넙치(Paralichthys olivaceus)사료 내 Probiotic의 첨가에 따른 성장, 사료효율, 비특이적 면역력, 수질정화능력 및 Streptococcus iniae에 대한 질병저항성에 미치는 영향

Abstract

Two consecutive studies were conducted to investigate the effects of dietary supplementation of probiotics on growth performances, feed utilization, innate immunity, disease resistance against Streptococcus iniae and water quality for juvenile olive flounder (Paralichthys olivaceus). In experiment I, four experimental diets were prepared by supplementing three different Bacillus spp. probiotics (B. subtilis, B. pumilus and B. licheniformis) into a control diet at the level of 0.5 %. Fish (initial mean body weight, 25 g) were randomly distributed into twelve 150 L capacity polyvinyl circular tanks (25 fish per tank). Triplicate groups of fish were fed the experimental diets to apparent satiation (twice a day, 09:00 and 17:00 h) for 8 weeks. To analyze innate immune responses, blood samples were collected at the end of the feeding trial and after three days of challenge with S. iniae. At the end of the feeding trial, weight gain, specific growth rate, feed conversion ratio and protein efficiency ratio of the fish fed B. subtilis supplemented diet were significantly (P<0.05) higher than those of the fish fed the control diet. Respiratory burst activity significantly increased in the group fed B. pumilus supplemented diet at both sampling times and in the group B. subtilis after challenge test in comparison to the control group. The lysozyme activity did so in the group fed B. subtilis only after challenge and significantly higher levels of superoxide dismutase were observed in the fish fed B. pumilus and B. licheniformis diets. However, the serum myeloperoxidase level was not significantly affected by dietary supplementation of probiotics. The fish fed the control diet showed the highest cumulative mortality rate, and it was significantly higher than that of the fish fed B. pumilus and B. licheniformis supplemented diets. In experiment II, the fish were fed a commercial diet to apparent satiation and they were transferred to 64 L plastic tanks with three replications at a density of 30 fish per tank. The water of the tanks was not exchanged for 5 days and the ammonium concentration of each tank was measured with 24 h intervals from the addition of probiotics. Three different types of probiotic microbes including Bacillus subtilis (1), Bacillus subtilis (2) and Bacillus licheniformis were used. Each probiotic solution was prepared to 104 CFU/ml. The ammonium concentration was significantly decreased when B. subtilis was added to the rearing water indicating improvement of water quality in the presence of probiotic. Also, significantly lower mortality was observed by addition of B. subtilis to the water in comparison to the control group. In conclusion, the use of B. subtilis can be suggested for enhancement of growth and innate immune responses as well as improvement of water quality for olive flounder.