White-leg shrimp (Litopenaeus vannamei) can tolerate a wide salinity range (0.5 – 40 ppt), which makes it possible to cultivate this species under different water salinities. Shrimp farmers however face problems, among others with reduced shrimp performances at low salinity. This might be caused by changes in water ion profiles, including potassium (K+) concentrations. K+ is the primary intracellular cation directly involved in maintaining ionic regulation in shrimp. Insufficient availability of K+ can potentially have negative effects on the osmoregulation ability of shrimp and thereby affect shrimp performances. To improve ionic profiles of the rearing water, shrimp farmers add mineral fertilizers, which can be costly, particularly for large shrimp farms. One possible alternative could be dietary K+ supplementation. To date, studies on dietary K+ requirement of L. vannamei under iso-osmotic and challenging conditions (e.g., low salinity) are limited. This study aims to determine dietary K+ requirement of L. vannamei by investigating growth, feed utilization, and body mineral compositions under iso-osmotic (25 ppt) and challenging (5 ppt) salinity conditions.
Five semi-purified diets were formulated with different levels of K+ supplementation: 4, 8, 12, 16, and 20 g/kg diet. The diets were tested in 4 replicates at two different salinity levels using the same shrimp in two periods; 8 weeks under 25 ppt salinity (Period I, iso-osmotic condition) and 2 weeks under 5 ppt salinity (Period II, challenging condition). In period I, 30 shrimp (mean initial weight of 5.51 g) were stocked per tank, and in period II, the number of shrimp was reduced to 15 shrimp/tank (mean weight of 15.44 g). At the end of period I, salinity was gradually decreased from 25 ppt to 5 ppt (over 8 hours). Shrimp were fed to apparent satiation. Whole shrimp and hemolymph samples were collected at the end of periods I and II to determine mineral compositions. These samples are being analysed in the lab at this moment.
Repeated measure ANOVA showed that dropping salinity from 25 to 5 ppt affected shrimp survival (p < 0.05). In both water salinity conditions, no effect of dietary K+ levels was observed on shrimp survival (Fig. 1-A), growth (Fig. 1-B), feed intake, and FCR (p > 0.05). These findings indicate that the different dietary K+ levels used in this study did not influence survival, growth, and feed intake of white-leg shrimp both in iso-osmotic and challenging conditions. Hemolymph and body mineral composition, and body mineral retention will be presented during the presentation, but are now being analysed.