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Item Influence of hydrology on water quality and trophic state of irrigation reservoirs in Sri Lanka(Lakes & Reservoirs: Science, Policy and Management for Sustainable Use, 2019) Nadarajah, S.; Wijenayake, W.M.H.K.; Amarasinghe, U.S.Many reservoirs provide multiple benefits to people around the world, in addition to primary uses such as irrigation. Thus, reservoir management should address their multiple uses. The water quality of ten irrigation reservoirs in Sri Lanka was examined in the present study with the objective of better understanding the effects of hydrological regimes on reservoir water quality and trophic state. Basic limnological parameters pertinent to the nutrient loads to, and trophic state of, the reservoirs were collected from June 2013 to February 2016. The sampling period was arbitrarily divided into two periods of approximately similar duration (period 1 = June 2013–September 2014; period 2 = October 2014–February 2016) to investigate whether or not there was a seasonal variation in the water quality parameters. Although temporal and spatial variations were observed, most water quality parameters were within the levels acceptable for drinking water standards. The 10 reservoirs were also ordinated by principal component analysis (PCA) on the basis of the water quality parameters of the two sampling periods in a two‐dimensional score plot. Reservoirs in the first principal component (PC1) axis were represented by negative scores attributable to the dissolved oxygen concentration and pH and, to a lesser extent, by electrical conductivity and chlorophyll‐a concentration. Positive scores in PC1 were represented by reservoirs with a score loading attributable to alkalinity, nitrate concentration, Secchi depth, temperature and seston weight and, to a lesser extent, from the total phosphorus concentration. There was a significant negative correlation of PC1 scores with relative reservoir water‐level fluctuation (RRLF; the ratio of mean reservoir waterlevel amplitude to mean reservoir depth). Furthermore, Carlson's trophic index also were influenced by RRLF, although not by hydraulic retention time (HRT), indicating allochthonous nutrient inputs into the irrigation reservoirs were mainly governed by RRLF, but not by HRT. Thus, the results of the present study provide useful insights into achieving desirable reservoir water quality through the manipulation of the hydrological regime.Item Length-based stock assessment of Oreochromis mossambicus and O. niloticus (Actinopterygii: Perciformes: Cichlidae) in multi-mesh gillnet fisheries in reservoirs of Sri Lanka. Acta Ichthyologica et Pischatoria(An International Journal for Ichthyology and Fisheries, 2017) Amarasinghe, U.S.; Jayasinghe, R.P.P.K.; Moreau, J.Background. Gillnets of mesh sizes (6.9 to 12.7 cm) are used in individual boats in Sri Lankan reservoirs targeting exotic cichlid species, Oreochromis mossambicus (Peters, 1852) and Oreochromis niloticus (Linnaeus, 1758), which dominate the fisheries. The mesh composition of gillnets varies from boat to boat and from season to season making catch samples in individual boats are under the influence of different selectivity patterns. As such, the conventional procedure of correcting length frequency samples using the overall mean selectivity pattern in the fishery is not precise in estimating growth parameters. The presently reported study aimed at assessing cichlid stocks in three tropical reservoirs accounting for gillnet selectivity of individual sampled boats. Materials and methods. Length frequency data obtained from each boat were corrected for the combined selectivity of the fleet of gillnets of different mesh sizes used in the boat during the sampling occasion. For predicting optimal fishing strategies using a yield-per-recruit approach, probabilities of capture determined from the array of fishing mortality in the length-structured virtual population analysis was incorporated. Results. Length frequency data corrected for overall gillnet selectivity in sampled boats gave reliable estimates of von Bertalanffy growth parameters. Based on these estimates, mortality parameters were determined and the overall gear selection pattern was deduced from the array of fishing mortality in the length-structured virtual population analysis. Optimal fishing strategies were determined incorporating overall probabilities of capture in the relative yield-per-recruit (Y′ × R–1) analysis. Conclusion. The overall gillnet selection pattern of a sampled boat offers a methodology for reliable estimation of growth parameters. Although both cichlid species are caught in gillnets simultaneously, Y′ × R–1 analyses predicted increase of exploitation rate for one species while its decrease for the other species perhaps due to influence of other factors such as fish behaviour and fishers’ choice of specific areas for fishing. This analysis provides a methodology of a more precise length-based stock assessment in future research.Item Food and feeding of Ehirava fluviatilis (Osteichthyes, Clupeidae) in Rajanganaya Reservoir, Sri Lanka(Sri Lanka Journal of Aquatic Sciences, 2014) Mihindukulasuroiya, I.D.; Amarasinghe, U.S.Ehirava fluviatilis is of marine origin and has colonized some freshwater reservoirs of Sri Lanka. A small scale fishery of E. fluviatilis exists at the outflow LB canal of Rajanganaya reservoir. This study was carried out to investigate food and feeding habits of E. fluviatilis collected from this location. Sampling was done fortnightly from the landings. From the length-weight relationship, an allometric growth is evident. The theoretical weight of 50 mm E. fluviatilis calculated from length weight relationship was 928 mg, a comparable value to that of the estuarine population in Bolgoda Lake, Sri Lanka but a higher value than that of reservoir population in Parakrama Samudra, Sri Lanka, indicating that the population in Rajanganaya is as healthy as estuarine population. The highest index of relative importance in the stomach contents was recorded for diatoms. In addition, two genera of copepods, two genera of cladocerans, and two genera of rotifer, one genus each of insect larvae and crustaceans other than copepods and cladocerans were also present. Diatoms were also an important food item. As colonization success of E. fluviatilis in inland reservoirs seems to be due to the presence of their preferred food, micro-crustaceans, findings of the present analysis will be useful for investigating physiological advantages of being a stenophagous fish species.