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Effects of probiotics on growth, stress tolerance and non-specific immune response in Japanese flounder Paralichthys olivaceus were evaluated in a closed recirculating system. Survival and growth of flounder treated by supplying commercial probiotics either in the diet (the probiotic diet group), or into the rearing water (the water supply group), were higher compared to the untreated group (the control group). Water quality parameters, pH, NH 4 -N, NO 2 -N and PO 4 -P showed lower concentration in the probiotic diet group compared with the control group and the supply group. Plasma lysozyme activity in the probiotic diet group and the water supply group was significantly higher (P < 0.05) than that in the control group. In heat shock stress tests, flounder in the probiotics-treated groups showed greater heat tolerance (measured by 50% lethal time, LT50) than the control group. Pathogen challenge tests with Vibrio anguillarum (2 × 10 7 c.f.u./mL) resulted in significantly higher survival in the probiotics-treated groups than the control group. Results indicated that probiotics supplied in the rearing water and the diet of fish enhanced the stress tolerance and the non-specific immune system of Japanese flounder, providing them a higher resistance against stress conditions and pathogens.

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This experiment was conducted to determine the optimum dietary protein level for juvenile olive flounder Paralichthys olivaceus (Temminck et Schlegel) fed a white fish meal and casein-based diets for 8 weeks. Olive flounder with an initial body weight of 4.1 + 0.02 g (mean + SD) were fed one of the six isocaloric diets containing 35%, 45%, 50%, 55% and 65% crude protein (CP) at a feeding rate of 4±5% of wet body weight on a dry-matter basis to triplicate groups of 20 fish per aquarium. After 8 weeks of feeding, per cent weight gain (WG) and feed efficiency ratios of fish fed the 55% CP diet were not significantly higher than those from fish fed the 50% and 65% CP diets, but significantly higher than those from fish fed the 35% and 45% CP diets. Fish fed the 50%, 55% and 65% CP diets had significant higher specific growth rates than did fish fed the 35% and 45% CP diets; however, there was no significant difference among fish fed the 50%, 55% and 65% CP diets. The protein efficiency ratio was inversely related to the dietary protein level; that is, maximum efficiency occurred at the lowest dietary protein level. Broken-line model analysis indicated that the optimum dietary protein level was 51.2 + 1.8% for maximum weight gain in juvenile olive flounder. The second-order polynomial regression analysis showed that the maximum WG occurred at 57.7% and it revealed that the minimum range of protein requirement was between 44.2% and 46.4%. These findings suggest that the optimum dietary protein level for maximum growth could be greater than 46.4%, but less than 51.2% CP in fish meal and casein-based diets containing 17.0 kJ g À1 energy for juvenile olive flounder.

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