Modeling the potential of periphyton based fish production in pond culture system
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To evaluate the potential of fish production from Periphyton-based aquaculture system, a simple dynamic simulation model was constructed. The model consists of three state variables, periphyton biomass (PB; g), fish biomass (FB; g) and nutrient stock and six rate variables (nutrient inflow, nutrient uptake by periphyton, periphyton grazing by fish, periphyton degradation rate, fish harvesting and mortality rates). In the model, it was assumed that PB is minimum before fish were stocked and that fish grazing would cease whenever PB would be lower than that minimum biomass. This model was implemented in Stella 8 and run with a time- step of 0.05 day. Parameter values were derived from the literature. We assumed a maximum periphyton density of 100 g dm m-2. PBmax was derived from this value by multiplying with the substrate area. Simulated PB increased from 10 g m-2 initially to 100 g m-2 after 24 days. Before day 30, periphyton productivity was greater than the consumption of the periphyton by fish. After day 105, fish grazing exceeded periphyton productivity as a result of increased FB and PB decreased steadily until reaching a value of about 75 g m-2 on day 182. The scenario in the model also showed that the optimum application rate of nutrient is at 15 g m-2 urea per two weeks. In the model a 1:1 ratio of substrate area to pond size tends to produce larger FB which was 1000 kg ha-1. Therefore, periphyton can increase the productivity and efficiency of aquaculture systems; however more research is needed for optimization.