Feed management may involve a nursery and a growout phase after the hatchery, but the principles are much the same. Nurseries are a means to (Persyn and Aungst, 2006):
- shorten production cycles in growout ponds and achieve more crops per year;
- regulate the flow of PL from hatcheries to farms;
- shorten hatchery cycles;
- acclimate PL to pond conditions;
- in temperate climates, in indoor nurseries provide a spring head start for PLs;
and - The condition and survival of shrimp PL from the nursery can be well established and nursery juveniles should not suffer much further mortality; while it is difficult to estimate the PL survival in a large growout pond.
Overall survival using nurseries followed by growout stages is often lower than with direct stocking into a large pond, so today nursery usage is not standardly applied. The rationale for direct stocking is that better and more natural production can be achieved in a large pond, so that both faster growth and better survival are achieved. The decision to use a nursery will be site specific. Various nursery configurations are discussed by Persyn and Aungst (2006).
Rearing densities of up to 2 000 kg/ha are considered semi-intensive production that does not require aeration (Figure 26). With large water exchanges to maintain oxygen levels, yields of close to 3 000 kg/ha are possible without aeration, but as soon at night-time oxygen levels fall below 3 ppm and especially if below 2 ppm, supplemental aeration is needed. Yields of over 5 000 kg/ha are considered intensive (Table 16). Modern intensive farming technologies employ low to zero water exchanges with heavy aeration and the promotion and development of bacterial bioflocs that convert excess nitrogen into an organic biomass that shrimp can consume (Avnimelech, 1999; Thakur and Lin, 2003). Harvesting can be done through drain harvesting and partial harvesting (Figure 27)
Natural production
In a well-prepared pond, natural production will provide a significant part of the postlarval shrimp’s nutritional needs and improve initial growth rates. This contribution of natural pond biota decreases over the course of the production cycle, with increasing shrimp size and biomass (standing crop) (for an overview, see Hunter, Pruder and Wyban, 1987; Allan and Maguire, 1992; Hunter, 1996; Gautier et al., 2001; Lee and Lawrence, 1997; Moriarty, 1999; Focken et al., 1998; Villamar, 1999; Nunes and Parsons, 2000; Browdy et al., 2001). Meiofauna are more abundant in a pond prepared with a good fertilization programme (Allan, Moriarty and Maguire, 1995).
Erler et al. (2005) demonstrated that the addition of carbon in the form of molasses resulted in greater growth and improved FCR of shrimp. The water had significantly higher concentrations of bacteria, Total Organic Carbon (TOC) and Total Dissolved Nitrogen (TDN). Phytoplankton abundance was not significantly different between tanks receiving molasses and the control. The increased bacterial biomass appears to be the contributing factor to increased shrimp growth and lower FCR. A higher C:N results in greater availability of feed in the form of proteinaceous bacteria (Avnimelech, 1999; Burford et al., 2003a,b; Ebeling, Timmons and Bisogni, 2006).
Application of formulated (compound) aquafeeds
Daily feed rations are calculated based on estimates of density, mean individual animal weight, survival and feed as a percentage of body weight (Table 7a,b,c). Feed management is now generally computer based. As well as commercial farm management programmes, each manager can establish his own management system. The feed as a percentage of biomass calculates sufficient feed, assuming standard growth, to achieve the target FCR during the growout cycle and provide enough food to achieve optimal growth. The total daily ration is divided into four or five feedings (e.g. 6h00, 25 percent; 12h00: 25 percent; 18h00: 35 percent; 24h00: 15 percent).
Feed as a percentage of biomass must be calculated based on the shrimp mass in grams and not the days of culture. For P. indicus, the formula below has been applied effectively to commercial growout (Sara, 2007):
Feed as percentage of biomass = 100 × (10^(-1.05 - 0.446 × @LOG(prawn weight in grams))) × 0.8 (modified from Green et al., 1997a,b)
Feed tables are a guideline to prevent over and under feeding and target an acceptable FCR over the planned growout period.
Feed trays must be used to monitor actual feeding behaviour (Figure 28). Moulting, temperatures over 35 oC, oxygen levels below 2 ppm, changes in feed brand and health can all effect feeding behaviour. Tables 7a,b,c provide an average feed level and provide the necessary feed increment to sustain growth.
A standard growth curve for optimal growth of P. indicus is:
Shrimp weight (g) = 40 × (1 - EXP(-0.013 × (DOC + 9)))^3.5
Where DOC is the day of culture and EXP is e raised to the power of the number (Excel).
Shrimp are weighed each week, survival estimated based on feed tray records and feed adjusted to prepare a feed guideline for the following week (Table 7a,b,c).
