The Fishery and Population Dynamics of Oreochromismossambicus and Oreochromis niloticus (Osteichthyes, Cichlidae) in a Shallow Irrigation Reservoir in Sri Lanka

Abstract

The fishery of a shallow irrigation reservoir in Sri Lanka was investigated. Catch and effort data of all species and length frequency data of cichlid species, O. mossambicus (Peters) and O. niloticus (L.) which form over 80% of landings, collected over a period of 13 months were analyzed. Yield-per-recruit analyses of the two cichlid species, performed using FiSAT computer software package indicated that both cichlid stocks in the reservoir are optimally exploited. Annual fish yield and monthly mean fishing intensity (FI in boat-days per month) were estimated for two scenarios, i.e., reservoir area at full supply level (FSL) and actual monthly mean reservoir area. They indicate that the estimates of yield (228 kg�ha-1�yr-1) and FI (2.9 boat-days�ha-1�month-1), which are computed for the reservoir area at FSL are about 23 and 21% respectively of the estimates of yield (985.5 kg�ha-1�yr-1) and FI (13.3 boat-days�ha-1 �month-1) based on actual monthly mean reservoir area. Fish stock assessment and fisheries management strategies, which are based on Y and FI estimated on the basis of reservoir area at FSL are therefore bound to be inaccurate. Potential implications of difference of yield and FI estimates based on monthly mean reservoir area and area at FSL, in determining the optimal fishing strategies are discussed.