Zoology
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Item Multi-mesh gillnet selectivity of Oreochromis mossambicus and O. niloticus (Cichlidae) in the fishery of three large perennial reservoirs in Sri Lanka(Sri Lanka Journal of Aquatic Sciences, 2017) Jayasinghe, R.P.P.K.; Amarasinghe, U.S.; Moreau, J.As in many tropical reservoir fisheries, the major fishing gear in the reservoirs of Sri Lanka is gillnet. Gillnets of a wide range of stretched mesh sizes (6.9 - 11.4 cm) are used in individual boats in Sri Lankan reservoirs targeting mainly two exotic cichlid species, Oreochromis mossambicus and O. niloticus, which dominate the fisheries accounting for over 80% of the landings. Although the filament characteristics and dimensions of gillnets of different mesh sizes are uniform, their mesh composition varies from boat to boat making catch samples in individual boats are under the influence of different selectivity patterns. As such, an approach is presented for constructing the overall selection curves in the sampled boats during different sampling occasions for the two cichlid fish species in the three reservoirs. For this purpose, gillnet selectivity of each mesh size was determined using Baranov-Holt method. Probabilities of capture of mesh-wise gillnet selection curves were then summed up giving weight to the contribution of each mesh size to the total number of net pieces in the sampled boat, to determine the overall gillnet selection from gillnets of all mesh sizes. The importance of the findings of the present study for length-based stock assessment methods and for imposing mesh regulations to the gillnet fisheries in reservoirs of Sri Lanka is discussed.Item Yield predictive models for Sri Lankan reservoir fisheries(Fisheries Management and Ecology, 2000) Nissanka, C.; Amarasinghe, U.S.; de Silva, S.S.Tropical reservoirs are primarily constructed for irrigation, generation of hydroelectricity and water supply schemes. Development of inland fisheries is a secondary use of most reservoirs. In Sri Lanka, most reservoirs are scattered in the rural areas of the country so that investigation of the fisheries of individual reservoirs with a view to developing management plans is prohibitive. The present study was instigated to explore the possibilities of developing suitable yield predictive models, which can be used in developing management strategies for the Sri Lankan reservoirs. The study was carried out in 11 perennial reservoirs of Sri Lanka. Basic limnological parameters (conductivity, dissolved phosphorus, total phosphorus, chlorophyll a [chl a] content and alkalinity) were determined in each of these reservoirs. Daily data on fish catch and fishing effort were collected in each reservoir. Data on catchment areas (CA), reservoir area (RA) and reservoir capacity (RC) were obtained from the irrigation and survey departments. It is evident that chl a is positively influenced by nutrients (dissolved phosphorus and total phosphorus), morphoedaphic indices derived as alkalinity to mean depth (MEIa) and conductivity to mean depth (MEIc) ratios and CA/RC ratios. MEIa and MEIc are also positively influenced by CA/RC ratios. All these morphological and edaphic parameters were found to positively influence fish yield in reservoirs. As fishing intensity (FI) is also a major determinant of fish yields, fish yield was better accounted by multiple regression models in which FI and individual morphological and edaphic parameters were used as independent variables. Of these multiple regression relationships, the best predictive power for fish yield (Y in kg ha?1 yr?1) was found by Y=18.9+6.78 FI+0.0073 CA/RC where FI is expressed as boat-days ha?1 yr?1 and CA and RC are in km2 and km3, respectively. In this relationship, FI and CA/RC account for about 68% of the variation in fish yield.Item Seasonality in Dietary Shifts in Size-Structured Freshwater Fish Assemblages in Three Reservoirs of Sri Lanka(Environmental Biology of Fishes, 2003) Weliange, W.S.; Amarasinghe, U.S.We studied seasonal changes in feeding habits of size-structured fish assemblages in three freshwater reservoirs in Sri Lanka. We obtained fish samples for diet analysis from three reservoirs during the months of rising water level (RIWL) and receding water level (REWL). During RIWL, peripheral areas with terrestrial/semi-terrestrial macrophytes get inundated, which resulted in increased food availability for macrophyte feeding fish. During REWL, detrital food sources increase due to decaying terrestrial plant material, which has inundated. Based on the dietary habits of individual species in the three reservoirs during RIWL and REWL, it is evident that detritivorous and phytoplanktivorous fish species, which belong to low trophic levels do not show significant variation in dietary habits between the two seasons (e.g. Oreochromis mossambicus, Oreochromis niloticus and Amblypharyngodon melettinus). Stenophagous species such as benthic invertebrate predators (Puntius chola and Puntius dorsalis), macrophytophagous species (Etroplus suratensis and Tilapia rendalli) and zooplanktivorous Hemirhamphus limbatus do not exhibit significant variation in dietary habits between seasons. Also their feeding habits do not vary with body size. Euryphagous species such as Puntius filamentosus on the other hand, exhibit variations in dietary habits between seasons as well as with body size. Stenophagous species, which feed on detritus and phytoplankton and euryphagous species, which exhibit temporal dietary plasticity, are therefore abundant in fish assemblages of these reservoirs.Item Some aspects of photosynthetic characteristics in a set of perennial irrigation reservoirs located in five river basins in Sri Lanka(Hydrobiologia, 2002) Silva, E.I.L.; Amarasinghe, U.S.; de Silva, S.S.; Nissanka, C.; Schiemer, F.Phytoplankton primary productivity of eleven irrigation reservoirs located in five river basins in Sri Lanka was determined on a single occasion together with light climate and nutrient concentrations. Although area-based gross primary productivity (1.43?11.65 g O2 m?2 d?1) falls within the range already established for tropical water bodies, net daily rate was negative in three water bodies. Light-saturated optimum rates were found in water bodies, with relatively high algal biomass, but photosynthetic efficiency or specific rates were higher in water bodies with low algal biomass, indicating nutrient limitation or physiological adaptation of phytoplankton. Concentrations of micronutrients and algal biomass in the reservoirs are largely altered by high flushing rate resulting from irrigation release. Underwater light climate and nutrient availability control the rate of photosynthesis and subsequent area-based primary production to a great extent. However, morpho-edephic index or euphotic algal biomass in the most productive stratum of the water column is not a good predictor of photosynthetic capacity or daily rate of primary production of these shallow tropical irrigation reservoirs.