Asian Fisheries Science

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Author: search.filters.author.Ajith Kumara, P.A.D.

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    Life-History Strategies of Nile Tilapia, Oreochromis niloticus(Linnaeus, 1758) (Teleostei, Cichlidae), in Irrigation Reservoirs of Sri Lanka
    (Asian Fisheries Society, 2020) Bandara, K.V.S.N.; Ajith Kumara, P.A.D.; Amarasinghe, U.S.
    As in many tropical countries, reservoir fishery of Sri Lanka is dominated by the exotic cichlid, Oreochromis niloticus(Linnaeus, 1758). Oreochromis niloticuspopulations in irrigation reservoirs are expected to be under the influence of various disturbances such as changes in hydrological regimes, trophic conditions and fishing pressure. In the present study, an attempt was made to investigate the effect of biological productivity and fishing pressure on life history patterns of O. niloticuspopulations in 10 irrigation reservoirs of Sri Lanka. Oreochromis niloticuspopulations in environmentally stressed reservoirs, exhibited low age at maturity than those inhabiting reservoirs with relatively low biological productivity. Hydraulic retention time (HRT) in reservoirs also had negative influence on size and age at maturity of O. niloticus. As length at first capture of O. niloticusis positively related to mean length at maturity, these attributes have significant implications on the optimal fishing. Fishing intensity (FI) in O. niloticuspopulations also had negative influences on mean length and age at maturity. Hence, the present study supports the opinion that altricial or opportunistic traits having faster growth and achieving maturity early in the life dominate disturbed environments and that precocial or equilibrium traits exhibiting delayed maturity occur in more stable environments. Possible shifting of life history strategies of O. niloticusto opportunistic traits due to disturbance events could be manipulated by increasing FI for smaller individuals in the population.
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    Population Dynamics of Nile Tilapia, Oreochromis niloticus(Linnaeus, 1758) (Teleostei, Cichlidae), in Some Irrigation Reservoirs of Sri Lanka
    (Asian Fisheries Society, 2020) Bandara, K.V.S.N.; Ajith Kumara, P.A.D.; Amarasinghe, U.S.
    Oreochromis niloticus(Linnaeus, 1758) exhibits differences in demographic parameters across the reservoirs, resulting in inconsistencies in optimal fishing strategies. This study aimed at investigating O. niloticuspopulations in 10 reservoirs. Length-frequency data of O. niloticusfrom fishlandings of 10 irrigation reservoirs in Sri Lanka were corrected for multi-mesh gillnet selection and were analysed by FiSAT II stock assessment software. Asymptotic total length ranged from 42.8 cm to 53.4 cm and growth constant ranged from 0.20 yr-1to 0.49 yr-1. As growth performance indices of 10 reservoir populations of O. niloticuswere within the range reported previously and as the estimated growth parameters fall within the “growth space” of the species, these estimates were consistent with those reported in the literature. Length-converted catch curves of 10 reservoir populations of O. niloticusalso gave reliable estimates of total mortality. Exploitation rates of the 10 populations ranged from very low (0.14) to very high (0.76) values. From the relative yield-per-recruit analyses, it was apparent that in some reservoirs, long-term fish yields can be optimised using gillnet mesh sizes of 7.6 cm. While optimal long-term fish yields of O. niloticusin other reservoirs could be achieved by increasing the permissible gillnet mesh size above the legal mesh size of 8.4 cm. Hence, the gillnet mesh regulations in the Sri Lankan reservoir fishery should not be uniform but reservoir-specific. Accordingly, site-specific management options, which are defined in consultation of resource users are necessary to be implemented in reservoir fisheries of the country.