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Fig. 6.3 Typical hatching curve of high quality cysts (solid line), cysts with a lower hatching output (dotted line), cysts with low hatching synchronicity (dashed line) and appearance of second stage nauplii for the good-quality cysts (grey line).

the price differences. Therefore, attention should be paid to selecting Artemia cyst lots with good hatching synchrony (< 7 h between hatching of first and last nauplii), fast hatching rate (<24h) and high hatching efficiency (>200 000 nauplii per gram of product) (Fig. 6.3). When hatching synchrony is low, first-hatched nauplii will have consumed much of their energy reserves by the time that the last nauplii will have hatched and harvesting is done. If the total incubation period exceeds 24 hours, the hatchery operator is unable to restock the same hatching containers for the next day's harvest, which implies higher infrastructural costs.

After hatching, and prior to feeding them to the larvae, Artemia nauplii need to be separated from the hatching wastes. After switching off the aeration in the hatching tank, cyst shells will float and nauplii will concentrate at the bottom of the tank. They should be siphoned or drained within 5 to 10 minutes and thoroughly rinsed with seawater or fresh water, using submerged filters (Sorgeloos & Leger 1992) to prevent physical damage to the nauplii.

6.2.3 Use of nauplii and metanauplii

In their first stage of development, Artemia nauplii do not feed but consume their own energy reserves (Benijts et al. 1976). At the high water temperatures that are applied during cyst incubation, freshly-hatched Artemia nauplii develop into the second larval stage within 6 to 8 hours. It is important to feed first-instar nauplii rather than starved second-instar metanauplii, which are transparent and less visible. Second instar metanauplii are about 50% larger in length and swim faster than first instars. As a result they are harder to catch and less acceptable as prey for those larvae with a critically small mouth size. Furthermore, they contain lower amounts of free amino acids so they are less digestible and their lower individual dry weights (DW) and energy content (Garcia-Ortega et al. 1998) reduce the energy uptake by the predator per unit of hunting effort (Leger et al. 1986). All this will be reflected in reducedlarval growth in the face of increased Artemia cyst consumption: for instance, 20 to 30% more cysts are needed to feed the same weight of starved metanauplii to freshwater prawn larvae. On the other hand, second instar stages may be more susceptible to digestive enzyme breakdown in the gut of the predator, since these enzymes can also penetrate the digestive tract of the Artemia through the open mouth or anus.

Artemia nauplii that are not immediately fed to the larvae can be conveniently stored at low temperature. Storing freshly-hatched nauplii at temperatures near 4°C, in densities of up to 8 million nauplii per litre for up to 24 hours greatly reduces their metabolic rate, and precludes moulting to the next instar stage (Leger et al. 1983). The drop in individual DW at 4°C is only 2.5 versus 30% at 25°C. The 24 hour cold storage technique allows for a rationalisation of Artemia cyst hatching effort and costs (e.g. fewer tanks, larger volumes, a maximum of one hatching and harvest per day). It also allows for a constant supply of a high-quality product and more frequent food distributions. This is beneficial for prawn larvae because food retention time in larviculture tanks can be reduced and hence the growth of Artemia in the culture tank minimised. With poor hunters, such as the early larval stages of M. rosenbergii, feeding cold-stored (and thus less active) Artemia results in a more efficient food uptake.

6.2.4 Nutritional features

A variety of physical and nutritional factors are relevant, as detailed in the following sub-sections of this chapter.

The nutritional effectiveness of any food organism is in the first place determined by its ingestibility, which is determined by its size and form. Although naupliar size varies greatly from one geographical source of Artemia to another (Vanhaecke & Sorgeloos 1980), this is often not critical for crustacean larvae since these can capture and tear apart larger food particles with their feeding appendages. As long as prey size does not interfere with the hunting ability of the predator, the use of larger nauplii (with a higher individual energy content) will be beneficial since the predator will spend less energy in taking up a smaller number of larger nauplii to fulfil its energetic requirements. For

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