Effects of C a and M g concentrations in seawater on the growth and energy budget of juvenile P aralichthys olivaceus ( T emminck & S chlegel, 1846)

Summary Effects of seawater calcium ( C a) and magnesium ( M g) concentrations on the growth and energy budget of juvenile P aralichthys olivaceus ( T emminck & S chlegel, 1846) were investigated at 20°C and a salinity of 30 psu. The C a/ M g ratio ( C a :  M g = 1 : 3) in the experimental water was kept constant. Five treatments were set: C 1, C 2, C 3, C 4, and C 5, and the Ca and Mg concentrations were 700, 1100, 1600, 2200, and 2800 (mg L −1 ), respectively. After a 60‐day feeding trial the P . olivaceus survival rates were not affected significantly by C a or M g concentrations in the range of 700–2800 mg L −1 . However, the specific growth rates ( SGR ) and weight gain ( WG ) of all treatments decreased with ascending C a and M g concentrations. The C a and M g concentrations had a strong influence on various energy parameters, namely, energy deposited for growth ( G ), energy lost for respiration ( R ), and energy lost in excretion ( U ), but had little influence on energy lost in feces ( F ). This indicated that the growth rate differences under different concentrations of C a and M g mainly resulted from differences of assimilation efficiency and energy channeled into respiration. The highest net growth efficiency was attained at the lowest concentration of C a and M g in seawater, and the higher concentrations of C a and M g increased the energy channeled to respiration and excretion, which led to significant reduction in growth. Therefore, the optimal growth of P . olivaceus could be obtained by regulating the concentration of C a and M g at 700 mg L −1 , where the average energy budget was: 100C (100% energy consumed in food) = 47.21 (±0.63) G + 32.89 (±0.42) R + 16.71 (±0.56) F + 3.19 (±0.05) U.