Zoology
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Item Estimation of maximum sustainable fish yield and stocking densities of fish fingerlings in fresh water lakes and reservoirs(University of Malaya, Kuala Lumpur, 1982) Wijeyaratne, M.J.S.; Amarasinghe, U.S.Item Growth-overfishing: a potential danger in Sri Lankan reservoir fishery(Reservoir Fishery Management and Development in Asia, 1988) Amarasinghe, U.S.This publication presents the results of an IDRC-funded workshop held in Kathmandu, Nepal, 23-28 November 1987. Representatives from 15 countries reviewed the status of reservoir fishery research in Asia under the following topics: existing fisheries, limnological aspects, biological and resource aspects, management aspects, and culture. Papers were presented on these topics, but the discussion sessions were the main element of the workshop. Summaries of these discussions as well as a series of general recommendations that were generated during the final discussion are presented in this book. The potential for increased fish production in reservoirs and the need for early involvement of fisheries scientists in the planning and preimpoundment studies before dam construction are emphasized.Item Status of the fishery of Pimbmrettewa wewa, a man-made lake in Sri Lanka(Aquaculture and Fisheries Management, 1987) Amarasinghe, U.S.The status of the fishery of Pimburctlewa wewu, a man-made lake in Sri Lanka, was studied from January 1985 to January 1986. The annual fish yield in the reservoir is 441 kg/ha, one of the highest in the south-east Asian region. Oreochromis mossambicus (Peters) is the dominant species in the fishery and accounted for about 90% by weight of the total catch. Although there were monthly variations in catch per unit effort, the daily catch per fisherman was not adversely affected. A possibility of having a more productive fishery is suggested In reducing The minimum permissible mesh size in the gillnet fishery during the peak recruitment season in May?June. The possible effects of reducing the mush size are discussed. Using the length-frequency data of O. mossambicus, the asymptotic length and the growth constant were estimated to be 39.3cm and 0.34 respectively. The estimates of total mortality (2.42). natural mortality (0?82) and mean selection length (24.6 cm) were based on these growth parameters Yield-per-recruit analysis indicated that the O. mossambictus fishery in the reservoir was optimally exploited.Item Assessment of fishing effort in Parakrama Samudra, an ancient man-made lake in Sri Lanka. Fisheries Research(Fisheries Research, 1986) Amarasinghe, U.S.; Pitcher, T.J.As in most artisanal fisheries, assessment of fishing effort is crucial to the management of the artisanal fishery of Parakrama Samudra, an ancient man-made tropical lake in Sri Lanka. Since there were few similarities between alternative ways of expressing effort for the gill-net cichlid fishery is the lake, this study aimed to find the most effective formulation of catch per unit effort (CPUE). The variability of preliminary estimates showed that, before analysis, the data needed to be stratified for different seasons with low and high water levels, and for three geographically distinct areas of the lake. Standardised catch per man was higher in boats operated by smaller numbers of fishermen, but catch per net did not change with the number of nets per boat, and is therefore recommended as the best measure of CPUE. The measure of fishing effort also needs to take account of fishermen who increased their catch by a water-beating technique. The incidence of beating varied with season and area, but could account for a significant portion of the catch. The highest catch efficiencies derived from one-man boats using this technique. Using this method of assessing effort, total annual yield from Parakrama Samudra was estimated at about 120 kg ha?1, a lower value than previously reported.Item Studies on the Exploitation of minor Cyprinids in Parakrama Samudra, A man-made lake in Sri Lanka, using Gillnets(Journal of National Aquatic Resources Agency Sri Lanka, 1985) Amarasinghe, U.S.Item Limnology and Fish production potential of some reservoirs in Anuradhapura District, Sri Lanka(The Ministry of Fisheries, Democratic Socialist Republic of Sri Lanka, 1983) Amarasinghe, U.S.; Costa, H.H.; Wijeyaratne, M.J.S.Item Estimation of maximum sustainable fish yield and stocking densities of fish fingerlings in fresh water lakes and reservoirs(Stuttgart : E. Schweizerbart'sche Verlagsbuchhandlung, 1964-, 1987) Wijeyaratne, M.J.S.; Amarasinghe, U.S.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.Item Some Factors Affecting Contribution of Cichlid Species Etroplus suratensis and Tilapia rendalli to the Gill-net Catches of a Man-made Lake in Sri Lanka(Asian Fisheries Society, 1988) Amarasinghe, U.S.; Samarakoon, J.I.In the gill-net fishery of Pakakrama Samudra, a man-made lake in Sri Lanka, some factors affecting species composition of the commercial landings were studied from February 1985 to February 1986, with particular reference to the cichlid species, Etroplus suratensis and Tilapia rendalli. The proportions on the catches of a water beating technique, a modified gill-net fishing method, were found to be significantly different from that of normal gill netting. These differences in the catches of E. surantensis and T. rendalli are suggested to be due to their restricted movements, associated with reproductive behavior. The increase of catches of two cichlid species with elevation of water level may have been brought about by their aggregation in recently inundated peripheral areas of the lake to feed on macrophytes. The importance of stratifying data according to fishing method and mesh size of gill net in order to determine species composition in discussed. Using this method of analysis, annual fish yield in Parakrama Samudra in 1985 was estimated to be about 46 kg?ha-1.Item Empirical Determination of a Desirable Mesh Size for the Gill net Fishery of Oreochromis mossambicus (Peters) in a Man-made Lake in Sri Lanka(Asian Fisheries Society, 1988) Amarasinghe, U.S.Experimental fishing with bottom-set gill nets of different mesh sizes was carried out in Parakrama Samudra, a man-made reservoir in Sri Lanka, May to September 1983. the optimum retention length of Oreochromis mossambicus, the dominant fish species in the reservoir fishery, was estimated for each mesh size using the Baranov-Holt method. The optimal length of O. mossambicus is a function of mesh size and is described by the equation: Y= 3.4763 + 2.0237X (r=0.992; p Natural mortality (M) was estimated to be 1.10 using L? = 31.3 cm and K = 0.48 year-1 estimated from length-frequency data of the catches. The long-term biological effects of the increase in mesh size in the gill-net fishery on the harvests and catch rates are discussed using length-structured yield-per-recruit and biomass-per-recruit analyses based on L? and M/K for different levels of exploitation rates and different sizes of first capture corresponding to various mesh sizes.