Submitted to: Journal of the World Aquaculture Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2007
Publication Date: 9/1/2007
Citation: Danaher, J., Tidwell, J., Coyle, S.D., Dasgupta, S., Zimba, P.V. 2007. Effects of Two Densities of Caged Monosex Nile Tilapia Oreochromis Niloticus on Water Quality, Phytoplankton Populations and Production When Polycultured With Macrobrachium rosenbergii in Temperate Ponds. Journal of the World Aquaculture Society. Interpretive Summary: Intensive aquaculture requires oxygenation from algae or via aeration. These dense blooms of algae can reduce water quality. One option to improve water quality is to grow fish that remove algae from ponds. We assessed the utility of this technique in growing freshwater shrimp with tilapia. Tilapia removed algae, as indicated by lower pH changes and decreased algal biomass in co-culture ponds. Growth of shrimp was increased from co-culture. The production of a second product from this operation improved financial returns relative to shrimp culture alone.
Technical Abstract: Elevated pH levels from the photosynthetic activity of phytoplankton can be a major problem in semi-intensive freshwater prawn (Macrobrachium rosenbergii) production systems. Phytoplanktontiverous fish may be able to graze excess phytoplankton and lower pH. The effect of different densities of caged (1 m3) Nile tilapia (Oreochromis niloticus) on water quality phytoplankton populations, prawn production, and total pond production was evaluated in freshwater prawn production ponds. The experiment was conducted in nine 0.04-ha ponds and consisted of three treatments with three replicates each. All ponds were stocked with graded, 60-day nursed juvenile prawn (0.93 ± 0.58 g) at 69,000/ha. Control ponds contained only prawns. Low-density polyculture (LDP) ponds also contained two cages (1-m3 each), containing 100/cage of monosex male tilapia (115.6 ± 22 g). High-density polyculture (HDP) ponds had four cages of tilapia. All ponds contained circulators and artificial substrate. Tilapia were fed a 32% protein floating pellet once daily to satiation, while the prawn were fed a 32% protein sinking prawn diet according to a feed chart. Total culture period was 106 days for tilapia and 114 days for prawn. Over the duration of the study, mean afternoon pH levels (surface and bottom) were significantly lower (P </= 0.05) in polyculture ponds than in prawn monoculture ponds, but pH levels were not significantly different (P > 0.05) between polyculture treatments. The impact of tilapia on phytoplankton was demonstrated by reduced phytoplankton biovolume in polyculture treatments. Tilapia in the LDP treatment had a significantly higher (P </= 0.05) specific growth rate (SGR) and average harvest weight than the HDP treatment. Prawn average weight, SGR, and production size index (PSI) for both polyculture treatments were significantly higher P </= 0.05) than prawn grown in monoculture. Prawn production was increased 12% in the LDP and 28% in the HDP compared to prawn monoculture, while total pond production (prawn and fish) increased 300% and 492%, respectively. Economic analysis indicate that LDP would increase net returns 339% compared to prawn monoculture, while HDP would increase net returns by 165% and 561% compared to LDP and monoculture, respectively. These data indicate that a tilapia and freshwater prawn polyculture system may provide pH control while maximizing pond resources, even under the temperature constrained growing season found in temperature climates.