Shrimp Culture
Part of the shell left after a moult.
Cannibalism occurs when shrimp has a soft shell.
Early post moult and post moult
Newly moulted shrimp will have limited energy; it cannot swim or walk far and can make just two or three jumps to escape from a predator. The moulted shrimp needs to extract minerals from water (which is the key supplier of minerals) to harden their new shell as quickly as possible, normally less than 1 hour. If it takes longer the chances of being cannibalised is higher.
Essential factors for moulting A proper mineral ratio, pH lower than 8.3 and double the amount of oxygen are prerequisites for moulting. Some farmers might try using some moulting stimulant or hormone, but I would strongly not recommend this. If we force shrimp to moult when they are not ready, it will lead to mortality. The farmer will not know that there are less shrimp in the pond, but might find less feed consumption and low survival rate at harvest.
General moulting problems In moult mortality, it is common to find in shrimp, white muscles along with clear and thin and soft shells. The possible causes are: shortage of oxygen, shortage of a major mineral, improper mineral ratio, low alkalinity, or exposure to H2S toxic gas. Farmers need to maintain a proper alkalinity of at least 120 ppm and keep a correct ratio of Mg/Ca/K. It is imperative to ensure that there is plenty of oxygen and absence of H2S at the pond bottom. It is recommended to use probiotics to eliminate H2S effectively. Thin shell and/or soft shell is mainly caused by a shortage of minerals. Even though shrimp in the earlier moulting are not seriously affected, this is an indicator that the remaining shrimp in the pond will be badly affected in the subsequent moult. The farmer should mix some trace elements into the feed to provide the major minerals during moulting. At the same time, the alkalinity should be maintained at 120 ppm. A plankton crash can also occur when a mineral that the phytoplankton needs in the day has been used up by the shrimp during moulting at night. On the first day after a moult, the farmer can detect a small change in pH. On the second day, the water colour is paler and the pH drops slightly and on the third day, the water colour becomes darker, and there is more foam at the water surface and the pH drops more than on the previous day. A plankton crash usually leads to many serious problems later. To prevent this from occurring, the farmer needs to monitor the mineral content, pH and alkalinity. Incomplete moulting is mainly caused by either an imbalance, improper mineral ratio or shrimp with insufficient energy to finish moulting. Some shrimp with a wound on the shell will not complete a moult. An example is when there is taura syndrome virus infection (TSV) or vibriosis infection on the shell. Rainfall can also trigger a moulting problem. Rain water with a low pH will make shrimp moult but at the same time, most of the pond will also have less oxygen, high toxic gases, cold water
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May/June 2018 AQUA Culture Asia Pacific Magazine
and shortage of minerals. Farmers will need to apply lime to maintain the pH and to prevent shrimp from moulting. To do this, the farmer should not stop aeration but cut down on feeding, and check pH and alkalinity every 2-3 hours during heavy rainfall. If there is an out of control situation, a probiotic that can control H2S should be applied. In some cases, a scar on the shell caused by the rostrum of another shrimp will no longer be there by the next moulting. Possible causes for deformity are malnutrition such as the lack of essential vitamins or an infection such as the bamboo shrimp virus. If this happens, feed supplements should be used and if this not does work, it is worth abandoning the crop.
Proper mineral management Shrimp needs some 23 minerals. The major minerals are calcium (Ca), magnesium (Mg), potassium (K), phosphorus (P) and sodium chloride (NaCl). The minor elements are iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), iodine (I), cobalt (Co), caesium (Cs), nickel (Ni), selenium(Se), fluoride (F), molybdenum (Mo), tin (Sn), chromium (Cr), strontium (Sr), vanadium (V) and silica (Si). The Mg/Ca/K ratio must be 40/15/13. For example, at 10 ppt, the farmer must maintain the water with 400 ppm Mg, 150 ppm Ca and 130 ppm K. Therefore, checking the mineral value constantly is essential especially for a farm stocking higher density than 100 PL/m2. All of the minerals above are required in specific amounts. The absence of one mineral could lead to the deactivation of other minerals as shown in the mineral wheel table (Figure 1). Companies market minerals in two ways. First the minerals are supplied in the form of compounds such as MgSO4, CaCl2, MgCl2 or lime etc. The compounds are blended in the correct ratio with all the essential major minerals (Ca, P, K, Mg, S, Na and Cl) in a 20 kg pack and used by direct application to the water. The second product consists of 16 trace elements and essential minerals comprising Fe, Mn, Cu, Zn, I, Co, Cs, Ni, Se, F, Mo, Sn, Cr, Sr, V and Si mixed in a 1 kg pack for top dressing of feed at the farm site. Essentially, farmers must understand the need for these minerals and provide a good quality product for shrimp to avoid a problem with mineral shortage. Furthermore, to benefit the shrimp, minerals should be applied at the right time when shrimp needs them, such as when it is moulting. The form and source of the minerals also require attention and farmers should seek guidance from experts.
Salt
Salt (sodium chloride) is a mineral source that farmers can use even though the salinity in the grow-out pond can be high (over 35 ppt.) At a high stocking density such as 150 PL/m2 or more, even though in highly saline conditions, trace minerals are still required. In cases where some minerals are inactive, special minerals to compensate for these are needed. A general rule is to mix 20g salt with 100 mL of water, and top dress 1kg of feed.