Effects of Lonic Concentrations on Survival and Growth of Penaeus monodon Reared in Low-Salinity Waters

Abstract

A comparative study was made of two black tiger shrimp (Penaeus monodon) farms operating under low-salinity water conditions located in the same vicinity. Each farm had 12 earthen ponds with an area of 4 rai (6,400 m2). Farm 1 achieved normal production over the past six years, while farm 2 operated for only two years had low production in each growout cycle. Postlarvae (PL) were stocked at a rate of 80,000 shrimp/rai (50 shrimp/m2). After harvesting, farm 1 had an average production of 899 kg/rai, shrimp body weight of 14.69 g, growth rate of 0.129 g/day and feed conversion rate (FCR) of 1.38. These results were significantly different from farm 2 which had an average production of 560 kg/rai, body weight of 11.66 g, growth rate of 0.096 g/day and FCR of 1.91 (P<0.05). The average survival rate in farm 1 was 70.8±12.45 % compared with 60.1±17.1% in farm 2. However, there was no significant difference (P>0.05). Analysis of the six major ions (Ca2+, Mg2+, Na+, K+, Cl- and SO42-) one day before stocking  PL found that the ion concentrations in both farms were approximately the same and the salinity five days after stocking was approximately 5 parts per thousand (ppt). At day 30, the ion concentrations in the low-production farm started to decrease lower than those of the normal- production farm, especially Na+ and Mg2+. Sixty days after stocking, Na+ and K+ concentrations from the low-production farm were significantly lower than the normal- production farm (P<0.05). Two probable causes were that the seepage in the low-production farm was higher due to a lower percentage of clay particles in the soil and that the level of the pond bottom was higher than the freshwater canal which surrounded the ponds. More freshwater was added to replace the seepage in the low-production farm than in the normal- production farm, so that salinity was diluted more quickly and affected all major ion concentrations. In particular, Na+ and K+ concentrations in the low-production farm were significantly lower than in the normal-production farm (P<0.05). Based on the results from this study it could be concluded that in order to achieve normal survival and growth of black tiger shrimp reared under low–salinity waters, the salinity for stocking should not be lower than 5 ppt and the ionic profiles should be similar to the seawater. During the first 30 days, salinity should be maintained or not allowed to drop quickly. The minimum levels of  Na+, Mg2+ and K+ during the culture period should be maintained not less than the concentration of seawater at 3 ppt.