Holding systems

Holding systems vary in size and configuration, depending on how many broodstock need to be held and the length of time required for holding. Holding systems can be relatively small and stocked at high densities, unless prawns need to be maintained for extended periods of time, such as for overwintering in temperate climates. For overwintering, holding systems must be able to maintain broodstock for up to 6 months. Water temperature must be maintained above 25°C to prevent loss of eggs and water quality parameters maintained to prevent stress and maximise broodstock health.

The size of the system depends on the total number of broodstock needed to produce a given quantity of larvae. Survival rates at broodstock facilities at Mississippi State University typically ranged from 50 to 60% for females and from 30 to 40% for males (originally stocked as a mix of

BC and OC males) over a 5 to 6 month holding period when stocked at 1 g of broodstock per litre of culture water in 6.1m diameter by 1.2 m deep tanks (W. Daniels, unpublished). Usually no more than 5 to 10% of the females are ready to spawn at any given time (i.e. all females spawning within a 3 day period). Tidwell et al. (2005) reviewed broodstock holding in temperate environments and provided similar stocking (1 g per litre) and survival rates (50-60% females; 20-30% males) based upon the work in Mississippi and Kentucky. To produce batches of 500 000 larvae, approximately 11 egg carrying female prawns averaging 45 g each would be needed (a smaller number of larger prawns would also be effective). Therefore, a conservative estimate would require that at least 440 female prawns be stocked to provide 500 000 larvae in 5 to 6 months (50% survival, 5% of females spawning at any given time). At least 110 males (44 BC and 66 OC males) would also be required (see section 4.2.3) to service these females. Although the number of broodstock required may seem excessive and cost prohibitive, it should be remembered that this system would support a continual supply of larvae from different batches of females (e.g. 5% of females available every few days or at least once weekly). Therefore, the hatchery design (tank size and numbers) would support a system of multiple batches of larvae.

Holding systems may be static, flow-through or recirculating, depending on the water source and the amount of time that the animals are held. Systems may be tanks or ponds. Ponds can be enclosed for over-wintering to maintain water temperature. To maximise usage, structures (e.g. netting or shelters) should be provided in the water column and on the bottom which allow the animals to spread out and occupy the water column as well as the benthic area. The added benefits are a reduction in cannibalism and an increase in fecundity due to reduction in aggressive encounters. Hanging small-mesh nylon netting vertically or horizontally in the water column is one method of providing additional surface area. These structures may also be buoyed using PVC piping and floats. Smith & Sandifer (1979) described different types and arrangements of structures which were tested for juvenile M. rosenbergii production and found that a layered habitat design consisting of alternating open and strip layers was best. They utilised a modified version of this (previously described in Sandifer & Smith 1977) for maintaining broodstock at their facility (Sandifer & Smith 1978). For experimental purposes, Cavalli et al. (2001a) maintained prawns in recirculation units containing one 150 L holding tank, divided into nine compartments (each housing one female or male) and one 40 L biological filter. Approximately 10% of the water was exchanged daily after the removal of wastes and uneaten feed. Freshwater was maintained at 28°C and the photope-riod was set at 12L/12D (Chavez Justo et al. 1991). Three

18 W fluorescent lamps placed 0.9 m above the water level of each holding tank provided a light intensity of 600 lux.

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