A Thesis Submitted in Fulfillment of the Requirements for the Award of the Degree of Doctor of Philosophy (Fisheries Science) in the School of Pure and Applied Sciences of Kenyatta University August, 2020

Nile tilapia (Oreochromis niloticus) culture in Kenya is mainly conducted in low input ponds where supplementary feed is given alongside pond fertilization. Excess nutrients in the culture water in form of nitrogen (N) and phosphorus (P) may make the culture environment susceptible to invasion by disease-causing microorganisms. In the recent past, probiotics have received much attention as a new strategy in fish health management and have been documented to improve fish growth performance and immunity in fish cultured in recirculating systems. However, their effect in tilapia cultured in low input ponds is still relatively unknown. In this study, monosex O. niloticus fingerlings with a mean weight of
39.75 ± 0.05 g were randomly stocked at 50 fish m-3 in 1.25 m3 cages in low input earthen ponds. The fish were fed twice daily at 3% body weight on seven isonitrogenous (28%
crude protein) diets supplemented with either Saccharomyces cerevisiae (1×1010 CFU g-1) or Bacillus subtilis (1×109
CFU g-1) at different levels as follows: No probiotic (Diet 0); S.
cerevisiae at 2 g kg−1 (Diet 1), 4 g kg−1 (Diet 2) and 6 g kg−1 (Diet 3); and B. subtilis at 5 g kg−1 (Diet 4), 10 g kg−1 (Diet 5) and 15 g kg−1 (Diet 6) for a period of 7 months. The fish
were sampled monthly for weight and length measurements. Hemato-immunological parameters were determined by blood sampling and hematological analysis for red blood cells, white blood cell and haemoglobin counts. Blood serum assay was conducted using commercial enzyme-linked immunosorbent assay (ELISA) kits to determine the serum protein, albumin, globulin levels and lysozyme activity. Microbiological samples were analyzed through sub-culturing to obtain pure cultures on nutrient media and enumerated through standard methods. Results of the trials indicated that the highest performance was
achieved with Diet 2. The highest final weight (255.31 ± 3.19 g), Specific growth rates (SGR) (0.77±0.01% day-1) and feed conversion ratio (FCR) (1.61 ± 0.02) were recorded in
fish fed Diet 2. This was followed by fish fed Diet 5. Results of growth performance analysis indicated that fish fed on probiotic-supplemented diets had significantly better
growth, nutrient utilization and FCR than fish fed on the control diet (P < 0.05). Probiotic supplementation significantly affected the body composition of the fish (P < 0.05). Saccharomyces cerevisiae at 4 g kg-1 (Diet 2) led to significantly high protein (86.06%) (P< 0.05) while B. subtilis at 5 g kg-1 (Diet 4) led to significantly higher protein (89.40%) (P< 0.05). Crude lipid and ash content were significantly lower in the fish fed probioticsupplemented diets (P < 0.05) compared to the control. Results of hemato-immunological
analysis indicated that haemoglobin (Hb), red blood cells (RBC), white blood cells (WBC), serum protein, albumin, globulin and lysozyme activity were higher in fish fed on
probiotic-supplemented diets and lower in the control group (Diet 0). Probiotic significantly affected hemato-immunological parameters (P < 0.05). Fish fed on probioticsupplemented diets retained the probiotics in their guts and had lower microbial load in their muscle (P < 0.05). This study shows that incorporation of probiotics in diets of Nile tilapia in low input ponds promotes growth performance, enhances body composition, improves immunity and manipulates gut microbiota of fish. The two probiotics differ in effect at different levels of application. Probiotic S. cerevisiae exhibited the best
performance at 4 g kg-1 while B. subtilis had the best performance at 10 g kg-1. Probiotics are therefore recommended for use in low input fish culture systems for better nutrient utilization, higher yields and improved fish health for increased aquaculture production.