A total number of two hundreds and seventy apparent healthy Oreochromis. niloticus (30.0±2.0 g) was used to evaluate the effect of dietary supplementation with probiotic bacteria (commercial and isolated Bacillus amyloliquefaciens) at a level of 0.1x1010g/ kg diet and/or overcrowding stress on fish growth performance, immune status and economic efficiency.
Fish were divided into three groups. The first group (G1) was fed on basal diet (D1) without probiotic supplementation. The second group (G2) was fed on basal diet supplemented with commercial probiotic (D2). The third group (G3) was fed on basal diet supplemented with isolated probiotic (D3). Each group was subdivided into two subgroups, subgroup A was maintained at density of 10 fish/ aquaria (considered as optimum density), and subgroup B was maintained at density of 20 fish/aquaria (considered as high density).
The obtained results showed that thedifference in initial and final body weight (IBW & FBW), total body weight gain (TWG), feed conversion ratio (FCR), and specific growth rate percentage SGR(%) values of Nile tilapia fed non supplemented or supplemented with probiotic (commercial and isolated) were not significant. Total feed intake (TFI) and survivabilityvalues were decreased significantly (P<0.01) when Nile tilapia was fed diet supplemented with probiotic bacteria (Bacillus amyloliquefaciens) sources. Differences in plasma total protein (TP), plasma albumin (ALB) and plasma anti-protease, bacteria count and survivability (%) were significant (P <0.05 or P <0.01). However, the other immunological parameters were not significant.
The differences in stocking density (10 or 20 fish / aquarium) on IBW, FBW and TFI values were not significant. However, there were significant differences (P<0.05) in TWG, FCR and SGR values of Nile tilapia. Stocking density 10 fish /aquarium improved significantly (P<0.05) the TWG, FCR and SGR values as compared to stocking density of 20 fish /aquarium. Feed cost required to produce 1kg weight gain for stocking density at 10 fish / aquarium) decreased as compared to stocking density at 20 fish / aquarium).
The interaction effect between probiotic sources and stocking density revealed that there were significant difference (P <0.05) in FBW and TWG ofNile tilapia, while IBW value was not significant. The highest values of FBW and TWG values were obtained when Nile tilapia was fed diet supplemented with probiotic and stocking density at 10 fish /aquarium. On the other hand, the lowest values of FBW and TWG values were observed when Nile tilapia was fed diet non-supplemented with probiotics sources and 20 fish / aquarium.
Conclusively, from the previous results, it could be concluded that diet supplemented with isolated probiotic and stocking density 10 fish /aquarium was the best values in growth performance, immunity and economical efficiency of Nile Tilapia (Oreochromis niloticus).