C/N-controlled periphyton-based freshwater prawn farming system: a sustainable approach to increase pond productivity

Three technologies showed to improve productivity and sustainability of pond production: (1) C/N ratio control, (2) providing substrates for periphyton development, and (3) fish driven re-suspension. The novelty of this PhD research is to combine these technologies, with the goal to raise pond productivity above levels obtained with each one of these technologies separately, and to increase the nutrient use efficiency in ponds above levels presently achieved, further enhancing sustainability. This combined technology is further referred to as C/N controlled periphyton (C/N-CP) technology. A series of experiments (Chapter 2-6) were conducted to develop such technology. The first step (Chapter 2) evaluated if increasing C/N ratio (from 10 to 20) in combination with providing vertical substrates for periphyton development in freshwater prawn monoculture ponds can enhance overall pond productivity. The results were encouraging due to the 75% increase of production; in addition it seemed that natural foods were underutilized by freshwater prawn. Therefore, the next step (Chapter 3) was further analysis of the above mentioned experiment investigating how C/N ratio control and addition of substrates influenced the natural food communities in freshwater monoculture ponds. This study suggested further investigation on the possibility of increasing stocking density of freshwater prawn and inclusion of tilapia due to its both sediment re-suspension and periphyton grazing activity. Therefore, in thethird step (Chapter 4) increasing stocking densities of prawn (from 2 to 3 m−2) and addition of different levels of tilapia (0, 0.5 and 1individual m−2) were tested. This study concludes that both stocking densities (2 and 3 juveniles m−2) of prawn with the addition of 0.5 tilapia m−2 resulted in higher fish production, good environmental condition and economic return. In the fourth step (Chapter 5), the effects of addition of periphyton substrates and tilapia driven bioturbation were tested in C/N controlled (C:N=20) system. This study showed that addition of tilapia (0.5 individual m−2) and periphytonsubstrates in C/N controlled ponds benefitedfreshwater prawn production and recommended that economicsustainability could still befurther enhanced by identifying cheaper on-farm carbohydratesources. Therefore, in the last step (Chapter 6) maize flour (Zea mays) is considered as a cheaper on-farm carbohydrate source and compared with tapioca starch. In addition, in this study considering the importance of rohu (Labeo rohita) as an indispensable species in south Asian aquaculture, both tilapia and rohu are considered to determine the suitability of either species in C/N-CP ponds. In added finfish (0.5 individual m−2),100% tilapia were found to be beneficial in C/N-controlled (C:N=20:1) prawn farming system compared to 50% tilapia+50% rohu or 100% rohu. In conclusion, a significant improvement of system environment, productivity and economic benefits was observed due to synergism among C:N ratio control, addition of periphyton substrates and tilapia driven bioturbation. Therefore, C/N-CP technology is a promising technology, improving the sustainability and productivity of present prawn farming by simple and affordable means.