Carbon-nitrogen ratio plays an important role in enhancement of natural food and subsequent fish production in ponds while carbohydrate type (starch vs NSP level) has an impact on the nutrient digestibility of fishes. This study investigated the effect of feed carbon-nitrogen ratio (C:N) and carbohydrate type (CT), on the fish production in carp-tilapia polyculture ponds stocked with 36 fish of each three carps (rohu, catla and silver carp) and 12 tilapia. Three dietary C:N ratios (low 7.6; medium 9.5; high 12.8) and two carbohydrate types (High starch - Low NSP SHNSPL; Low starch – High NSP SLNSPH) were tested in a 3x2 factorial random design .
Survival was not affected by the experimental factors. Survival of all species was good, except for catla (21%). This was possibly due to high interspecies competition for feed among catla, tilapia and silver carp. Tilapia, though low in number, clearly monopolized the feed resource supplied, shown by its high growth rate (5.5 g.d-1) averaged over all treatments (figure 1). This indicates that even low inclusion of tilapia can hamper carp production.
The dietary C:N ratio did not affect total fish production while carbohydrate type influenced fish production showing improved biomass gain with diets containing SLNSPH carbohydrate (P<0.05). Interaction effect of CT x species showed that growth rate of tilapia improved with SLNSPH containing diets (figure 1). Feed conversion ratio was significantly lower with SLNSPH diets (1.9) than with SHNSPL diets (2.5). However, the body protein content of all fish species was lower with SLNSPH diets (156 g.kg-1) than with SHNSPL diets (161 g.kg-1). Average fat content of all fish species increased with increasing C:N ratio from low (77 g.kg-1) to medium (80 g.kg-1) to high (90 g.kg-1). Increasing the C:N ratio and NSP content in the diet considerably lowers the feed cost, while maintaining or increasing fish production in ponds.