From the spawning tank, smaple of eggs are counted to determine the number of eggs spawned per female. In normal condition, fertilized eggs hatched within 12–15 hours. The hatching rate is measured by assessing the number of hatched nauplii. Nauplii are then transferred directly into the 40-ton tanks if the number of nauplii is between 0.5 million and 1 million. They are then reared directly in the large tank up to the 25th post larval day. On the other hand, if the number of nauplii is less than 0.5 million, they are stocked in 2.5-ton indoor tanks at a density of 100–150 larvae/liter. The larvae are reared either to the third mysis stage (M3) or one day old (P1) post larvae. They are then transferred to the outdoor 40-ton nursery tanks for further nursing (Fig. 31).
Fig. 30. Harvesting of nauplii and transport containers.
stop aeration
allow nauplii to swim toward the surface then collect nauplii using beaker
drain the water through a filter net for total harvest
jerrycan
plastic jar
plastic bag
After hatching, the newly hatched nauplii are stocked at a density of 100–150 nauplii per liter in the 2.5-ton larval rearing tanks with fresh filtered seawater filling up to 3/4 of tank capacity. No feed is required at the nauplii stage since the nauplius still utilizes its yolk as food. However, diatom are inoculated immediately after stocking to ensure availability of feed when the nauplii molt into the protozoea stage.
Protozoea stage
This is a critical stage of larval rearing. The larvae at this stage start feeding on external food and feed on minute and easily digested microscopic algae such as Skeletonema costatum, Chaetoceros sp. and Tetraselmis sp. The optimal feeding of phytopla ton in the rank is 50,000 cells/ml for Skeletonema or Chaetoceros and 10,000 cells/ml for Tetraselmis.
On the other hand, there is a bright prospect in the use of wet or dry processed invertebrates tissue or encapsulated or microencapsulate feeds in shrimp hatchery feeding strategy. The feeding scheme advocated is based either on the exclusive use of a marine invertebrate wet or dry processed tissue as feed for all the shrimp larval stages or the exclusive use of microencapsulated The use of these types of feed can reduce production cost as well as make the feeding regime of shrimp larvae more convenient especially for small-scale or backyard hatcheries which can cot afford to have a phycology laboratory. The use of marine invertebrates as food organisms can be purchased at low prices and in large quantities since these are available locally. The commonly used food organisms are paste shrimp (Acetes sp.), rock shrimp (Metapenaeus sp.), stomatopod (Oratosquilla sp.) blood cockle (Anadara sp.) and mussel (Perna sp.).
The shrimp larvae are fed with either wet or dried processed crustacean tissue during protozoea stage. However, the time of metamorphosis is usually uncertain. Thus, feeding must start one day ahead of the expected time of metamorphosis, that is, feeding starts from nauplii four. The quantity of feed given is 10 μg/larva/day and 50 μg/larva/day for dry and wet processed crustacean tissue respectively. The amount of feed given is increased by 20% daily. The particle size used are 50>125u, 125>250u and 250>350u for protozoea, mysis and early postlarvae, respectively. Based on opbservations, feeding in outdoor tanks appears to be better than that in indoor tanks.
Fig.31. Schematic diagram of prawn production from hatchery to grow-out ponds.
Microencapsulate diets on the other hand is the latest research product in feeding strategy. However, studies using this type of feed are still in progress. The recommended feeding rate according to Jone (1984) is 16 μg/larva/day and increased daily by 20%.
Mysis stage
The larvae at this stage will start feeding on rotifers (Brachionus plicatilis) or the brine shrimp nauplii required depends on the density of shrimp larvae being reared. Each mysis larvae consumes about 100–200 rotifers or about 20–50 artemia nauplii per day or a standard ratio of about 5 grams dry Artemia cysts is required per cubic meter of rearing water.
During this stage, the tank bottom is already filled with dead organisms and must be siphoned out daily. Once the larvae reaches the first day of postlarval stage (P1), they can be transferred to the bigger nursery tanks.
One day before transferring the postlarvae to the outdoor tanks, the nursery tanks should be first filled up with fresh filtered seawater to allow adequate blooming of diatom. The postlarvae is stocked at a density of 15–20 larvae/liter.
The initial water level in the 40-ton nursery tanks during stocking is 100 cm. The nauplii density is usually about 20–50 per liter. Immediately after stocking, diatom starters are inoculated to ensure bloom of the desirable species. Technical grade fertilizers can be used directly to enhance algae growth. The fertilizers used are:
| KNO3 | 3 ppm |
| Na2HPO4 | 0.3 ppm |
It is pertinent to monitor the types and density of algae in big tanks to ensure that the optimal density is maintained.
During the protozoea stage, about 10–20 cm of fresh filtered seawater is then added daily. However, the amount of water added is dependent on the diatom growth. When diatom density is below the desired level in the culture tank, more cultured diatom and fertilizers are added to accelerate algal bloom. On the other hand, over-blooming of algae should also be controlled by shading or by draining out a portion of water and replenished with fresh seawater.
Mysis stage
The same operational procedure for rearing of mysis stage used in the small-tank system is employed in the outdoor nursery tanks. After water level in nursery tank has been filled to its full rearing capacity, approximately 30% of the water is changed daily.
Postlarvae stage
Early postlarval stages (P1-P6) are fed with brine shrimp nauplii at a rate of 100–200 per postlarvae per day.
Once the postlarvae reached the sixth day (P6), they are fed with finely minced mussel or cockle meat or larval pellet feed while Artemia feeding is stopped at P9. Beyond this stage, the larvae are fed solely on minced mussel or cockle meat or articicial diets 3 to 4 times daily. In the meantime, polyethyelene nets were provided as a substrate for the larvae.
Good water quality should be strictly maintained especially during this phase of larval rearing. While 30–40% of the rearing water is being changed daily, efforts must be made to ensure saturation of dissolved oxygen concentration in the water and low concentration of ammonia (0.1 ppm). Hence regular siphoning of tank bottom to remove excess feeds, metabolic wastes and dead algae is an important and routine hatchery function. When 50% of the rearing water has been drained, continuous flow-through of fresh seawater is maintained for 2–3 hours. Such flow-through operation enables the suspended (solid) particles to be drained out and water clarity maintained.
Sometimes, when over-blooming of diatom becomes uncontrollable, or the sediments accumulated at the tank bottom is too thick, which deteriorate water quality rapidly, it is necessary to transfer the larvae to another well-prepared tank.
It is highly important to ensure that there is no drastic fluctuation of environmental conditions. Efforts must be made to maintain stable condition in the tank and the water quality maintained within the allowable limits:
| Salinity | 30–32 ppt |
| Dissolved oxygen-saturation | |
