Post-larvae are removed from larval- rearing tanks and concentrated for counting, packing and shipping. Shrimp PL are shipped at densities of 1,000 to 2,000 per L in polyethylene bags containing seawater (30 to 35 ppt) that is saturated with oxygen and cooled to 18 °C (64.4 °F) before packing.
The bags are usually packed in insulated foam coolers to maintain temperature during shipping. Some farmers save on the high cost of freight by transporting high densities of PL (more than 1 million) in live-haul trucks with oxygenated tanks.
Acclimation:
When a PL is at least 8 days old (PL8), it is sufficiently robust to handle the stress of transport and acclimation. The culture system must be more than 15 ppt salinity and warmer than 20 °C (68 °F) for the acclimation to proceed.
Acclimation is accomplished in small, temporary tanks near the pond (direct stocking) or in a nursery tank (a temporary tank for holding animals before they are released into the grow-out system). During acclimation, the density of shrimp is more than 500 per L. Pure oxygen is provided to the tanks and bags during acclimation.
As a general rule, the acclimation temperature is changed by 1 °C (1.8 °F) every 15 minutes until it matches the culture system temperature. Once temperature is adjusted, salinity can be changed by 1 ppt every 15 minutes until it matches the culture tank salinity.
If the pond has less than 15 ppt salinity and/or a temperature of less than 20 °C (68 °F), it is advisable to use a nursery phase and stock older, heartier juveniles later on.
Zero exchange intensive nurseries:
Intensive nurseries (1,000 to 50,000 PL/m2) with biomass capacities of 2 to 4 kg/m2 operated under zero exchange have become an important tool for shrimp farmers. Zero exchange nursery systems can have survivals of more than 80 percent and a harvest biomass of 1 to 7 kg/m2, depending on the skill of the management and the investment in capital infrastructure (filters and oxygenation).
Basic systems consist of a green house enclosure; a 20- to 60-mil, high-density, polyethylene (HDPE) liner; a sand or bead filter; a regenerative blower for aeration; a propane or electric heat source; and oxygen injection for large biomasses (more than 2.5 kg/m2) and emergencies. With increased experience and capital expenditure, farmers may increase production densities to more than 2.5 kg/m2.
Intensive nurseries increase growout survival by allowing farmers to stock larger shrimp (0.25 to 1.0 g) into grow-out systems. In the temperate climate of the southeastern U.S., greenhouse-enclosed nurseries are used to give shrimp a head start in early fall.
A “head start” nursery is an effective way to increase the number of crops each year and have better control over production by stocking known quantities of heartier juveniles. In regions where early spring temperatures are cooler than 20 °C (68 °F), greenhouse nurseries are essential so that the crop can be held until water in the grow-out system warms up.
Nursery management:
Nursery culture water is maintained at a temperature of 28 to 29 °C (82.4 to 84.2 °F), a dissolved oxygen level of 5 to 7 mg/L, a pH of 7.5 to 8.5, and a Secchi depth of 25 to 30 cm. Ten to 14 days before PL arrive from a hatchery, the culture tanks are filled with seawater (20 to 35 ppt).
Then the water is sterilized with sodium hypochlorite (household bleach at 10 ppm) to prevent the introduction of pathogens from incoming waters. The treated culture water is allowed to sit for 24 hours without aeration to prevent a rapid reduction of chlorine.
During this period the chlorine concentration will remain above 1 ppm to provide the necessary disinfection level. After 24 hours, the water is heavily aerated to remove residual chlorine, a process that takes 3 to 5 days depending on temperature and organic load. The dechlorinated culture water is then fertilized and inoculated with algae.
The raceway water is fertilized with inorganic fertilizer (5 mg/L urea, 0.5 mg/L triple super phosphate and 1 mg/L sodium metasilicate). Water from an adjacent raceway with a healthy algal bloom is generally used to inoculate the new raceway with algae.
Moderate aeration is maintained in the raceways as the algal bloom develops over the next 7 days to a Secchi reading of approximately 25 to 30 cm. It is important to check pH as the bloom develops. The high pH (higher than 8.75) created by the algal bloom can be a problem for young PL.
To counteract it, carbon dioxide from a compressed CO2 cylinder is added to the culture water with an air stone to decrease the pH to 8.0 before PL are stocked. Nursery aeration is then reduced to a minimum and PL are acclimated and stocked in the nursery system.
Author:
Ryan L. Gandy
