Science

Permanent URI for this communityhttp://repository.kln.ac.lk/handle/123456789/1

Browse

Author: search.filters.author.de Silva, S.S.Has files: No

Search Results

Now showing 1 - 10 of 29
  • No Thumbnail Available
    Item
    A rationale for introducing a subsidiary fishery in tropical reservoirs and lakes to augment inland fish production: case study from Sri Lanka
    (2016) Amarasinghe, U.S.; Kumara, P.D.A.; de Silva, S.S.
    An extra 30–40 million tonnes of food fish will be required by 2050 to meet the increasing need for a growing population. In the wake of plateauing of the traditional food fish supplies from marine capture fisheries, the gap in the supplies will have to be met from aquaculture and other plau- sible strategies, including increased inland fish production. The existing fisheries in tropical reservoirs and lakes tradition- ally tend to target only table-sized fish, often exotics and/or translocated species, using a single type of gear, mostly gill nets. In such fisheries, many small indigenous species (SIS) are unexploited due to the prohibition of the use of suitable fishing gear. The status of fisheries for small, indigenous species of four Sri Lankan reservoirs was investigated with a view to identifying regulatory con- straints to the establishment of SIS fisheries without adversely impacting existing commercial fisheries. It is estimated that a potential of 7.5 t per fisher per annum of SIS through the introduction of a new fishery is obtainable. This study is significant as SIS are recog- nized as important sources of essential macro- and micronutrients which can play a crucial role in combating malnutrition and micronutrient deficiencies in rural popula- tions of many South and Southeast Asian countries.
  • No Thumbnail Available
    Item
    A comparative assessment of the fisheries in lacustrine inland waters in three Asian countries based on catch and effort data
    (Fisheries Research, 1991) de Silva, S.S.; Moreau, J.; Amarasinghe, U.S.; Chookajorn, T.; Guerrero, R.D.
    The catch and effort data of the artisanal fisheries of the natural lakes and reservoirs in the Philippines, and reservoirs in Sri Lanka and Thailand are evaluated. The yield (kg ha?1 year?1) to effort relationship of the Philippines lakes and Thai reservoirs conformed to a second-order curve, and that of Sri Lankan reservoirs was linear. Similarly, the catch per unit effort (CPUE) to effort per unit area relationships for the Philippines and Thai fisheries were similar and conformed to the Fox model. The relationship of the mean annual yield (kg ha?1) to mean effort in the Sri Lankan reservoirs when considered as a whole, on a yearly basis, conformed to the Schaefer model. On the basis of these relationships the effort which gave the maximum yield per annum was computed for each fishery. The yields were 515, 256 and 93 kg ha?1 for efforts of 20, 6.4 and 10 fishermen for the fisheries of the Philippines, Sri Lanka and Thailand, respectively. An attempt is made to explain the reasons for the apparent differences in these three artisanal fisheries, and the importance of the models in the management of the respective fisheries are discussed. 1 Present address: Deakin University, Warrnambool Faculty of Applied Science and Environmental Technology, P.O. Box 423, Warrnambool 3280, Australia.
  • No Thumbnail Available
    Item
    Use of GIS tools to develop a scale for selection of non-perennial reservoirs for culture-based fisheries activities
    (GIS/Spacial Analysis in Fishery and Aquatic Sciences, 2004) de Silva, S.S.; Wijenayake, W.M.H.K.; Gunaratne, A.B.A.K.; Amarasinghe, U.S.
  • No Thumbnail Available
    Item
    Fish yield prediction based on catchment features , quantified using Geographical Information Systems, in lowland reservoirs of Sri Lanka
    (GIS/Spacial Analysis in Fishery and Aquatic Sciences, 2004) Amarasinghe, U.S.; de Silva, S.S.; Nissanka, C.
  • No Thumbnail Available
    Item
  • No Thumbnail Available
    Item
    Fluctuations in Water Level in Shallow Irrigation Reservoirs: Implications for Fish Yield Estimates and Fisheries Management
    (Reservoir and Culture-based Fisheries: Biology and Management, 2000) Amarasinghe, U.S.; de Silva, S.S.; Nissanka, C.
    Due to fluctuations in water level, reservoir surface area changes considerably. Despite this effect, fish yields (Y) and fishing intensities (FI) in these reservoirs are often estimated for the reservoir area at full supply level (FSL). This paper compares the estimate of optimal fishing strategies according to this conventional method with those based on Y and FI calculated for actual mean monthly reservoir area. Catch and effort data, collected at least for 20 days a month from 10 individual reservoirs in Sri Lanka, were analysed to estimate mean annual fish yields (kg/ha/yr) and total FI (boat-days/ha/yr). Reservoir areas at FSL were used to estimate these values. Using the hypsographic curves and mean monthly water levels in individual reservoirs, actual mean reservoir area in each month in each reservoir was determined. Annual fish yields (kg/ha/yr) and total FI (boat-days/ha/yr) in individual reservoirs were then estimated, based on these actual reservoir areas. In both estimates, Y was linearly related to FI, indicating that the fish stocks were perhaps exploited at suboptimal levels. The results appear to indicate that the conventional method of using reservoir area at FSL to estimate Y and FI in multi-purpose reservoirs, instead of actual reservoir area, may have serious implications for fisheries management. An alternative method for estimating fish yields and FI is suggested for reservoirs with heavy draw-down.
  • No Thumbnail Available
    Item
    Population Dynamics of Oreochromis mossambicus and O. niloticus (Cichlidae) in Two Reservoirs in Sri Lanka
    (Asian Fisheries Society, 1992) Amarasinghe, U.S.; de Silva, S.S.
    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.
  • No Thumbnail Available
    Item
    Spatial Changes in Growth and Mortality and Effects on the Fishery of Oreochromis mossambicus (Pisces, Cichlidae) in a Man-made Lake in Sri Lanka
    (Asian Fisheries Society, 1989) Amarasinghe, U.S.; de Silva, S.S.; Moreau, J.
    Growth and mortality were estimated from the length-frequency data of Oreochromis mossambicus from the gillnet fishery of Parakrama Samudra, a man-made lake in Sri Lanka, separately for its three basins. The asymptotic length (L?) and the growth constant (K) of O. mossambicus in the northern basin of the lake (34.9 cm and 0.3, respectively) are appreciably different from those in other tow basins (middle basin, L? = 38.8 cm and K = 0.24; southern basins are 2.22, 1.64 and 1.07, respectively. The spatial differences in growth are suggested to be due to different fishing pressure and environmental conditions. Yield-per-recruit analysis indicate that in the populations with low L? and high K, exploitation level should be maintained at a low level. Fish yield could be optimized by increasing the size of first capture and exploitation rate in the population with high L? and low K.
  • No Thumbnail Available
    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.
  • No Thumbnail Available
    Item
    Selectivity patterns in the Multi-mesh gillnet fishery for Cichlid Species in a Sri Lankan Reservoir
    (The Third Asian Fisheries Forum, Asian Fisheries Society, Manila, Philippines, 1994) Amarasinghe, U.S.; de Silva, S.S.