Effects of Dietary Mannan Oligosaccharides on Growth, Nonspecific Immunity and Tolerance to Salinity Stress and Streptococcus iniae Challenge in Golden Pompano, Trachinotus ovatus

Abstract

The effects of dietary supplementation with mannan oligosaccharide (MOS) on growth performance, haematological parameters, abundance of intestinal Vibrio spp., immune response, and tolerance to low salinity stress and Streptococcus iniae challenge were evaluated in pompano (Trachinotus ovatus). Pompano (3.24 ± 0.45 g) were fed six diets including a basal diet as control, and the basal diets supplemented with 0.1%, 0.2%, 0.4%, 0.6%, or 0.8% MOSs for 8 weeks. The results showed that pompano fed 0.1%–0.4% MOS had significantly improved specific growth rates ( $P\le 0.039$ ). Dietary MOS did not affect the survival rate of pompano ( $P=0.192$ ). WBC count was significantly higher in fish fed diets containing 0.1%–0.6% MOS. Dietary MOS increased lymphocytes count ( $P\le 0.042$ ) and reduced monocytes and basophils counts ( $P\le 0.019$ ); however, no effects were found on neutrophils and eosinophil numbers ( $P\ge 0.064$ ). Intestinal Vibrio spp. counts reduced in fish fed diets containing 0.1%–0.4% MOS ( $P\le 0.035$ ). The phagocytic ratio significantly increased in pompano fed diets containing 0.1%–0.6% MOS ( $P\le 0.015$ ), while the phagocytic index and serum lysozyme activity in fish fed 0.1%–0.4% MOS were significantly higher than the control ( $P\le 0.035$ ). There were significant positive correlations between MOS levels and fish growth on days 14, 28, 42, and 56 ( $P\le 0.049$ ). In addition, dietary MOS levels were highly correlated with blood parameters, abundance of intestinal Vibrio spp., and immune responses. Optimal dietary MOS requirements for maximal growth were estimated to be 0.440% on day 21 (R2 = 0.861), 0.385% on day 28 (R2 = 0.877), 0.371% on day 42 (R2 = 0.891), and 0.365% on day 56 (R2 = 0.750), showing decreasing tendency in MOS requirement as fish size increased. Furthermore, the optimal MOS concentration for maximal immunity based on lysozyme activity was estimated to be 0.431% (R2 = 0.817) on day 56. In addition, fish fed 0.1%–0.4% MOS showed better resistance to low salinity stress and Streptococcus iniae challenges ( $P\le 0.014$ ). In conclusion, MOS supplementation effectively reduced the prevalence of intestinal Vibrio spp. and enhanced the growth, immune responses, and tolerance to low salinity stress and Streptococcus iniae in juvenile pompano.