Dietary non-starch polysaccharides influenced natural food web and fish production in semi-intensive pond culture of Nile tilapia

Dietary non-starch polysaccharides (NSP) changes the nutrient digestibility and faecal characteristics in fish. This study assessed the effect of the type of dietary NSPs on fish production and the contribution of natural food to the total fish production in semi-intensively managed tilapia ponds. Twelve ponds, each divided into three equally-sized compartments, were assigned to test the effect of the type of dietary NSPs (i.e. PecHem-Diet, a diet with easily fermentable NSP, vs LigCel-Diet, a diet with slowly fermentable NSP). Fish were restrictively fed, based on the crude protein content of the feed. Three feeding levels (no = 0, low = 9 g.kg(-0.8).d(-1) and high= 18 g.kg(-0.8).d(-1)) nested in pond were analysed in a split plot design. Initial fish biomass was 3084 g.compartment(-1) and the experiment lasted 56 days. With the LigCel-Diet biomass gain was higher (2599 vs 2192 g.compartment(-1)) and feed conversion ratio (FCR) was lower (1.4 vs 1.9; P <.001) than with the PecHem-Diet. Diet had no effect on fish survival and specific growth rate (SGR). For both diets, increasing feeding level increased (P <.001) biomass gain, fish survival, FCR and SGR. There was a significant interaction effect (P <.05) between diet and feeding level on FCR. Fish body composition was the same in both diets. With the LigCel-Diet, the apparent digestibility coefficient (ADC) was higher (P <.001) for crude protein, fat, phosphorus and calcium and lower (P <.05) for ash compared to the other diet. Neither feeding level nor the interaction between diet and feeding level influenced the apparent digestibility coefficient (ADC) of any nutrient. Diet composition did not alter the organic matter (OM) composition of the faeces. delta C-13 and delta N-15 from the stable isotope analysis revealed that N gain in fish originated from both feed and natural food of the pond. Natural food abundance in the pond increased over time for both diets. Chlorophyll-a was higher in the pond fed with LigCel-Diet. Fish gut content and calculated N gain indicated an enhanced contribution of natural food to fish growth in ponds fed with LigCel-Diet. In conclusion, the type of dietary NSP determines tilapia productivity in semi-intensive managed ponds by altering food web productivity.