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Growing and harvesting black tiger prawns

Growout

Pond preparation

Between harvests, stocking ponds are dried out 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). Prior to filling, 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 per square metre. The post-larvae are weaned from plankton to manufactured feed over the first 4 weeks.

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. Consumption is measured at each feed by using feed trays that are submerged along the pond edges. Growth rates are monitored via weekly sampling of the prawns. This feed management enables farms to minimise feed wastage and maintain good feed conversion rates.

Feed conversion ratios (kilogram of feed to produce 1kg of prawns) range from 1.6:1 to 2.2:1.

Water quality

Stable pond conditions and good water quality are necessary to maximise survival and growth rates. Paddlewheels and aspirators are normally used for aeration. The aeration generates a current causing the sediments to accumulate in the centre of the pond. This maintains a clean feeding area around the pond 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. Water exchange is minimised to help maintain stable water conditions.

Wastewater is removed from the bottom of the pond and drained into sedimentation or treatment ponds before being reused or released to the environment.

Harvesting

Crops are normally ready for harvest in 120-150 days; however, the time will depend on stocking rates and water temperature.

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.

Normally, prawns are harvested when they are 25-35g each. 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.

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General enquiries 13 25 23