Browsing by Author "Nissanka, C."

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Effect of hydrological regimes on fish yields in Sri Lankan reservoirs
(Reservoir and Culture-based Fisheries: Biology and Management, 2001) Nissanka, C.; Amarasinghe, U.S.
The need for empirical models for predicting fish yields in lakes and reservoirs, both in tropical and temperate regions, has long been recognised because investigation of the fisheries of individual water bodies for management purposes is prohibitive. In a previous study, morphological and edaphic factors, including extents of catchment areas in reservoirs of Sri Lanka, were found to influence fish yields. Hydraulic retention time is reported to be another factor influencing fish yields in tropical reservoirs. This paper attempts to investigate the effect of hydrological regimes on fish yields in irrigation reservoirs of Sri Lanka. Daily catch and effort data were collected from 10 shallow irrigation reservoirs from December 1997 to September 1999. Nitrate, phosphate and chlorophyll-a content in each reservoir were determined once in two months. Hypsographic curves (i.e. area-water depth relationships) and monthly mean data on reservoir capacity, water level, reservoir area and total outflow volume were obtained from the Department of Irrigation. Flushing rate (outflow/reservoir capacity) had little influence on water nutrients, chlorophyll-a and fish yields in reservoirs. As irrigation authorities control the hydrological regimes of these reservoirs, strong co-ordination between fisheries and irrigation authorities is useful for augmenting fish yields in the reservoirs of Sri Lanka.
Evaluation of the robustness of predictive yield models based on catchment characteristics using GIS for reservoir fisheries in Sri Lanka
(Fisheries Management and Ecology, 2002) Amarasinghe, U.S.; de Silva, S.S.; Nissanka, C.
Land-use patterns in the catchment areas of Sri Lankan reservoirs, which were quantified using Geographical Information Systems (GIS), were used to develop quantitative models for yield prediction. The validity of these models was evaluated through the application to five reservoirs that were not used in the development of the models, and by comparing with the actual fish yield data of these reservoirs collected by an independent body. The robustness of the predictive models developed was tested by principal component analysis (PCA) on limnological characteristics, land-use patterns of the catchments and fish yields. The predicted fish yields in five Sri Lankan reservoirs, using the empirical models based on the ratios of forest cover and/or shrub cover to reservoir capacity or reservoir area were in close agreement with the observed fish yields. The scores of PCA ordination of productivity-related limnological parameters and those of land-use patterns were linearly related to fish yields. The relationship between the PCA scores of limnological characteristics and land-use types had the appropriate algebraic form, which substantiates the influence of the limnological factors and land-use types on reservoir fish yields. It is suggested that the relatively high predictive power of the models developed on the basis of GIS methodologies can be used for more accurate assessment of reservoir fisheries. The study supports the importance and the need for an integrated management strategy for the whole watershed to enhance fish yields.
Extent of the catchment of reservoirs of Sri Lanka: an important aspect in the management of their capture fisheries
(Asian Fisheries Forum, 1998) Nissanka, C.; Amarasinghe, U.S.; de Silva, S.S.
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.
Fisheries in perennial reservoirs of Sri Lanka: Strategies for management
(Sri Lanka Association for Fisheries and Aquatic Resources, 2008) Amarasinghe, U.S.; Nissanka, C.
The inland capture fishery in Sri Lanka is essentially based on the multitude of perennial reservoirs in the country. It was developed after the introduction of the exotic cichlid species, Oreochromis mossambicus into Sri Lankan freshwaters in 1952. O. niloticus introduced in 1975 is also well-established in the reservoirs of the island. These two cichlid species, which form over 90% of total landings in the perennial reservoirs of Sri Lanka, are sources of high quality, affordable sources of animal protein for rural communities. The other unique characteristics of the reservoir fishery of Sri Lanka include, (a) the use of non-mechanized fibreglass canoes, approximately 6 m in length manned by two persons; and (b) the exclusive use of gillnets as the fishing gear (6-20 net pieces per craft), range of mesh size of which is from 7.5 cm to 12.7 cm, and each net piece being of a standard length and height of 60 m and 1.5 m, respectively. Small sized cyprinids are also abundant in Sri Lankan reservoirs. They can be differentially exploited using small-mesh (15-52 mm stretched mesh size) gillnets without catching juvenile cichlids because juvenile O. mossambicus and O. niloticus are only found in shallow (<1.5 m in depth), littoral areas of reservoirs and their adult counterparts, which are targeted by the fisheries, are found in deeper waters. These small cyprinid species (i.e., Amblypharyngodon melettinus, Puntius chola, P. dorsalis and P. filamentosus) can be utilized to make fishmeal or dried fish. Resource-based approaches such as length-based fish stock assessment methodologies have been employed to determine the optimal fishing strategies in the perennial reservoirs of Sri Lanka. Empirical models have also been developed to estimate potential fish production based on catchment land-use characteristics, which can be quantified by GIS methodologies. However, these resource-oriented approaches are inadequate for effective management of fisheries in perennial reservoirs of Sri Lanka. For this purpose, it is imperative that fishing communities be made active partners in decision-making process. A co-management procedure in which the centralized fisheries administration authority and fishing community share the responsibilities of making decisions for fishery resource management should be introduced for effective management of the reservoir fishery of Sri Lanka
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.
