Effects of salinity on the growth, plasma ion concentrations, osmoregulation, non-specific immunity, and intestinal microbiota of the yellow drum (Nibea albiflora)

The yellow drum (Nibea albiflora) is a coastal fish species that is intensively farmed in China. The aquaculture of this species is mainly restricted to coastal ponds and net cages in areas that may experience a wide variation in salinity due to rainfall or freshwater input. However, our knowledge regarding the impacts of salinity on the growth and health of yellow drum is limited. In this study, we evaluated salinity tolerance of yellow drum based on their survivals at various salinity treatments in a 7-day experiment, and investigated the effects of salinity on the growth performance, blood ion concentrations, Na+-K+-ATPase and non-specific immune enzyme activities, and intestinal microbiota composition of this species in a 7-week experiment. The median lethal time (LT50) revealed that yellow drum reared in water with salinities ranging from 0 to 60 parts per thousand (ppt) survived within a salinity range of 2 to 45 ppt and that large-sized fish (body length = 25.70 +/- 0.51 cm) showed a higher salinity tolerance than small-sized fish (body length = 9.52 +/- 0.74 cm). Among juvenile fish (small size) maintained for 7 weeks in water with salinities of 6, 12, 18, 30, and 42 ppt, we observed that growth was restricted only at 42 ppt. Although the parameters measured in the blood ion concentrations and activities of Na+-K+-ATPase, antioxidant enzymes, and immunomodulatory enzymes (alkaline phosphatase and acid phosphatase) were mainly non-significant among the fishes in the 6, 12, 18, and 30 ppt groups, in most instances, these parameters were significantly different from those in the fish from the 42 ppt group. This indicates that high salinity would probably represent a stress factor for the yellow drum. Comparison of the intestinal microbiota composition among fish in the 6, 30, and 42 ppt groups revealed that the composition was altered by the salinity level. Notably, the relative abundance of several probiotic taxa that can promote the growth and enhance the immunity of fish was found to be relatively higher in the 6 and 30 ppt groups. However, the abundance of Vibrio species, which are potentially detrimental to fish health, was relatively higher in the 42 ppt group. The findings of this study will enable us to gain a more comprehensive understanding of the effects of salinity on the growth and health of yellow drum and contribute to improvements in the aquaculture of this species.