Potential locations for prawn farms are rare, due to the specific requirements needed. While most northern coastal areas of Australia are suitable for prawn farming, many are populated residential areas making environmental approvals difficult. Coastal areas remote from residential areas may lack the necessary infrastructure (like roads or reliable power), or are too far from markets, to make farming viable.
Prawn farming has been trialled unsuccessfully in the Northern Territory and it is thought appropriate farm sites exist in the north of Western Australia between Geraldton and Kununurra where some investment has taken place.
However, prawns are only farmed commercially in northern New South Wales (around Yamba) and in clusters along the entire Queensland coast at Gold Coast, Bundaberg, Mackay, Townsville and Cairns. Ninety-five per cent of Australian farmed prawn production takes place in Queensland.
Before establishing an aquaculture enterprise, potential operators need to understand the relationship between air temperature and water temperature in their region. Temperature will have an impact on the growth rates of prawns and therefore the number of harvests in any one year.
Prawn farms require temperatures above 25°C during the production season and the water temperature should not fall below 22°C.
Australian prawn farms located south of Townsville in Queensland are usually limited to one crop a year due to the colder winters. Farms at higher latitudes may produce two crops by stocking ponds in winter, typically aiming for a harvest around Christmas when prices are higher and restocking in January.
Prawns are housed in earthen ponds in a layout that allows water to flow in from a natural sea water source, such as tidal rivers or creeks, through the ponds before being processed through an effluent treatment system and draining back to the natural source.
Ponds are generally a square or rectangle shape, with sloping walls and rounded edges to maximise water movement. Pond walls are often lined with a polyethylene black liner to protect against erosion. Ponds can vary in size from 500m2 to 2ha, but tend to average around 1ha in size.
Delivery of water to the ponds is undertaken by an aqueduct system that uses gravity and pumps to move water from a storage reservoir around the ponds.
To grow prawns successfully, expertise in pond ecology, biology and water quality dynamics is required. Prawns must have clean water and they are highly sensitive to any changes in water quality. This makes the management of ponds the most critical aspect of prawn farming.
The optimum location for a prawn farm is on flat land that is less than 1km from access to estuarine or marine (sea) water. At this close proximity to water, elevation of the land becomes important and an understanding of the location’s highest astronomical tide (HAT) will be required. The farm elevation should be more than 1m above the HAT to ensure ponds can still be drained for harvest during high tide, but less than 10m above the HAT to minimise the energy costs of pumping water through the farm ponds.
The farm site should also have access to mains electricity (three-phase) and good roads that make transporting produce easy.
To ensure the necessary water quality, the site should have access to an unpolluted water supply, with an optimum salinity range of 15 to 36 parts per thousand (ppt). It is important to understand the impacts of seasonal events, like rainfall and evaporation, on salinity noting that salinity should not be less than 1ppt or greater than 40ppt (which is the average salinity level for seawater).
The optimum range for pH of the water source is 7.5 to 8.5. The pH of estuarine waters can be affected by local soil factors. Also ensure water sources are not polluted by industry, urban areas, agricultural activities or water treatment facilities.
Ensuring adequate volumes of water is also important to prawn health. Determine whether sufficient daily volumes of water will be available to supply a prawn farm in any location.
Soil type is another important consideration in the establishment of a prawn farm. Soils will need to have adequate clay content to create a water seal to reduce the loss of water by seepage, low organic matter content and a pH of 5.5–8.5. It is recommended that soil sampling be undertaken to determine the site’s suitability for pond construction. In particular, understanding the load bearing capacity, permeability and presence of contaminants, is important.
Ponds should be filled with water to a depth of 1m, 21 days before prawns are added. This allows for the establishment of a healthy and stable bloom of algae and other plankton that makes up a large proportion of the larval prawn’s diet.
After the prawns are harvested, it is recommended that the pond is drained and dried out for a minimum of two weeks. This allows for the removal of any decomposed organic material and enables soil remediation if necessary. Some farmers till the soil at the bottom of the pond to breakdown any organic residues and nutrients. It also exposes more surface area to sunlight, which will kill any algal spores or fish eggs. Heavy rollers may then be used to compact the soil.
The Australian prawn farming manual includes detailed information about pond establishment.