Black tiger prawn aquaculture

Market and industry

Black tiger prawns are Queensland's largest, fastest growing aquaculture activity.

Prawns are harvested early in the year up to May, and on demand. Most of black tiger prawn crops are sold on the domestic market. Prawns are washed, graded and generally cooked before marketing. Some prawns are sold green and others are individually quick–frozen for storage and sale at a later date.

Successful prawn farming requires clean and sustainable coastal resources. The industry is regulated by local, state and, in some areas, commonwealth laws to ensure farmers meet high standards for environmental protection.

Read the annual aquaculture industry report for the latest prawn valuation and production data.

You can also contact the Australian Prawn Farmers Association (APFA) for advice and support.

Culture and production systems

The intensive culture of marine prawns uses hatcheries and purpose–built earthen ponds, constructed on coastal lands or next to the estuarine parts of river systems.

Most ponds are about 1 hectare in size and have a gently sloping bottom to allow drain harvesting of the prawns and full draining for a dry-out period between crops. Depth of a pond varies from 1.5–2.0m.

Read the Australian prawn farming manual for detailed information and guidance on all aspects of prawn farming.

Water salinity

Black tiger prawns grow best in warm brackish waters (water that is more saline than freshwater, but less than seawater) and can grow quickly under a range of salinities. Prawns can survive in zero salinity (freshwater) for short periods.

Maximum growth rates occur in 15–20 parts per thousand (ppt) salinity—seawater is normally 35ppt.

Water temperature

Black tiger prawns are a tropical species that achieve the best growth rates when water temperatures are 25–30°C. Lower temperatures will cause feeding to stop at 20°C, and deaths around 14–15°C.

Provided you have controlled stocking rates and selective harvesting in place, this temperature range limits production to:

• one crop during summer in areas south of Mackay
• two crops between Cardwell and Cooktown.

Crops are normally ready for harvest in 120–150 days (when prawns are 25–35g each) however, the time will depend on stocking rates and water temperature.

Stocking density

Ponds are stocked with post–larvae at rates varying from 25–40 per square metre. The higher the stocking density, the slower the growth rate. With water temperatures above 24°C prawns should reach 30–35g each in 120–150 days.

Harvesting from ponds

Ponds are sometimes partially harvested using traps or seine nets, but more often a drain harvest is used. The water is released through the outlet structure, which has a net fitted over the pipe and the prawns are then caught in this net. Partial harvests may be used early in the season to reduce the density of prawns in the pond and allow the prawns remaining to grow to a larger size.

Breeding

Prawn larvae are produced in hatcheries, which use wild-caught spawners or broodstock collected mainly from the coastal waters between Cooktown and Innisfail.

The majority of these spawners are gonadally mature and when they are transferred to the hatchery they either spawn spontaneously or are induced through the use of eyestalk ablation. Each spawner will produce between 200,000 and 500,000 eggs.

Post larvae stage

The eggs are hatched and pass through a number of larval stages until they reach the post–larvae stage. The post–larvae are normally sold to the grow out farms for stocking ponds when they are 15–20 days old. The quality of the post–larvae will vary between spawners and the time of the year the spawners are collected.

Feeding and growing

Stable pond conditions and good water quality are necessary to maximise survival and growth rates. Paddlewheels and aspirators are normally used to aerate ponds. Aeration generates a current causing the sediments to accumulate in the centre of the pond. This maintains a clean feeding area around the pond’s edge.

As the quantity of prawns (biomass) increases, the level of aeration required increases to maintain the levels of dissolved oxygen in the water. A minimum of 1 kilowatt of aeration is required for each tonne of prawns in the pond.

Exchanging water controls the density of algal blooms and ammonia levels in the ponds. Wastewater is removed from the bottom of the pond and drained into sedimentation or treatment ponds before being reused or released to the environment. Water exchange can be minimised to help maintain stable water conditions.

Prepare ponds for grow out

Stocking ponds are dried out between harvests, and accumulated sediments from the previous crop are removed. These sediments contain organic matter (dead plankton, faeces, uneaten food and old moults) and mineral matter (soil particles eroded from pond walls by aeration).

Before being filled, the ponds are limed to encourage plankton growth and minimise disease problems. The ponds are filled through screened inlets, fertilised with both organic and inorganic fertilisers and allowed to stand for 7–10 days to allow the plankton to develop.

Feeding

Ponds are stocked with post-larvae at rates varying from 25–40 animals per square metre. The post–larvae are weaned from plankton to manufactured feed over the first 4 weeks. Feed conversion ratios (kilogram of feed to produce 1kg of prawns) range from 1.6:1 to 2.2:1.

Prawns are fed 3–4 times per day, with the last feed being given close to dark. Feed is blown into the pond using an engine-driven blower. You can measure consumption at each feed by using feed trays submerged along the pond edges.

Growth rates should be monitored via weekly sampling of the prawns. This feed management helps minimise wasted feed and maintain good feed conversion rates. Generally, prawns are harvested when they reach 25–35g each.