Prawns - aquaculture prospects

Introduction

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.

Diet

Larval prawns are fed on cultured phytoplankton and zooplankton, including brine shrimp (Artemia salina). Some formulated, microparticulate diets are also available for larval prawns. Formulated diets are used as soon as postlarvae are stocked into nursery or growout ponds. Initially, diets developed for poultry or trout were used, but these have now been completely replaced by specially formulated penaeid diets because of better performance.

Penaeid prawn diets are usually based on fishmeal and Soya bean, and often include small quantities of squid or other mollusc meal. These are well digested and utilized by prawns and supplies a well-balanced mix of essential amino acids for growth and development. Although there are abundant supplies of protein rich meals from terrestrial animals and plants in Australia, very few fishmeal or squidmeal products or other aquatic protein meals are produced.

Diseases and predators

To protect native species and reduce the risk of vertical transmission of diseases, no live penaeids are allowed into Australia. So far, this appears to have prevented the introduction of major Penaeid viral diseases such as Infectious Hypodermal Haematopoietic Necrosis Virus (IHHN), Yellow Head and White Sport viruses, which have caused very serious disease problems in countries that permitted importation. Monodon Baculo Virus (MBV) had been recorded in Australia although this appeared to be a problem in hatcheries where prawns are cultured under sub-optimal conditions. Although viral, bacterial, fungal and protozoan infections have occurred in Australian prawn hatcheries and ponds, there have been no serious epidemics reported.

Predatory fish are usually excluded from ponds by proper screening of incoming waters, but predatory birds, especially the black cormorants, have caused major problems in some areas.

Commercial farm development potential and sustainable aquaculture strategy

Ten major estuaries from the Tweed River down to Manning River, have recently been investigated by the New South Wales Fisheries following a joint program of government agencies and representatives of the aquaculture industry, to evaluate areas suitable for land-based commercial aquaculture development. The project involved risk analysis of potential sites as well as formulation of specific environmental codes of practice as a strategic process to ensure a viable and environmentally sustainable aquaculture industry.

Geographical Information Systems (GIS) technology was utilised to map suitable sites. It is envisaged that this risk mapping process will help direct the investment community to short-listed areas in these estuaries where the proponent can undertake specific site assessment. This approach will be the basis for the future commercial development of viable and sustainable aquaculture industries for the estuarine farming of endemic prawns and finfish species in NSW.

Marketing

For annual nationwide commercial prawn catches the Commonwealth Bureau of Rural Science's Fishery Status Reports. Much of the fishery's catch is exported. In NSW, black tiger prawn farms recorded a 463 tonne production valued at more than $6 million for 1998. Mostly, the prawns are marketed as a value-added product (cooked prawns) for local markets, mainly Sydney and Brisbane.

Conclusions

There now exists excellent opportunity for the prawn industry to boom in NSW because of the Government's recent approaches to streamline regulatory frameworks controlling aquaculture development, to ensure an economically viable and ecologically sustainable aquaculture industry. Problems such as urgent establishment of commercial prawn hatcheries in NSW, identification of suitable sites for prawn farm development, bridging the gap between research and the industry, and code of best practice, were being addressed to by the Government's inter-agency Steering Committee also recently established.

The challenge for the prawn farming industry throughout Australia is to reduce production costs, maintain high product quality and remain competitive.

Problems with broodstock availability and quality, and the maturation of broodstock in captivity, will also need to be solved to ensure that availability and quality of postlarvae do not restrain the prawn farming industry.

FURTHER INFORMATION

For further information, call NSW DPI Port Stephens Research Centre on 0249 82 1232

REFERENCES

This information was edited by James Dominisac, NSW DPI

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