Reasons for the Sudden Fish kills in Uyanwewa Reservoir, Sri Lanka: A Remote Sensing Approach

Abstract

Sudden large-scale fish kill incidednts have been reported in Uyanwewa reservoir located in Hakmana area in Sri Lanka during last few years and the last fish kill incident was recorded on the 17th July 2020 and the reason for sush events have not been revealed yet. According to fishermen, the majority of the reported dead fish were bottom dwellers including Cirrhinus mrigala. This reservoir has not been subjected to any previous scientific study and therefore, the baseline information is also lacking. The main objective of this study was to investigate the possible reason for the sudden fish kill in 2020 incident in Uyan wewa reservoir. Sampling was carried out on 25th January, 09th and 25th February 2021. Water quality parameters; Nitrate, Total Phosphate, Dissolved Oxygen (DO), Total suspended solid (TSS) and temperature were measured in water samples collected from the surface, middle, and bottom layers of the water and Chlorophil-a (Chl-a) was determined in 12 surface water samples collected using a Rutner sampler. Landsat 8 satellite data were retrieved for the dates of sampling as well as the date nearest to that of the past fish kill (14th July2020). Chl-a is an indicator of the mass of phytoplankton and is a reference for estimating the eutrophication in an aquatic ecosystem. The past Chl-a concentrations of the reservoir were determined by using the regression model, which was developed using the band ratio and Chl-a concentrations in 2021, using ArcGIS 10.6.1. For the development of the Chl-a retrieval algorithm, the most suitable band ratio was received for (B3+B4)/B5 with an R2 value of 0.7277 (p = 0.000). Results indicated that during the last sudden fish-kill of Uyanwewa in 2020, the mean Chl-a concentration was 5.64±0.86 μgL-1 which is quite higher than the values obtained in sampling days (4.13±2.25, 5.13±1.28, 4.65±0.72 μgL-1). Based on TSI (Chla) = 41.71), the reservoir was in mesotrophic condition on the 14th July 2020. One-way ANOVA was used to compare the water quality data between the layers at the sampling dates. Mean water temperature and DO levels showed a significant difference between the water layers and the mean DO concentration was lowest in the bottom layer (3.05±0.46 mg/L). Mean Chl-a (4.13±2.25, 5.13±1.28, 4.65±0.72 μg/L) and phosphate results (0.044±0.03, 0.058±0.03, 0.064±0.03 mg/L) of Uyanwewa were characteristic of being in the mesotrophic and eutrophic range respectively according to Nürnberg, 1996. DO levels in the middle and bottom layers (4.48±0.46, 3.05±0.46 mg/L) also indicated that the algal bloom formation may be a probable cause of fish kills in Uyanwewa reservoir through the depletion of DO concentration in middle and bottom layers.