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Population dynamics of two cichlid species, Oreochromis mossambicus and O. niloticus in two reservoirs in Sri Lanka were studied using length-based methods. Asymptotic total length (L?) and growth constant (K per year) of O. mossambicus (L? = 43.7 cm and K = 0.52 in Kaudulla; L? = 45.0 cm and K = 0.45 in Minneriya) and O. niloticus (L? = 54.5 cm in both reservoirs; K = 0.35 and 0.43 in Kaudulla and Minneriya, respectively) were used to estimate mortality and exploitation rates (E). The growth performance of O. mossambicus in both reservoirs was better than that of O. mossambicus populations in various geographical areas. This may be due to introgressive hybridization between O. mossambicus and O. niloticus or a very favorable environment for O. mossambicus in Sri Lanka reservoirs which provide a variety of nutritious food sources. Length-structured Virtual Population Analysis indicates that fishing mortality rates are approximately constant throughout the recruited stocks, suggesting that the effects of gill-net selectivity on the catch samples are small.
Relative yield-per-recruit analyses incorporating probabilities of capture inducate that O. mossambicus and O. niloticus yields in Minneriya could be increased by increasing size at first capture. In Kaudulla, any increase in E of O. mossambicus stock would adversely affect the O. niloticus stock. Also, increase in size at first capture of O. niloticus to optimize yield would leave the O. mossambicus stock underexploited. |
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