Fluctuations in Water Level in Shallow Irrigation Reservoirs: Implications for Fish Yield Estimates and Fisheries Management

Abstract

Due to fluctuations in water level, reservoir surface area changes considerably. Despite this effect, fish yields (Y) and fishing intensities (FI) in these reservoirs are often estimated for the reservoir area at full supply level (FSL). This paper compares the estimate of optimal fishing strategies according to this conventional method with those based on Y and FI calculated for actual mean monthly reservoir area. Catch and effort data, collected at least for 20 days a month from 10 individual reservoirs in Sri Lanka, were analysed to estimate mean annual fish yields (kg/ha/yr) and total FI (boat-days/ha/yr). Reservoir areas at FSL were used to estimate these values. Using the hypsographic curves and mean monthly water levels in individual reservoirs, actual mean reservoir area in each month in each reservoir was determined. Annual fish yields (kg/ha/yr) and total FI (boat-days/ha/yr) in individual reservoirs were then estimated, based on these actual reservoir areas. In both estimates, Y was linearly related to FI, indicating that the fish stocks were perhaps exploited at suboptimal levels. The results appear to indicate that the conventional method of using reservoir area at FSL to estimate Y and FI in multi-purpose reservoirs, instead of actual reservoir area, may have serious implications for fisheries management. An alternative method for estimating fish yields and FI is suggested for reservoirs with heavy draw-down.