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.