Low-Salinity Rice-Straw Bioflocs Retain Nitrogen-Removal Capacity After Salinity Transfer and Improve Survival of Pacific White Shrimp

Biofloc technology promotes dense microbial communities that remove nitrogenous waste and suppress pathogens, but the combined effects of salinity and carbon–nitrogen source selection on biofloc maturation and disease mitigation remain unclear. We evaluated two salinity levels (5 ppt and 25 ppt) and two carbon–nitrogen source combinations (rice straw with ammonium chloride or brown sugar with shrimp feed) in a 60-day biofloc trial. Under low salinity (5 ppt), the rice straw and ammonium chloride treatment reduced ammonia to undetectable levels by day 17 and nitrite to zero by day 40, while also establishing a stable, functionally diverse microbial community. When this mature biofloc was transferred abruptly to 15 ppt and 25 ppt, nitrite was again removed to zero within four days. Pre-inoculation of shrimp tanks with mature biofloc suppressed nitrite accumulation and lowered Vibrio loads in the hepatopancreas and gut, yielding a survival rate of 41% under natural infection compared with 0% in the control. In a Vibrio harveyi challenge, biofloc-treated groups achieved 90–93% survival versus 40–57% in non-inoculated groups and exhibited reduced tissue damage alongside increased abundance of beneficial Bacillus. Pre-stocking biofloc inoculation markedly reduces Vibrio infection risk and enhances shrimp survival, offering a cost-effective, eco-friendly strategy for improving water quality and biosecurity in high-density shrimp culture.