Dietary protein and energy requirements of juvenile Japanese flounder, Paralichthys olivaceus.
Eight test diets of four protein (41, 44, 47 and 50%) and two energy levels (20 kJ g-1 and 19 kJ g-1) were formulated to investigate the proper dietary protein and energy levels for the growth of juvenile Japanese flounder, Paralichthys olivaceus. Squid liver oil concentration was used to adjust energy levels and brown fishmeal was used as the protein source. Weighing about 5.9 g, each duplicate group of flounder was fed test diets twice a day to apparent satiation, for 45 days. Performance of fish fed the different diets was evaluated for survival, percent weight gain, relative growth rate, feed efficiency and protein efficiency rate. Survival was over 85% for all treatments. Growth and feed efficiency of flounder increased as dietary protein increased in both energy levels of 19 kJ g-1 and 20 kJ g-1 diet, but no evidence of reaching a plateau for growth data was found. Lowest rates of cumulative ammonia nitrogen excretion as proportion of ingested nitrogen were recorded in fish fed the 50% protein-20 kJ g-1 and 50% protein-19 kJ g-1 diet (2.78 and 2.60%, respectively). The digestibility rates of the experimental diets with the high energy level (20 kJ g-1) were higher than those with the lower levels (19 kJ g-1). Digestion efficiencies in all experimental groups ranged from 89 to 92% for protein, from 60 to 85% for lipid and from 73 to 89% for energy, while those for the total digestibility ranged from 51 to 72%. The results indicate that Japanese flounder juveniles with 6 g mean weight need at least 50% dietary protein for best growth when brown fishmeal is the sole protein source. Furthermore, it can be concluded that Japanese flounder juveniles can utilize dietary energy up to 20 kJ g-1 efficiently, under the conditions applied in this study.