Importance

Salinity is the concentration of all mineral salts in water. The salinity of water is very important, greatly affecting the existence, development and helping to maintain the physiological functions of shrimp. Through the process of regulating penetrating pressure, shrimp must maintain the level of mineral salt dissolved in their bodies at a stable level. Like the temperature, each fish and shrimp have the appropriate salinity to grow and develop. The salinity of TTCT is 5 – 35 ‰. When the salinity changes for a few minutes or a few hours, once more than 10 ppt, the shrimp will not be able to withstand it. According to some studies, salinity also affects the antibacterial activity and immunity of shrimp; However, the immune system can recover from salinity changes within 6 days. The enzymes that catalyze the activity in the body are also sensitive to the increase in salinity. The enzyme phenoloxidase, in particular, catalyzes melanosis (the formation of dark spots on shrimp shells). This is the mechanism against bacteria of shrimp in the immune system, but it makes a difference in shrimp, which reduces shrimp economic value.

The suitable salinity for whiteleg shrimp farming is 5-15‰, black tiger shrimp is 15-20‰. Intraday volatility is no more than 5 ppt. In brackish water aquaculture, changes in salinity have an effect on the growth of white leg shrimp at seed stage. However, there are also studies that show that fluctuations in salinity in the appropriate range will stimulate shrimp to molt and grow faster. Research Su et al. (2010) showed that, a variation of 2 days/times vannamei cultured at ±5‰ and ±10‰ salinity had better growth than shrimp cultured at unchanged or ±15 salinity. Due to climate change, extreme weather events such as prolonged heavy rain can cause a sudden decrease in salinity in ponds, making it difficult to care for shrimp; especially during the stocking period, changes in salinity can greatly affect the growth and survival of shrimp.

Effects when salinity changes

whiteleg shrimp is increasingly being cultured at low salinity, due to their high adaptability. During the breeding stage, whiteleg shrimp always live in an environment with relatively high salinity, but after domestication thanks to high tolerance, in low salinity areas, shrimp still live and grow well. In case the salinity is too low, below 5‰, the ions Ca2+, Mg2+, Na+, K+… in the water with low content make the molting process of shrimp take place unevenly. increase the loss rate many times over. After it rains, the sudden decrease in salinity of the pond water also greatly affects the shrimp, especially the molting process of shrimp is stimulated, but the nutrients and minerals necessary for the shell softening process are not enough to provide. As a result, shrimps have reduced resistance and are easily attacked by bacteria, causing dangerous diseases, then making shrimp more sensitive to nitrogen-containing toxins such as NH3, NO2 …

On the contrary, when whiteleg shrimp live in an environment with too high salinity, above the tolerance level, the shrimp will be stunted, grow slowly, even shocked and die in mass. At the same time, when the salinity is high, white feces disease and acute hepatopancreatic necrosis (early mortality of shrimp – EMS) will develop very complicatedly, causing disease outbreaks that cause great economic losses. High salinity will cause changes in some environmental parameters such as pH, alkalinity; It also causes algae in shrimp ponds to grow quickly, producing a lot of toxic gas… In particular, the oxygen content in the water will increase sharply during the daytime but decrease to a minimum at night. At that time, the environment will lack oxygen, leading to shrimp often floating at midnight.

The study (published in the Vietnam Journal of Agricultural Science No. 2/2018) was carried out at the experimental farm of the Faculty of Fisheries, Can Tho University. The experimental subject is PL12 black tiger shrimp which has been tested as disease free. The experimental water source is tap water with salinity 0‰ and sparkling water with salinity 80-90‰ purchased from Bac Lieu. Of the 7 treatments performed, 5 shrimp treatments were directly stocked at salinities of 5‰, 10‰, 15‰, 20‰ and 30‰; The remaining 2 treatments were divided into 2 subgroups for rapid domestication from salinity 20‰ to 5‰ within 3 hours and slow domestication from salinity 20‰ to 5‰ in 3 days. Each treatment was repeated 3 times and was arranged completely randomly in a plastic tank containing 70 liters of water, density of 2 fish/liter. Shrimp were fed 4 times/day with C.P’s No. 1 (40% protein) feed with a diet based on body weight. The results show that, when the salinity shock is greater, the growth in length of the crayfish is lower, the survival rate of shrimp is affected by sudden changes in salinity (decreased from 10 – 15 ) and shrimp from salinity. 20‰ of direct stocking (without domestication) to salinity of 5‰ and 10‰ had lower survival rates than the other treatments after 20 days of culture. Besides, shrimp growth in length, weight and survival rate was highest in treatment 20‰ and salinity shock threshold from 15-30‰ did not affect shrimp growth.

