Effect of fish size on nitrogen utilization in Japanese flounder, Paralichthys olivaceus (Temminck & Schlegel)

Information on nitrogen utilization of varying size of Japanese flounder is essential in the formulation of cost-effective and low-pollutant diets. To do that, total ammonia nitrogen (TAN) excretion (mg-N/100 g fish) of different size of Japanese flounder (small size, SS = 13.5 ± 0.40 g; medium size, SM = 41.2 ± 1.15 g; large size, SL = 119.0 ± 2.74 g) was monitored every 2 h following 24 h after feeding with the diets containing 40, 46, 52, 58 or 64% protein. Results indicated that the TAN excretion (mg-N/100 g fish/d) of SS and SM were significantly (P<0.05) higher than that of the SL in each dietary protein level. The only peak on post-prandial TAN excretion occurred 2 - 4 h after feeding in SS and SM. However, the fish of SL tended to reach to the peak with slightly longer time than those of SS and SL. TAN excretion significantly (P<0.05) increased with increasing nitrogen (protein) intake in all fish size. At the same protein level, TAN excretion decreased with increasing fish size. No interactive effect between dietary protein level and fish size was observed on TAN excretion. The relationship between nitrogen intake and TAN excretion was identified by linear regression in each size, and intercepts of the equations represented the ration of ingested nitrogen diverted to ammonia. SLshowed more effective nitrogen utilization compared to SS and SM. The results indicated that nitrogen utilization of Japanese flounder was size-dependent. In conclusion, to optimize the protein level in diets for Japanese flounder, size dependent absolute dietary protein requirement must be established.