In any shrimp farming, management of water quality is of primary consideration particularly in ponds with higher stocking rates. Degradation of water quality is detrimental to shrimp growth and survival. Good quality water is usually defined as the fitness or suitability of the water for survival and growth of shrimp.
Younger shrimps appear to tolerate a wider fluctuation of salinity than the adults. The post-larvae of many Penaeid species can tolerate wide salinity fluctuation which has little effect on their survival or growth. In pond condition, P. monodon can tolerate wide range of salinity from as low as 5 ppt to a high of 40 ppt. Many Metapenaeus also tolerate high salinity. P. merguiensis and P. indicus prefer brackish water while P. semisulcatus and P. japonicus require more saline condition for growth (27–32 ppt).
Due to high evaporation rate in some countries, salt concentration in ponds gradually increases during the summer months. Salinity may increase to beyond 40 ppt and thus retard growth. This should be taken into consideration when brackishwater species are used since salinity may increase above their limit of tolerance. In such cases, the species cultured should be changed during the summer months to conform with increasing salinity or the water should be changed frequently either by pumps or through tidal exchange.
Table 4. Methods of Measuring the Physico-chemical Parameters of Ponds.
|Parameters||Method of Measurement||Remarks|
|1. Dissolved oxygen||Winkler titration method||A traditional procedure which is generally conducted in the laboratory.|
|Polarographic method||This involves the use of a portable DO meter for field use. It is reliable and accurate. It is important to calibrate using the data obtained form Winkler Method. from time to time.|
|2. pH||Electrometric method||This uses portable pH meters which are standardized using standard buffer solutions. It is gives accurate results especially with glass electrodes.|
|Colorimetric method||This makes use of comparators such as Lovibond. The samples after addition of indicators are compared with colored solutions of known pH.|
|pH papers||It does not give accurate values but sufficient enough for fast baseline monitoring in the field.|
|3. Total Ammonia Nitrogen||Nesslerization methods||Widely used method for water sampels but tedious.|
|Phenate method||It is less cumbersome than nessler. Fast and accurate. It can be used using a specific-ion meter or a spectrophotometer.|
|4. Nitrate||Cadmium reduction method||Quite cumbersome but relable.|
The pH of the pond water is indicative of its fertility or potential productivity. Water with pH ranging from 7.5 to 9.0 are generally regarded as suitable for shrimp production. The growth of shrimps is retarded if pH falls below 5.0. Water with low pH can be corrected by adding lime to neutralize the acidity.
Water of excessive alkalinity (pH values > 9.5) may also be harmful to shrimp growth and survival. In ponds which are excessively rich in phytoplankton, the pH of pond water usually exceeds 9.5 during late afternoon. However, at daybreak, the pH is usually lower. Excessive plankton growth can be corrected by water exchange.
Maintenance of adequate level of dissolved oxygen in pond water is very important to shrimp growth and survival. Prolong exposure to the stress of low concentration of oxygen lowers their resistance to disease and inhibits their growth. In most cases, oxygen depletion often resulted in mass mortality (anoxia) of shrimp stock. This is particularly common in intensive culture operation.
Dissolved oxygen in the pond water comes from two sources. Most of it comes as a by-product of photosynthesis. The other source is from the diffusion of atmospheric air. The amount of dissolved oxygen in the pond water is affected by many factors particularly water temperature, respiration and the level of organic matter. In tropical shrimp pond, the oxygen level in the pond water is normally low because of the higher temperature. However, tropical species are able to adopt to lower oxygen concentration than their temperature counterparts.
During daytime, more oxygen is produced through photosynthesis than is removed from the water by the respiration of animals. At night, both plants and animals continue to respire while oxygen is being added to the water only from the atmosphere. In some instances, the respiratory demand under certain circumstances cause total depletion of oxygen especially at daybreak causing anoxia of the cultured animals.
Depletion of DO in the pond can be controlled by the following measures:
Water exchange throug renewal of pond water with fresh water either by tidal flow or pumping.
Installation of aeration system. In the design of pond layout, it is essential to consider maximal utilization of the natural environment to maintain higher dissolved oxygen content in pond water such as:
Orientation of the long axis of the pond with the prevailing wind.
Construction of larger pond to allow a greater contact of water surface with atmospheric air.
Promote wind action on the pond in facilitating water movement and oxygen diffusion.
Avoid planting of trees on dikes.
Nitrogen in pond exists in different forms such as nitrate, nitrite, ammonia and various forms of organic nitrogen. Organic nitrogen ranges from relatively simple dissolved compounds such as amino acids to complex particulate organic matter. Nitrogen occurs in the mud in the same form that exist in water. In pond culture activities, ammonia nitrogen (in the form of un-ionized ammonia) is considered important since this compound is toxic to aquatic animals at certain concentrations. Ammonium ions which is another form of ammonia nitrogen is harmless except at extremely high concentrations.
Ammonia nitrogen is a product of fish metabolism and decomposition of organic matter by bacteria. The pH and temperature of the water regulate the proportion of total ammonia which occurs in un-ionized form. The highest concentration of total ammonia nitrogen usually occur after the peak of phytoplankton bloom when most of them died. The pH of the water is low because of high concentration of carbon dioxide. Studies have shown that exposure to ammonia concentration of 0.45 mg NH3 -N/1liter would reduce shrimp growth by 50%.
Water temperature plays a very important role in regulating the activities of the cultured animal. The rate of chemical and biological reactions is said to double every 10°C increase in temperature. This means that aquatic organisms will use twice as much dissolved oxygen and chemical reactions will progress twice as fast at 30°C than 20°C. It follows therefore that dissolved oxygen requirement of aquatic species is higher in warmer than in cooler water.
Many Penaeid species are tropical or subtropical species. The optimum temperature is about 25–30°C and hence many of the species such as P. indicus, P. monodon and P. merguiensis can be cultured throughout the year while P. japonicus and P. orientalis are limited to the summer growing seasons only.
Hydrogen sulfide can severely affect shrimp growth in pond. H2S is produced by chemical reduction of organic matter that accumulates and forms a thick layer of organic deposits at the bottom. The bottom soil turns black and a rotten smell is discharged if disturbed. High levels of hydrogen sulfide would affect directly demersal or burrowing shrimps such as P. monodon. At levels of 0.1–0.2 ppm in the water, the shrimps appear to loss their equilibrium and die instantly at a concentration of 4 ppm.
Using iron oxide (70% ferrous oxide) to treat the bottom soil containing high levels of H2S would not be economical. The cheaper means is by frequent exchange of water to prevent building up of H2S in the pond.