The study (published in the Vietnam Journal of Agricultural Science No. 2/2018) was carried out at the experimental farm of the Faculty of Fisheries, Can Tho University. The experimental subject is PL12 black tiger shrimp which has been tested as disease free. The experimental water source is tap water with salinity 0‰ and sparkling water with salinity 80-90‰ purchased from Bac Lieu. Of the 7 treatments performed, 5 shrimp treatments were directly stocked at salinities of 5‰, 10‰, 15‰, 20‰ and 30‰; The remaining 2 treatments were divided into 2 subgroups for rapid domestication from salinity 20‰ to 5‰ within 3 hours and slow domestication from salinity 20‰ to 5‰ in 3 days. Each treatment was repeated 3 times and was arranged completely randomly in a plastic tank containing 70 liters of water, density of 2 fish/liter. Shrimp were fed 4 times/day with C.P’s No. 1 (40% protein) feed with a diet based on body weight. The results show that, when the salinity shock is greater, the growth in length of shrimp will be lower, the survival rate of shrimp is affected by sudden changes in salinity (decreased from 10 – 15 ) and shrimp from salinity of 20 ‰ and direct stocking (without domestication) to salinity of 5 ‘ and 10 ‘ had lower survival rates than the other treatments after 20 days of culture. Besides, shrimp growth in length, weight and survival rate was highest in treatment 20‰ and salinity shock threshold from 15-30‰ did not affect shrimp growth.

According to a study also published in the Journal of Vietnam Agricultural Science; The experiment object is whiteleg shrimp raised in the form of an individual with an initial mass of 0.007 g/individual raised in 5L plastic glue containing 2l sea water 20 ‰, each test is repeated 30 times and fluctuations Salinity is ± 5 ‰, the experiment is performed in 45 days. As a result, after 45 days showed that shrimp raised in fluctuations ± 5 ‰ of the salinity fluctuations every 6 days and time with the shortest molting cycle (4.9 days/time) and the hundreds of shrimps participating in the molting. The highest is 22.1%/day. In contrast, the shrimp in the control solution does not fluctuate salinity, the longest molting cycle (5.3 days). This shows that the fluctuations of salinity have an impact on the molting of shrimp, with a cycle of 6 days/time the molting cycle of the shortest shrimp and the percentage of shrimp participating in the highest molting/day. After 45 days of farming, shrimp with the best growth growing (0.88 g/individual) in fluctuations ± 5 ‰ Saline fluctuations 6 days/time, followed by fluctuations ± 5 ‰ cycle Salinity fluctuations 4 days/time (0.85 g/individual) and shrimp raised in the control lot with unchanged salinity (0.83 g/individual). The survival rate of ± 5 ‰ cycles of salinity fluctuations every 6 days for the best results (80%) and higher than the control solution. This experiment shows that the salinity has a change in salinity and the salinity fluctuations are 6 days/time with short molting cycles, high molting rate/day, leading to growth in good volume. More than shrimp farming at salinity does not change.

>>So according to the research results, in practice, in shrimp farming, people should change the water with a new salt concentration that does not differ by more than 5‰ with a water change cycle every 6 days to stimulate shrimp to molt better. If the weather is rainy, try to maintain the salinity to no more than 5‰ by draining the surface fresh water when there is stratification of salinity due to heavy rain. If you want to change the salinity of the pond, it is extremely important to tame the shrimp to get used to the new salinity.

Source Phuong Dong- Thuysan247.com