The maintenance of optimal environmental conditions is necessary for maxima growth and survival of the cultured organisms.
Salinity - Biologically, most penaeid shrimps do not breed in brackishwater while mating, spawning and even hatching of eggs take place only in the open sea. Salinity in spawning grounds normally ranges from 30 to 36 ppt. Thus, seawater salinity in spawning tanks should be maintained at 30–32 ppt to ensure good hatching rates. Moreover, low salinity affects larval growth during the first 15 days of rearing. Though abrupt or extreme variations in salinity may adversely affect larval survival, slight variations in salinity is not detrimental.
Temperature - Temperature directly affects the metabolic system of any given species. In penaeid shrimps, eggs do not hatch at temperatures lower than 24°C. Larvae usually grow and molt faster at higher temperature. The optimum temperature is 26–31°C. Below this level, larvae do not grow well and molting may be delayed. The protozoea of P. monodon, for instance, molt to mysis stage within 4 days at temperatures ranging from 28°C to 31°C, however, molting takes 6 days when temperature drops to 24–26°C. Slight increase in water temperature above threshold may be lethal in the tropic species. Gradual variations in temperature throughout the day is not critical, however, sudden changes even as narrow as 2°C can cause high mortalities due to stress and temperature stock.
Dissolved oxygen - Dissolved oxygen is a critical factor in larval rearing. High mortalities can occur if aeration stops even for only one hour.
pH and nitrogenous compound - Normal pH of seawater ranges from 7.5 to 8.5. The pH value is a key indicator of changes in the water environment of the rearing tank relative to ionized and un-ionized ammonia. This is so because NH3 and NH4 ratio in water is pH dependent. If pH value is high, this signifies increased levels of un-ionized ammonia (NH3) which is toxic to larvae. Ionized ammonia (NH4 +) however, is not toxic because it is unable to pass through the gill membrane of the larvae. Safe ammonia concentrations in water should not exceed 1.5 ppm for NH4+ and 0.1 ppm for NH3. 4
Shrimp larvae at the first protozoan stage cannot efficiently seek food as the swimming appendages have yet to develop. Hence, the feeds must be present in sufficient quantity. On the other hand, diatoms often overbloom in the rearing tank especially those in the outdoor hatchery. This causes high mortality due to attachment of diatom on the appendages of the larvae which makes them unable to move and molt properly. In addition, overblooming of diatom collapses easily the next day and this results in water fouling. Therefore, programming of natural food culture and maintaining feeds at sufficient levels only is an important operational strategy. (Fig. 32A & B).
To monitor if the feed is sufficient in the rearing tank, the density of diatoms is counted daily before and after water management. Diatoms are counted by using a haemacytometer while Brachionus and Artemia densities are established by head count. Once diatoms in the larval rearing tank become brown, new diatom cultures are added to meet the density requirement of the larvae. The approximate density sufficient for larvae in the rearing tank is 50,000/ml for Chaetoceros sp. or Skeletonema costatum and 10,000/ml for Tetraselmis sp. Brachionus must be maintained at 20 individuals/ml and Artemia at 50 grams for every 100,000 postlarvae. Overblooming of diatoms during summer days is controlled by shading the larval rearing tank or by draining out a portion of the water and replenishing with fresh filtered seawater.
Environmental parameters such as water temperature, salinity and pH should be checked twice daily. Meanwhile, the estimated number of larvae at each stage of development should also be recorded. Count larvae in three 1-liter samples for small tanks and 10 times for big tanks. The average number of larvae per liter will give an idea of total amount of larvae. However, larval estimates can be done until P4 only because the larvae changes to demersal feeding habit after this stage. The precise number of larvae will be known during harvest when head counts are done.
