Population dynamics of Oreochromis mossambicus, Oreochromis niloticus and their hybrids/ introgressed individuals in three reservoirs of Sri Lanka

Abstract

Introduced cichlids Oreochromis mossambicus and Oreochromis niloticus are the major contributors to the reservoir fishery of Sri Lanka. Studies have shown hybridization/ gene introgression between these two species has resulted in three main groups; O. mossambicus, O. niloticus and hybrids/ introgressed individuals (hybrid x parent species) in many reservoirs. Population dynamics, growth performances, mortality and recruitment are important parameters in stock assessment and information on these is essential for better management of a fishery. Present study compares the growth performance and population dynamics of the three cichlid groups in three reservoirs of Southern Sri Lanka. Monthly length frequency samples of O. mossambicus and O. niloticus and hybrids/ introgressed individuals were taken from three reservoirs namely, Chandrikawewa, Ridiyagama and Lunugamwehera for a period of 14 months and the data were analysed using FiSAT software package. Growth performance indices of the hybrid genotypes were intermediate to parental genotypes indicating that the hybrids/ introgressed individuals are of intermediate performance compared to parental groups. O. mossambicus showed lower growth rate than other two groups. Except in Lunugamwehera reservoir, O. niloticus and hybrids/ introgressed groups did not show appreciable difference in growth rates. High natural mortalities were observed in all reservoirs for all groups while hybrid/ introgressed group showed high fishing mortalities compared to other two groups. With the exception for hybrids and O. mossambicus genotypes at Chandrikawewa reservoir, all other genotype groups were overexploited. This would lead to depletion of stocks which could adversely affect the fishery in long term. Present study shows that growth and population dynamic parameters are different in three groups within a reservoir and also among three reservoirs. Therefore, management strategies should be adopted after studying different reservoirs separately. A strategy common to all reservoirs may not be successful.