Nutrient Distribution in a Fish Pond at Rwasave Fish Farming and Research Station, Rwanda

Abstract

This paper reports on the study nutrient distribution in the fish pond carried out at Rwasave at Fish Farming and Research Station in Butare, Rwanda. The purpose of the study was to determine the nutrient distribution in a single fishpond supplied by Rwabuye River and fertilized by rabbit dung. Vertical and horizontal distribution of nutrients in the fishpond were assessed and the nutrient concentrations in the inflow and outflow of water from the fishpond were determined. Twelve parameters were monitored. Temperature, pH, Dissolved Oxygen, Electrical Conductivity, Turbidity and Transparency were measured directly in the field. Total Nitrogen, Total Phosphorus, Nitrate-Nitrogen, Nitrite-Nitrogen, Ammonium-Nitrogen and Orthophosphate were measured in the laboratory using different standard analytical methods. Eight sampling sites were located within the pond, in addition to inflow and outflows. The sampling was done fortnightly. The water samples were collected at the surface and bottom of the pond. Samples were taken using a Van Dorn Bottle water sampler and were preserved and stored in cooler boxes, and later analyzed for nutrients using colorimetric methods. Samples from sediments were taken using a metallic stick and fish using a fibre mesh. Both sediments and fish samples were analyzed for TN and TP content. Statistically, the results indicate that the nutrient concentrations in the pond were higher than those in the inflow. This is due to the accumulation of nutrients and additions due to rabbit droppings. The TN averaged 25.43±5.82 mg.l-1 and TP 1.25±0.95 mg.l-1 in the pond, in inflow TN was 25.1±9.1 mg.l-1 and TP 0.82±0.79 mg.l-1. The average concentration of TN in fish for the two times of sampling was
8.75% N and 8,827 ppm for TP. In sediment the mean concentrations were 0.2%N and 236.5ppm respectively for TN and TP. The mean Secchi disk measurement was 26 cm and indicates that water pond was turbid. The nitrate concentrations show that the pond was exposed to eutrophication. Previously reported figures also are generally low compared to the results of this study and urgent control is therefore required. It was observed that there is generally a high level of nutrients in the Rwasave fishpond. It is recommended that proper nutrient control practices be adopted for avoiding excess nutrients and environmental pollution.

Key words: Nutrient distribution, Fish farming, Fishpond, Rwabuye, Rwasave, Rwanda.