Farming Barramundi

Extensive Pond Culture

Extensive larval rearing is carried out in earthen ponds ranging from 0.1 to 1 hectare surface area. Ponds designed with a maximum depth of 1.8 metres and a central concrete raceway into which the entire pond drains, enable easy harvesting (see Shelley (1993) for details). Inlet water should be pre-filtered to 300µm to filter out potential predators and eggs of other fish species.

Ponds to be used for larval rearing should be filled and fertilised with a combination of organic and inorganic fertilisers 8 to 10 days before the larvae are stocked. Fertiliser regimes for extensive pond culture varies across Australia, and the natural productivity of the seawater and pond soil at each site must be taken into account when determining the correct fertiliser regime. Table 3 provides the recommended fertilisation rates for northern Queensland. Schipp (1996) details rates for the Northern Territory. Farmers should develop their own application schedule based on these two examples.

Larvae are stocked before the first feed, which is up to two days after hatching, and prey on naturally occurring pond zooplankton.

The need for inorganic fertiliser is monitored with a Secchi disk, which measures turbidity. A Secchi disk is a flat disk measuring 20cm in diameter, painted with alternating black and white quadrants (Figure 3). It is lowered into the pond and the depth at which it just disappears and reappears is the 'Secchi depth'. To maintain adequate phytoplankton density a Secchi reading of 30 to 40cm should be maintained (please refer to Rimmer (1991) for more information). Care must be taken in the later stages of the pond cycle because excess inorganic fertiliser may stimulate a bloom of undesirable algae such as filamentous algae or blue-green algae (Rimmer, 1991).

Figure 3 : Diagram of a Secchi disk, which is used to measure water turbidity

The later applications of organic fertiliser are designed to feed the zooplankton by stimulating microbial production while not creating a large oxygen demand. If dissolved oxygen levels drop severely, organic fertiliser should not be applied. In the case of particularly severe oxygen depletion (<3mg/L) emergency aeration should be employed (Rimmer, 1991).

Copepods are usually the major component of zooplankton in a brackish water pond. The nauplii of copepods are around 50 to 100 mm in size and are ideal food organisms for newly hatched fish larvae. Rotifers may also be introduced to ponds to be eaten by barramundi larvae (Rimmer, 1991).

The major advantage of extensive larval rearing, in comparison with intensive techniques, is that it requires less labour and facilities, resulting in cheaper production of fingerlings. Larvae also tend to grow faster and are generally more robust (Rimmer & Rutledge, 1991).

In 1996/7, barramundi fingerlings from intensive hatcheries were sold for about 80 cents each, while those reared in ponds sold for 20 cents each. The price for extensively reared fingerlings will probably decrease further in the next few years as the technique is more widely adopted (Lobegeiger et al., 1998).

Water quality in the pond should be measured at least daily. Ponds should be routinely monitored for stratification as well as temperature, salinity, dissolved oxygen, pH and turbidity. Zooplankton populations should be monitored at least twice weekly (Rimmer, 1991).

The recommended maximum stocking density for barramundi is 900,000 larvae per hectare. Growth is rapid and the barramundi reach 25 to 35 mm total length (TL) in about three weeks (Rimmer, 1991).

Fingerlings reared extensively in ponds are generally harvested at 25 to 35 mm TL. These fish are then transferred into weaning tanks at densities of 5,000 to 10,000 per cubic metre (m³) and virtually all are fully weaned within two days (Rimmer, 1991).

Ponds should be dried at least once per year to allow the release of nutrients bound up within the pond substrate, as well as killing potential pathogens. Ponds that can not be totally dried may need an addition of lime to act as a sterilising agent in the wet areas (Schipp, 1996).

Weaning & Nursery Phases

Barramundi larvae metamorphose into fingerlings at around 21 days old and are around 20 to 25 mm TL.

Once the larval stage is completed, barramundi fingerlings need to be weaned off live foods and onto an artificial dry pellet. An Australian company produces weaning diets for barramundi. Other diets are available from overseas but are very expensive and need to be imported (namely SEVBAR™ and Skrettings™) (Schipp, 1996; Phillips, 1998). Weaned fingerlings are usually retained in nursery systems/small tanks or fine-mesh cages in larger tanks until at least 80 mm (Barlow, 1998).

The weaning process often results in up to 90 per cent mortality due to cannibalism. Regular grading is the only way to control cannibalism (Shelley, 1993). Fingerlings less than 50 mm TL require grading every 3 to 4 days if grown in water temperatures of 28^o to 30^oC. Once they reach 50 mm TL grading is only required once a week until they reach 80-100mm TL. Fish may be graded again at 200mm TL and finally at 300 mm TL (Schipp, 1996).

Barramundi fingerlings are known to survive in waters with a salinity of over 50 ppt and at temperatures of up to 35^oC. They can also survive temperatures as low as 16^oC. The optimum temperature for growth is considered to be between 28^o to 32^oC and the optimum salinity range is 0 to 36 ppt (Schipp, 1996).