EFFECT OF A MIXED Bacillus PROBIOTIC ON SURVIVAL, GROWTH, AND MICROBIOTA OF COMMON SNOOK Centropomus undecimalis LARVAE

Disease is a major limitation to the expansion of the aquaculture industry due to high organic loads that favor the proliferation of opportunistic pathogens and high stocking densities resulting in stress and immune suppression of the animals. Fish maintain symbiotic bacteria or microbiota that not only combat these pathogens but are necessary for proper development, growth, and immune function. With the increasing prevalence of antibiotic-resistant bacteria and the knowledge of the importance of commensal bacteria to fish health, use of probiotics is seen as an alternative to controlling opportunistic pathogens where antibiotics used to be the standard. The common snook Centropomus undecimalis is a top sport fish species in Florida. Its economic value and high susceptibility to habitat loss, red tide, and cold events make it a target species for aquaculture. Unfortunately, marine fish larvae experience high (up to 99%) mortality in aquaculture systems. It is hypothesized that these mortalities are due to harmful fish-bacteria interactions.

This project administered probiotic strains of Bacillus licheniformis and B. amyloliquefaciens to common snook larvae in recirculating aquaculture systems in an attempt to reduce larval mortality. Systems included 1) control tanks that did not receive probiotics; 2) PBW treatments with probiotics administered directly to the tank water; 3) PBW_LF treatments with probiotics in the tank water and in rotifer and Artermia cultures used for live food. Initial (immediately following hatch) samples of tank water and larvae were taken for baseline data. Water and larvae samples were collected at 7, 14, and 28 dph and subjected to DNA extraction and Illumina sequencing to characterize differences in bacterial communities between treatments. Larvae were subsequently collected to monitor innate immune parameters including lysozyme (a bactericidal enzyme) and superoxide dismutase (SOD; an indicator of oxidative stress). Total length of larvae was measured at hatch and 3, 7, 14, and 28 dph. Survival was determined at the end of the trial.

Initial results indicate a significant increase in survival in the probiotics treatments (Figure 1) as compared to the control. Lysozyme could not be detected using typical turbidometric assays. SOD increased significantly with age and higher activity in the control after 28 dph suggests increased oxidative stress (Table 1). Microbiota analysis is forthcoming.