Abstract:
The fish yields of most of the perennial reservoirs are lower than the potential fish yields and fish yields are mainly affected by the productivity of the reservoir. The productivity levels are different in reservoirs even with similar morphometric characteristics. Therefore it is important to know the productivity and the potential fish yields of reservoirs in order to maximize the inland fish production of Sri Lanka. In this study the comparison of the nutritional status, present fish yields and potential fish yields of Devalahandiya and Wijayakatupotha reservoirs in Puttalam district were calculated. These reservoirs are primarily built for irrigational purposes and fisheries are developed secondarily. These two reservoirs are similar in size. However they have different mean depths, slope gradients and shoreline characteristics. During the studied period from April 2003 to December 2003 some limnological properties and morphological differences were determined. Morphological features such as surface area, capacity, mean depth, maximum depth, full supply level, and slope gradient of the two reservoirs were studied. The limnological properties of water such as surface temperature of water, pH, sechi disc visibility, conductivity, alkalinity, hardness surface and bottom dissolved oxygen levels, primary production levels, zooplankton and phytoplankton densities, detritus matter in the bottom sediment were compared. Commercial fish catch by species were taken from July 2003 to December 2003, two times weekly. Thereby the present fish yield was calculated. By conductivity and mean depth data inorpho-edaphic indices and the maximum sustainable yields were obtained for the two reservoirs. Oreochromis niloticus is the main contributer to the yield of both reservoirs. In addition to that Channa sp., Etropus sp., Danio sp., and sometimes Indian major carps Labeo rohita and Catla catla were present in the catch. The present commercial commercial fish catch of Devalahandiya and Wijayakatupotha reservoirs are 102.5 Kg/ha/Yr and 31.9 Kg/ha/yr. respectively. The potential fish yields of these reservoirs are well above the present yield. The potential fish yield of Devalahandiya and and Wijayakatupotha reservoirs are 270.0 Kg/ha/Yr and 199.6 Kg/ha/Yr respectively. The great difference in present fish yield and potential fish yield is due to the low abundance of fish stocks. Stocking of fish fingerlings would be a correct measure to increase the fish yields of these two reservoirs.
