The group of marine prawns that are of economic interest in Australia belong to the family Penaeidae.
Some of the commonly known species, such as the eastern king prawn (Penaeus plebejus), the school prawn (Metapenaeus macleayi) and the greasyback prawn (Metapenaeus benettae) are caught by commercial trawlers offshore and in many New South Wales estuaries.
In the New South Wales and Queensland, the most commonly cultured penaeid prawn is the black tiger or leader prawn, (Penaeus monodon). Smaller quantities of the Kuruma prawn (Penaeus japonicus) and the school prawn are also cultured.
In the past, attempts to culture several other species including the Eastern King prawn, the Brown Tiger prawn (Penaeus esculentus) and the Banana prawn (Penaeus merguiensis), met with limited commercial success because these species had inferior performance in growout cultivation resulting in a poorer economic return.
Improved technology and understanding of prawn farming will probably see increased interest in the commercial cultivation of these species in the future.
Reproduction:
With the exception of the greasyback prawn, which can complete its life cycle within an estuary, penaeid prawns migrate to oceanic waters to breed. Females of some species can produce more than 800,000 eggs at a time. Twelve to 20 hours after spawning the eggs hatch into nauplii, the first of three planktonic larval stages.
While nauplii do not feed, second stage larvae called protozoae graze on microscopic algae (phytoplankton). Protozoae in turn develop into final mysis stage larvae that progressively switch to a diet of other small planktonic animals (zooplankton).
Two to 3 weeks after spawning the larvae develop into postlarvae, which resemble small adult prawns. Young prawns move into estuaries and develop into juveniles. The life cycle is completed when the prawns, now half to 1 year old, move out to sea to mature and mate.
Most prawn farming operations in New South Wales are based on hatchery-reared postlarvae. Hatchery techniques for most species are well developed, and Australian hatcheries produce about 50 million postlarvae each year. Although there are a few multi-purpose marine hatcheries in New South Wales that could produce penaeid postlarvae, most postlarvae are currently produced in Queensland.
Broodstock black tiger prawns are usually trawled from northern Queensland and transported to hatcheries for land-based maturation (ablation). Alternatively, broodstock are reared in ponds from hatchery juveniles. Although the technology for induced maturation has already been achieved and perfected, wild spawners were still preferred by most hatchery operators over the hand reared stock, mainly because of better quality eggs and superior fry produced for pond stocking. To match this performance, it is believed that a superior maturation diet rich in unsaturated fatty acids (HUFA) and essential omega-3, has yet to be proved in the market for the land-based maturation of black tiger prawns.
Development of aquaculture:
In 1996, FAO acknowledged the tremendous increase of prawn production development from 0.4 million tonnes in 1986 to over 0.92 million tonnes in 1994. The overall increase was due largely from the continuous increase in the culture of black tiger prawns from 0.33 million tonnes in 1991 to 0.51 million tonnes in 1994. Black tiger prawns constitute more than half of the world supply of farmed prawns.
In general, prawn farming in Australia was slow to develop, in contrast with other countries in the Asian and Latin American regions. Apart from perceived government regulatory impediments seen to be retarding aquaculture’s further development, there are also few failed ventures during the late 1970′s, until early 1990′s when the industry was dominated by a small number of large producers. In 1995-96, ABARE estimated 83% of total production as being accounted for by 42% of farms. For NSW production figures see the NSW DPI aquaculture production reports.
Given the current NSW Government’s regulatory reform covering aquaculture industry, it is projected that a minimum of 500 hectares of prawn ponds will be constructed within a 5 year period. At full operation, this would generate revenue of $50 million and potential direct employment of more than 300 Australians.
Growth:
Growth of penaeid prawns in ponds is usually much faster than in the wild. Tiger prawns can be grown from postlarvae to a market size of 30g or more in 3-6 months, depending upon conditions in the pond, especially temperature and stocking density. The optimum temperature ranges for growth of tiger prawns and school prawns are 25-35ºC and 21-27ºC respectively. Outside these ranges growth rates are slowed down.
Growout methods:
Some farmers in New South Wales and Southern Queensland have built greenhouses over small (less than 25m²) nursery ponds or raceways to increase water temperatures during cooler months.
Optimum conditions are maintained within these ponds, and prawns are usually fed high quality artificial diets. The use of nursery ponds allows farmers to accurately estimate survival rates when prawns are transferred to growout ponds, and improves production capacity of the pond (shortened days of cultivation DOC).
Most farmers in Australia stock their ponds at densities of 15-25 prawns/m² when postlarvae are scarce, or if the ponds are not aerated and have minimal water exchange capability. The very high densities (for example, greater than 75 prawns/m², depending on species) used overseas have not been applied on commercial prawn farms in Australia because of its unsustainable nature.
The average pond size and production per unit area in New South Wales is about 1ha and 3.5t/ha/year. Over the past 5 years there has been a trend for pond sizes to decrease and aeration and water exchange capacity to increase. This has led to an improvement in production per unit area.
Generally farmers add inorganic fertilisers to their ponds to stimulate and maintain algal blooms of phytoplankton. These algal blooms are encouraged as they help stimulate a natural food chain within ponds. They also assimilate potentially harmful metabolic waste products (for example, ammonia), help maintain dissolved oxygen concentrations, and (by increasing turbidity) help prevent the colonisation of the pond bottom by benthic macrophytes. When blooms become too intense (indicated by rapid algae die-off) pond water environment is managed by proper water exchange.
Average water exchange rates range from 5-10%/day and usually increase up to 30%as prawn size grow. Most farmers have utilized 0.75 - 1.0 kW paddle-wheel aerators. These are operated during the night to prevent dissolved oxygen and pH to reach critical concentrations, and during the day for limited periods to help circulate pond water and prevent stratification.
Information Sourced From:
