Population Dynamics of Commercially Important Fish Species in Two Reservoirs of the Walawe River Basin, Sri Lanka

Population dynamics of dominant fish species in two lowland reservoirs of Sri Lanka were studied based on length-based stock assessment methodologies, as implemented in FiSAT II software. The estimated growth parameters of Oreochromis mossambicus in Udawalawe and Chandrikawewa reservoirs and those of O. niloticus and Labeo dussumieri in Udawalawe reservoir fall within the growth space of auximetric plots of cichlids and cyprinids indicating reliability of estimates. In the Chandrikawewa reservoir where O. mossambicus exhibited faster growth, increased fishing pressure might lead to push the population towards the extreme of ‘r-selected’ life strategy so that continuation of the fishery at the present exploitation level is advisable. Relative yield-per-recruit (Y’/R) analyses indicate that for O. niloticus and L. dussumieri in Udawalawe reservoir, exploitation levels can be increased to optimize Y’/R. However, since these two species and O. mossambicus are caught in the same fishing gear, exploitation rate for O. niloticus and L. dussumieri has to be increased to a level that would not lead to over-exploitation of O. mossambicus stock.