The Appropriate Salinity Level of Brine Water for Raising Black Tiger Shrimp Under Low-Salinity Conditions
The Appropriate Salinity Level of Brine Water for Raising Black Tiger Shrimp Under Low-Salinity ConditionsChalor Limsuwan1, Temdoung Somsiri2 and Somrudee Silarudee31 : Faculty of Fisheries, Kasetsart University, Bangkok, Thailand2 : Aquatic Animal Health Research Institute, Bangkok, Thailand3 : Rajamangala Institute of Technology, Chanthaburi, Thailand
In the past seven or eight years the use of low salinity conditions for raising Clack Tiger Shrimp has become more popular in almost every part of the country where shrimp farms are found. This is because shrimp farms that use water with low salinity have no problems of diseases from luminescent Bacteria or toxic dinoflagellate plankton. Problems from yellow head virus and white spot syndrome virus are also less severe than on farms where normal seawater, with a salinity of 30-35 ppt, is used. Seventy percent of the shrimp produced in Thailand, or about 300,000 tons a year, comes from farms using low-saline conditions, yet these farms account for only 50 percent of the total amount of area devoted to shrimp farming in the country. Shrimp farms using seawater cover about 50,000 hectare, approximately the same area as shrimp farms using low-salinity water, but they produce only 30 percent of the total output.Shrimp farms using low-salinity water obtain their water from two sources – either from brackish streams or rivers, or from salt farms. The water obtained from salt farms is called brine water. Shrimp farmers who use brine water dilute it to a salinity level of 3-5 ppt and then add more fresh water as the shrimp grow so that the salinity level is down to 0-2 ppt by the time the shrimp are harvested (Limsuwan, 2000).There is a great variance in the salinity of brine water obtained from salt farms at different stages of the salt making process or at different times of the year. The salinity may be as low as 100 ppt at some times, or as high as 300 ppt during the dry season. No previous studies were made of the appropriateness of different concentrations of brine water for using is shrimp farms, including the most beneficial levels of the major ions and the most appropriate level of salinity to prevent disease.For this reason, this study was undertaken to analyze the amount of major ions and the salinity level of brine water at different stages in the salt making process to determine which stage is most appropriate for drawing water to use in shrimp farms. The samples were taken from salt farms at Samut Songkram, an important salt production area. The salt farmers pump seawater into shallow ponds and allow it to evaporate until salt precipitates out. Usually brine water is pumped out for sale to shrimp farmers when it has evaporate to a high level of salinity. Water trucks with a capacity of 12-15 tons pump out the water and deliver it to shrimp farms, and the shrimp farmers then add fresh water to reduce the salinity to 3-5 ppt before releasing their postlarvae.Brine water with a salinity of 35, 45, 80, 120, 250 and 300 ppt was collected for analysis four times. The standard method of Clescari et al. (1998) was used to analyze for concentrations of major ions and the samples were also cultured for total bacteria count and Vibrio bacteria count, since Vibrio bacteria are the most common type to cause diseases in marine animals.The results of the bacteria tests are shown is Table 1. The total bacteria count dropped drastically when the salinity reached 200 ppt. The number of Vibrio bacteria (both the yellow colony and the green colony) dropped gradually with the increase in salinity, and no Vibrio bacteria were found when the salinity reached 200 ppt. When separate cultured of green colony Vibrio bacteria were raised on the specific substrate of Thiosulfate Citrate Sucrose (TCBS) agar, it was found that the yellow colony could survive in brine water with salinity of up to 120 ppt, but when the salinity level was increased to 200 ppt all the bacteria died.
The results of tests for concentrations of various ions in brine water of different salinity levels are shown in Table 2. The concentration of the major ions such as calcium, magnesium, sodium, potassium, chloride, bicarbonate and sulfate at a salinity level of 35 ppt were about the same as in normal seawater. However, as the salinity of the brine water solution increased, the concentration of most ions increased, with the exception of calcium and bicarbonate. The concentration of calcium ions increased with the salinity of the solution up to the point of 80 ppt, and then it remained constant up to the level of 200 ppt, when it decreased. The decrease, which was apparently due to precipitation, was more marked at the salinity levels of 250 ppt and 300 ppt, the volume of bicarbonate tended to decrease slightly with increasing salinity, while the volume of carbonate increased slightly with increasing salinity. Yet when the bicarbonate and carbonate ion concentrations of the other major ions magnesium, sodium, potassium, chloride and sulfate increased steadily with increased salinity.Other ions, such as orthophosphate, silicate, iron and zinc, were found in very low concentrations at every salinity level from 35 ppt to 300 ppt. The amount of iodide ions dropped gradually from the salinity level of 35 ppt, and at the level of 200-300 ppt. the amount of iodide ions was about half that of normal seawater. Usually, when brine water mixed with fresh water to achieve a salinity level of just 3-5 ppt to use for raising black tiger shrimp, the original salinity of brine water is around 100-300. This is because it is more cost efficient to transport highly saline brine water and then dilute it at the shrimp farm.According to the results of this study, brine water at every level of salinity contains sufficient concentrations of the major ions to support the needs of shrimp, although at salinity of 250 ppt or over the amount of calcium ions will drop. This is similar to the results reported by sturm (1980). However, the shrimp farmers must dilute the brine water solution before introducing the shrimp to the pond. If this makes the concentration of major ions such as calcium, magnesium, potassium or bicarbonate to low, the farmer can solve the problem by adding agricultural limestone (CaCo3) or dolomite limestone (MgCO3.CaCO3). In some cases, the fresh water used for diluting the brine water is low in potassium ions, so the farmer may add potassium chloride or potassium sulfate (K2SO4) to increase the potassium ion level (Boyd, C. E., personal communication). These chemicals are commonly used in aquaculture.While the salinity of water in the shrimp pond drops close to 0 ppt shrimp will weaken and will tend to eat each other during molting. In this case, farmers should add granular salt or brine solution to raise the salinity to a level where the shrimp can eat and grow normally. The salinity level should be at least 2 ppt if the concentration of shrimp is up to 6 tons/hectare, but can be as low as 1 ppt if the concentration of shrimp is only around 3 tons/hectare. In terms of bacterial problem this study found that no Vibrio bacteria was present when the salinity of brine water was up to 200 ppt, so brine water with a salinity of 200 ppt or over should be safe for shrimp farming and should not cause any bacterial disease problems. At 200 ppt, the amount of major ions is also just as appropriate for shrimp farming compose to regular seawater.Therefore, we can conclude that brine water with a salinity of 200 ppt, when diluted with fresh water to the level of 3-5 ppt, is appropriate for shrimp farming.