Bioflocs technology: an integrated system for the removal of nutrients and simultaneous production of feed in aquaculture
Future development of intensive aquaculture must deal with its impacts on the environment in the form of water pollution and the use of fish oil and fish meal. The bioflocs technology simultaneously addresses both problems co occurring with the further expansion of the industry. While maintaining good water quality within the aquaculture systems it produces additional feed for the cultured animals. In contrast to conventional water quality control techniques, the bioflocs technology offers a sustainable, economical and easy to implement alternative. Chapter 1 gives an overview of the literature concerning nitrogen removal techniques in aquaculture and bioflocs technology. In Chapter 2, the impact of the carbon source on the performance of biofloc reactors was studied. The carbon source influenced the capacity of the technique to control the water quality in the biofloc reactors and the nutritional properties of the flocs. The carbon source also affected the eukaryotic and prokaryotic community composition of the bioflocs, which offers great possibilities for fine tuning of the technique, more specifically concerning water quality control, feed production and/or costs. This prime importance of the choice of carbon source was confirmed in two further studies (Chapter 3 and Chapter 4) in which bioflocs grown on different substrates were fed to giant freshwater prawn (Macrobrachium rosenbergii) postlarvae and white shrimp (Litopenaeus vannamei) respectively. In both studies, glycerol grown bioflocs showed better results than glucose grown bioflocs. The potential significance of these results calls for further studies on the use of bioflocs as a feed in aquaculture, both in freshwater and saline systems. Parameters to consider in the future are accessibility, palatability or attractiveness of the bioflocs towards the animals, amino acid composition, essential fatty acids content and cost of the used carbon source as well as the overall cost of the technology (especially compared to conventional biofilter systems and feeding costs) In addition to the environmental, economical and sustainable considerations addressed above, a more specific problem was studied in Chapter 5, where aquaculture animals are exposed to lower temperatures during winter, possibly leading to mass mortality in industrial ponds. Covering the ponds with either plastic sheets or glass allowed solar heating of the culture water (thereby reducing the temperature decrease) and permitted to minimize water exchange. The application of bioflocs technology resulted in maintenance of good water quality, concomitantly providing additional feed to the animals, tilapia (Oreochromis niloticus x Oreochromis aureus) without compromising survival, growth and condition factor of the cultured species. At this moment, the aquaculture industry is most importantly faced with mass mortalities due to infectious diseases. To conclude this work, a potential extra added value feature of the bioflocs technology was studied in Chapter 6. In this study, bioflocs were found to be able to protect brine shrimp (Artemia franciscana) larvae from pathogenic Vibrio harveyi. These results indicate that in addition to water quality control and extra in situ feed production, the technique also has potential to protect the cultured animals from infections with pathogenic bacteria, which are responsible for major economic losses in aquaculture. To conclude, the last chapter (Chapter 7) provides a brief discussion of the performed studies. Directions for future in depth studies are raised based on the studies performed in this work that might contribute to further sustainable development of aquaculture.