm dams negates the need for costly purpose built ponds.
- Low labour costs. Due to the low stocking densities and nature of extensive farming, less attention to water quality issues and stock monitoring is necessary. Therefore, farm husbandry demands are reduced as are the needs for additional labour.
Disadvantages of extensive culture:
- Equipment is required to manage the stock, e.g. harvesting, purging tanks, aeration, transportation tanks.
- Poor control of stock management, most particularly predation by birds
- Low productivity e.g. extensive (500kg/Ha), intensive (upto 10t/Ha)
- A non-reliable water supply (e.g. drought)
Extensive culture in NSW:
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.
Information Sourced From:
