Effects of Water Velocity on Growth Performance of Juvenile Japanese Flounder Paralichthys olivaceus

Critical swimming speed (Ucrit) of juvenile Japanese flounder Paralichthys olivaceus with 12.5-cm body length was determined to be 1.8-body lengthkec. A feeding trial was conducted to investigate the effects of water velocity on growth and other nutritional parameters of juvenile flounder (initial mean body weight: 5.7 g/fish), which is a sedentary species. The fish were raised in three water velocities, < 0.3 body length/sec (control), 0.9 body length/sec (slow) and 2.1 body length/sec (moderate), for 8 wk. Over the course of the feeding trial, the weight gain of the control, slow and moderate groups of fish were. 634%. 671% and 564%. respectively (significant between slow and moderate groups). A second order polynomial suggested that the optimum water velocity occurred at about 1.0 body length/sec. The feed efficiency of the moderate group (1.48) was significantly lower than those of the control (1.56) and slow (1.56) groups, while there were no significant differences in feed intake (1.72–1.75% of wet biomass) among the treatments. Water velocity did not affect the proximate composition of the whole body; however, rearing the fish at the moderate water velocity did slightly but significantly reduce protein retention. Lipid content of fin muscles was significantly different among the treatments: control (14.9%), slow (17.6%) and moderate groups (11.6%). Unlike salmonids, water current does not seem effective for improving growth and feed efficiency in juvenile Japanese flounder.