Browsing by Author "Nadarajah, S."

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Effect of Hydrological regimes on Fish yield in Reservoirs of Kala Oya River basin, Sri Lanka
(Sri Lanka Association for Fisheries and Aquatic Resources, 2015) Nadarajah, S.; Amarasinghe, U.S.; Wijenayake, W.M.H.K.
Biological productivity of reservoirs is known to be influenced by abiotic factors such as reservoir morphology and hydrological regimes. Seasonal water level fluctuations in reservoirs influence physical, chemical and biological features in reservoirs and thus exert impacts on water quality. It is also a fact that such physio-chemical, biological parameters and the morpho-edaphic factors influence fish yield. As such, the present study was conducted to investigate the influence of reservoir hydrological regimes on fish yield. Water quality parameters were measured in 8 irrigation reservoirs of Kala Oya river basin from June 2013 to December 2014. The hydrological data of reservoirs were gleaned from the Department of Irrigation and Mahaweli Authority. Fish yield data were collected from the fishermen’s log books. Several empirical relationships were derived between abiotic and biotic variables such as total phosphorus, dissolved phosphorus, seston weight, organic weight and chlorophyll-a content against flushing rate (annual outflow/mean reservoir capacity). Also, conductivity, alkalinity, morpho-edaphic indices (MEI) defined as MEIa (alkalinity/mean depth), and MEIc (conductivity/mean depth) had significant positive influences on reservoir fish yield. Conductivity and alkalinity are more significantly related to than fish yield MEIs, possibly due to less variability of mean depth of reservoirs Dissolved phosphorus and seston weight showed strong positive correlation with flushing rate (p<0.05). However, the relationships of total phosphorus and organic weight with flushing rate were not significant at 5% probability level. The flushing rate had a significant second order influence on chlorophyll-a, which perhaps suggests that nutrient enrichment in reservoirs through release of phosphate from the sediments had a greater influence on biological productivity at intermediate levels of flushing rate. There appears to be a negative influence of flushing rate on reservoir fish yield and as such, it can be concluded that through manipulation of hydrological regimes in irrigation reservoirs, fish yields can be optimized. An effective dialogue between irrigation and fisheries authorities is therefore needed for reservoir fisheries management.
Impact of irrigation on reservoir fisheries: Antagonistic or supportive?
(Faculty of Science, University of Kelaniya, Sri Lanka, 2016) Nadarajah, S.; Amarasinghe, U.S.; Wijenayake, W.M.H.K.
Although reservoirs represent lacustrine ecosystems, they are drastically different from natural lakes for the main reason of prominence of anthropogenic perturbations, which affect ecosystem functioning. Many reservoirs in Sri Lanka are primarily used for irrigation of agricultural lands and are secondarily utilized for inland fisheries production. Effects of hydrological regimes on the biological productivity in reservoirs are therefore expected to have a significant bearing on the fish yield. Aim of this study is to investigate the influence of hydrological regimes on fish yield. In the present study, twelve limnological characteristics of 10 irrigation reservoirs in the Kala Oya river basin of Sri Lanka were investigated from June 2013 to February 2016. Fish yield data in these reservoirs were obtained from the log-book records of fisheries societies. Hydrological data were obtained from the irrigation authorities. Relative water level fluctuation (RWLF), defined as the ratio of amplitude of mean water level fluctuation to mean depth, showed a third order polynomial relationship with chlorophyll-a content (Chl-a) according to, Chl-a = -2.4046 RWLF 3 + 18.732 RWLF 2 - 39.487 RWLF + 44.052 (R² = 0.4351). There was also significant third order polynomial relationship of Chl-a, with fish yield (FY) as: FY = -0.047 Chl-a 3 + 2.689 Chl-a 2 - 45.186 Chl-a + 328.54 (R² = 0.410). As some of the reservoirs in the Kala Oya river basin are terminal reservoirs (e.g., Rajanganaya, Angamuwa, Siyambalangamuwa) from which water is released mainly for irrigation while some are, in addition to irrigation water supply, feeder reservoirs of several downstream reservoirs (e.g., Ibbankatuwa, Kalawewa), effect of hydrological regimes on the biological productivity were possibly represented by polynomial relationships. There was a positive linear relationship between RWLF and FY according to the equation, FY = 27.222 RWLF + 66.729 (R² = 0.586). This indicates that through manipulation of hydrological regimes, reservoir fish yield can be optimized. As such, an effective dialogue should be in place between irrigation authorities responsible for controlling hydrological regimes and authorities responsible for inland fisheries development.
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.