Influence of catchment characteristics on fish yields in reservoirs of Sri Lanka
(Sri Lanka Association for Fisheries and Aquatic Resources, 1999) Nissanka, C.; de Silva, S.S.; Amarasinghe, U.S.; Wijesuriya, W.A.D.D.; Fernando, M.J.J.
Investigation of the Negombo estuary (Sri Lanka) brush park fishery, with an emphasis on community-based management
(Fisheries Management and Ecology, 2002) Amarasinghe, U.S.; Amarasinghe, M.D.; Nissanka, C.
The Negombo estuary brush park fishery in Sri Lanka was investigated from June 1998 to March 1999. The mean fish yield (all species) in the fishery was 12.46 t ha?1 yr?1. Fin fish species formed about 91% of the yield and the rest was formed by penaeid shrimps and crabs. Fish yield and the twig density in the brush park exhibited a second order polynomial relationship indicating a minimum yield at an intermediate value of twig density in the brush parks. The relationship between fish yield and duration of implantation of brush parks showed an optimal period of 30?40 days for high fish yields. Income levels of fishermen, availability of construction material and suitable sites for implantation of brush parks are some of the factors determining size and number of brush parks per fisherman and duration of installation. Indigenous knowledge within the fishing community about the effect of salinity variations, twig density and mangrove species used on the harvests greatly contribute to effective operation of this fishing practice. Cultivation of mangroves to obtain twigs and branches is a unique feature in this estuary. Indigenous knowledge on mangroves has led to the adoption of sound silvicultural practices. Although cultivating mono-specific mangrove stands may not increase diversity of mangrove forests, it reduces denudation of naturally occurring mangrove forests because of brush park construction, and retains habitats for other organisms. Mangrove management plans in the Negombo estuary should therefore be viewed and treated in an integrated manner that takes into account both resource and social components.
Land-Water Linkages in Rural Watersheds Electronic Workshop
(2002) de Silva, S.S.; Amarasinghe, U.S.; Nissanka, C.
Population dynamics of Penaeus indicus (Crustacea: Penaeidae) in Rekawa Lagoon, Sri Lanka
(Sri Lanka Association for Fisheries and Aquatic Resources, 1997) Nissanka, C.
Recruitment of Penaeus indicus (Crustacea : Penaeidae) to the fishery of Rekawa lagoon Sri Lanka: Is artificial enhancement possible?
(Sri Lanka Association for Fisheries and Aquatic Resources, 1996) Nissanka, C.; Amarasinghe, U.S.; Jayakody, D.S.
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.
Use of geographical information systems as a tool for predicting fish yield in tropical reservoirs: case study on Sri Lankan reservoirs
(Fisheries Management and Ecology, 2001) de Silva, S.S.; Amarasinghe, U.S.; Nissanka, C.; Wijesooriya, W.A.D.D.; Fernando, M.J.J.
Use of geographical information systems (GIS) in inland fisheries has hitherto been essentially restricted to site evaluation for aquaculture development and assessment of limnological changes in time and space in individual water bodies. The present GIS study was conducted on the land-use pattern of the catchments of nine reservoirs in Sri Lanka, for which detailed fishery data, viz. yield, fishing intensity, landing size of major constituent species, together with selected limnological data such as conductivity and chlorophyll-a, were available. Potential statistical relationships (linear, curvilinear, exponential and second-order polynomial) of fish yield (FY, in kg ha?1 yr?1) to different land-use patterns, such as forest cover (FC, in km2) and shrub-land (SL, in km2), either singly, or in combination, and/or the ratio of each land type to reservoir area (RA in km2) and reservoir capacity (RC in km3), were explored. Highly significant relationships were evident between FY to the ratio of SL and/or FC+SL to RA and/or RC. Similarly, the above land-use types to RA and RC ratios were significantly related to limnological features of the reservoirs. The relationships of FY to various parameters obtained in this study were much better correlated than those relationships of FY to limnological and biological parameters used in yield prediction in tropical and temperate lacustrine waters previously.
Use of geographical system as a tool for fish yield prediction in tropical reservoirs : a case study on Sri Lankan reservoirs
(University of Kelaniya, 2000) Nissanka, C.; Amarasinghe, U.S.; de Silva, S.S.
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.