Research on combined cage-in-pond culture systems for South-East Asia.

Submerging a cage within a pond and intensively feeding fish in the cage is one method of using the waste products from cage culture in semi-intensive pond culture. Using hybrid walking catfish, Clarias macrocephalus x C. gariepinus, in cages and Nile tilapia, Oreochromis niloticus, at large in ponds, treatments of one or two cages per pond produced better results than higher stocking densities. These treatments resulted in net fish yields as high as 56 tonnes ha-1 yr-1, and the most efficient system in terms of nutrient retention was using one cage per pond. The systems with one cage per pond delivered 3.7 kg of nitrogen (N) and 1.0 kg of phosphorus (P) ha-1 d-1, and retained 48% of the N added in fish, whereas two cages resulted in 34% N retention. For P, the results were 61 and 42% retained in fish, respectively. This aquaculture system resulted in some reduction in nutrient enrichment of ponds compared to feeding alone. A second series of experiments conducted using large Nile tilapia in cages, with small Nile tilapia in the pond, showed the best results when cages were stocked at 50 fish m-3 and the pond at 1.4 fish m-3. Survival was 98.8% for caged tilapia, which grew from 120 to 456 g in 90 days, with a net annual yield of 18 tonnes ha-1. Open-pond tilapia grew from 16 to 124 g with 92% survival and a net annual yield of 6.2 tonnes ha-1. The daily waste loading rate was approximately 1.7 kg of N and 0.37 kg of P ha-1, lower than the optimal fertilization rate proposed by CRSP experiments of 4 kg of N and 1 kg of P. Caged and pond fish accounted for about 50% of the N applied to the pond and about 60% of the P. The dynamics of oxygen, ammonium and plankton were evaluated against total loading rates for all culture combinations. Oxygen levels declined dramatically (about 75-100%) in all loading rates of ponds and never reached levels approaching zero decline in oxygen. N increased with increased loading density of fish and a level of zero change in ammonium would occur at a loading rate of about 0.8 kg m-3. Plankton dynamics showed similar results, and a fish loading rate with zero change in chlorophyll a should occur at a loading rate of about 0.3 kg m-3. These results indicate that it may be possible to reduce N and plankton contents in effluent water, but doing so would require loading rates that are not economically feasible.