Further research into the immunological capacity and growth potential of dietary supplementation of poly-β-hydroxybutyrate (PHB) is warranted to assess it as a novel growth enhancement and disease mitigation strategy. Previous observations reported in this laboratory have demonstrated potential beneficial effects of bacteria-derived PHB on growth and health of juvenile Nile tilapia. Therefore, the goal of two subsequent 8-week feeding trials was to replicate these evaluations in both commercially relevant advanced stage and fingerling Nile tilapia. Advanced stage Nile tilapia (~207 g/ fish initial weight) were fed a commercial diet [~33% crude protein (CP)] top-coated with carrier consisting of menhaden oil, carboxymethyl cellulose and alcohol, either with or without crystalline PHB dissolved in it to provide PHB at 1.0% of dry diet weight. Each diet was fed to 30 advanced stage Nile tilapia in triplicate 1200-L round tanks fashioned as a recirculating aquaculture system with well water trickled through to maintain optimal water quality. Tilapia were fed to apparent satiation twice daily based on visually assessed feeding activity. After 4 weeks of feeding, advanced stage tilapia exhibited significant (P<0.05) enhanced percentage weight gain and feed efficiency with PHB supplementation (Fig. 1). However, no statistical differences in any measured growth performance parameter or body condition indices [hepatosomatic index (HSI), intraperitoneal fat (IPF) ratio, and fillet yield] were apparent after 8 weeks. In vitro/ex vivo analyses including superoxide anion production, bactericidal, and phagocytic activity of head kidney-derived leukocytes, and blood serum lysozyme are currently being determined and will be complete at the time of presentation.
An additional feeding trial is currently being conducted in which eight fish meal- and fish oil-free isoenergetic and isolipidic diets containing 40% CP and 10% lipid from practical ingredients are being fed to Nile tilapia fry (0.9 g/ fish initial weight). To compare with the efficacy of the bacteria-derived PHB previously evaluated, dietary treatments in this trial include a basal diet to which two levels of crystalline PHB (0.0 and 1.0%), chitosan (0.0 and 0.5%), and nucleotides (0.0 and 0.5%) were supplemented in a 2 x 2 x 2 factorial design. Each diet is being fed to triplicate groups of fingerling Nile tilapia stocked at 15 fish per 30-L aquaria operated as a recirculating system. Fish are being fed to apparent satiation twice daily based on a percentage of group body weight which is determined weekly. This trial is currently in progress with a scheduled completion in January of 2023.
In conclusion, advanced stage Nile tilapia showed a dramatic increase in weight gain and feed efficiency after short-term (4 week) exposure to PHB supplementation; however, such effects diminished in the subsequent 4 weeks. Immune response analyses of advanced fish are currently underway, and responses of fingerling tilapia in the ongoing trial will be presented.