Effects of Na/K ratio in seawater on growth and energy budget of juvenile Litopenaeus vannamei

Abstract

The effects of seawater Na/K ratio on growth and energy budget of juvenile Litopenaeus vannamei Boone, 1931 were investigated. Salinity (S=30 ppt) and the total concentration of sodium and potassium in the experimental water were kept constant. Seven treatments were set: R1, R2, R3c, R4, R5, R6 and R7, the Na/K ratios were 25.6, 34.1, 47.3, 85.2, 119.3, 153.3 and 187.4 (mmol/mmol), respectively. The shrimp in R7 (Na/K=187.4) died within 2 weeks after the experiment began. After the 30-day feeding trial, the molting frequency (MF) and feed intake in terms of wet weight (FIw) and energy content (FIe) of the shrimp under the other six treatments were not significantly affected by the different Na/K ratios in the seawater. The final wet weight, weight gain (WG), specific growth rates (SGRw and SGRe) and food conversion efficiencies (FCEd and FCEe) of the shrimp under Treatment R6 were significantly lower than those under the other five treatments. There was a trend that, with all the indices mentioned above, the values of the test shrimp under Treatment R2 was the highest, and the sequence was: R2 (Na/K=34.1)>R3c (Na/K=47.3)>R1 (Na/K=25.6)>R4 (Na/K=85.2)>R5 (Na/K=119.3)>R6 (Na/K=153.3). In the experiment, the percentages of energy deposited for growth (G), energy lost for respiration (R), energy lost in excretion (U) and energy lost in exuviae (E) to the energy consumed in food (C) were significantly affected by the different Na/K ratios in seawater, while the percentage of energy lost in feces (F) to C was not significantly affected by the ratios. The percentage of G to C of the test shrimp under Treatment R6 was significantly lower while the percentage of E to C was significantly higher than those under the other treatments. There were no significant differences among the energy allocations of the test shrimp under Treatment R1, R2, R3c, R4 and R5. According to regression analysis, optimal shrimp growth could be obtained by regulating the Na/K ratio of the seawater to 40–43 at a salinity of 30 ppt.