Intensifying aquaculture production through new approachesto manipulating natural food.

The bulk of the world aquatic production is comprised of herbivorous/omnivorous fish species that feed low in the food chain and are cultured in semi-intensively managed earthen ponds in the tropics and sub-tropics. This review summarizes the basic principles and mechanisms of natural food-based production and explores the potential of novel aquaculture technologies that aim to further improve nutrient efficiency and overall sustainability. There is a strong linear relationship between primary productivity and fish yields in earthen ponds. Primary productivity depends mainly on the availability of elementary nutrients (N, P and C) and sunlight. Fertilization and liming are common practice in aquaculture ponds to maintain natural productivity and water quality. The practical upper limit for net primary productivity (NPP) is 4 g C/m<sup>2</sup>/day but most of the aquaculture ponds remain within the productivities between 1 and 2 g C/m<sup>2</sup>/day. Pond food organisms are rich sources of protein and energy, containing 9-64% and 9-24 kJ/g, respectively, on a dry matter weight basis. The nutritional requirement of most pond fish species ranges from 17-55% protein and 15-30 mg/kJ protein: energy ratio. Considering an average pond productivity of 2 g C/m<sup>2</sup>/day, an estimated total fish production of 10 t/ha/yr can be achieved without any supplementary feed, but in practice this is rarely achieved. Polyculture and integrated agriculture-aquaculture (IAA) are traditional in Asia. The reported maximum polyculture production is about 15 t/ha/yr. The term integrated multi-trophic aquaculture (IMTA) has been used in developed countries as a synonym for polyculture. Aquaculture is integrated with other activities, most commonly domestic, agricultural or agroindustrial in using resources (water, nutrients and labour) to improve the use of otherwise underused or sometimes potentially damaging waste materials. Aquaculture using treated wastewater has also been successful in recent years: production being reported at 5-7 t/ha/yr. To enhance pond productivity in the form of periphyton, vertical hard substrates are introduced in ponds. The reported increase in fish production through the use of substrates ranged from 30-115% in carp monoculture and 30-210% in carp polyculture. An annual maximum production of 9 t/ha was reported from periphyton-based polyculture ponds without any supplementary feed. The activated suspension technique provides continual aeration and mixing of the organic matter throughout the water column, thus keeping the decomposition process aerobic and fast. It is based on the development of dense micro-organisms that function both as a bioreactor controlling water quality and as a protein food source for the cultured animals. The technique increases the feed protein utilization and water use efficiency, leading to reduction in overall costs. We conclude that the novel approaches to manipulating natural food can increase the productivity and efficiency of aquaculture production systems, and seem to be conformed to the criteria for ecological aquaculture and organic aquaculture with little modifications based on other specific criteria. More research is needed for optimization of the novel technologies. Directions for future research are recommended.