Effects of Saccharomyces Cerevisiae Postbiotics on Growth Performance, Haematological Parameters and Disease Resistance in Nile Tilapia (Oreochromis Niloticus) Fingerlings

Abstract

The use of antimicrobial feed additives and chemical substances in aquaculture systems to promote fish growth, prevent diseases, and treat infections has led to increased antibiotic resistance in infectious bacteria. Saccharomyces cerevisiae fermentation postbiotics (SCFP) are eco-friendly alternatives to antibiotics, which have shown strong potential for improving sustainability and enhancing aquaculture productivity. This research assessed the effect of dietary SCFP supplementation on growth performance, haematological parameters, and susceptibility to disease infection in Nile tilapia fingerlings. A feeding trial of 55 days was conducted at the Department of Veterinary Pathology fish-holding unit, University of Nairobi. Monosex tilapia fingerlings weighing 6.79 ± 0.11 g, were purchased from a commercial hatchery. A basal diet containing 30% crude protein and 3000 Kcal of digestible energy (DE) per kilogram was formulated and used in this trial. SCFP was added at 0g(control), 2g, 4g, and 6g per kilogram of feed to make four dietary treatments. The fingerlings were weighed and randomly assigned to the four diets containing different levels of SCFP. Each diet was fed to a group of 25 fish replicated 3 times to make 75 fish per treatment in a completely randomized experimental design. Water temperature and dissolved oxygen levels were maintained at 28ºC and above 4.5 ml/liter, respectively. Water quality parameters (dissolved oxygen, temperature, total ammonia, and pH) were monitored daily to ensure to make sure they were within the optimum range. The fish body weights and lengths were measured every 10 days to assess growth and feed utilization. The remaining 30 fish per treatment were subjected to a challenge experiment where twenty were injected intraperitoneally with Aeromonas veronii biovar sobria at a concentration of 0.5×108, while the other 10 fish were untreated controls. All the fish were fed on their allocated diets at 3% body

weight thrice daily and monitored every day for 12 days for deaths and/or clinical manifestations of the bacterial disease. After the feeding trial, blood samples were collected for haematology. A post-mortem examination was conducted on surviving fish, and organ samples were taken for recovery of inoculated bacteria. Fish-fed diets containing the postbiotic had significantly higher growth compared to those fed on the control diet. Those fed on diets containing 4g/kg postbiotic (T2) exhibited higher growth performance, followed by those fed on 6g/kg postbiotic diet (T3) and 2g/kg postbiotic diet (T1). Fingerling on the diet without SCFP supplementation had significantly lower growth performance. No significant difference was found in all growth performance and feed utilization indices between T2 (4g postbiotic) and T3 (6g postbiotic). After challenge infection, SCFP-supplemented fingerlings had higher red and white blood cell counts than controls, though not significantly different. Similarly, the former had a higher survival percentage (T1=50%, T2=80%, T3=70%) than the control (30%). The clinical manifestations of the bacterial disease induced were most severe in the control group when compared to the treatment groups and comprised lost appetite, lethargy, abnormal movement, distended abdomen, skin ulcers, erosions, haemorrhages darkening, pale gills, and inflamed vents. A post-mortem examination of clinically sick fingerlings revealed ascites and enlargement of visceral organs, especially the liver and spleen. The results of this study indicate that SCFP supplementation has the potential to improve tilapia growth performance, boost protective immunity, and increase the survival of O. niloticus in the face of bacterial challenge. Future studies should explore the residual impacts of postbiotic supplementation during the fingerling stage on the growth and performance of tilapia during grow-out in larger systems.