Effect of selected water quality parameters on the prevalence of  Poecilia reticulata (Guppy) population in  Sri Jayewardenepura canal system

Present study was carried out to determine the effect of some selected water quality parameters on the prevalence of P. reticulata (Guppy) population in Sri Jayewardenepura canal system. Fish and water samples were collected in six locations from January 2016 to December 2017. Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD), Electrical Conductivity (EC), Hardness, Alkalinity, Nitrate- Nitrogen (N-NO₃^-), Orthophosphate (PO₄^3-), Temperature, pH, Water velocity and depths in each sites were measured. Total lengths and weights of specimens (n-520) were measured and relative densities (RD) were calculated. Guppy was the most abundant species in all sites and high densities were recorded at slow moving and stagnated shallow waters. Results revealed that all the sites were subjected to moderate or high pollution and most conspicuous were the low DO (0.83±0.12-4.84±0.76 mg/l) and high BOD (1.78±0.4-9.10±0.6 mg/l) levels. Polluted water quality of studied locations were further confirmed by high values for (N-NO₃^-) (3.9±0.8-11.5±1.2 mg/l), PO₄^3- (1.9±0.4-3.9±0.3mg/l) and EC (257.6±12.8-801.6±42.0μS/cm). Other recorded water quality parameters pH (6.6±0.2–8.3±0.4), Temperature (29.9±1.1-8.3±0.4), Hardness (80.0±10.4-113.3±13.1 mg/l) and Alkalinity (70.7±11.5-179.6±20.4 mg/l) are in suitable range of fish and aquatic life. Despite the fact that the locations were polluted, RD of guppy (32.7±27.7-85.7±22.9) were considerable in all locations. The study showed that the RD of guppy significantly varied with some of the water quality parameters; decreasing with increasing DO, N-NO₃^-, PO₄^3- levels (P≤0.05) as well as with water depths and velocity of water and increasing with increasing pH, BOD and alkalinity of water. The study recorded that the average adult sizes of fish were smaller (♀-35 mm, ♂-25 mm) than the previous records (♀-60 mm, ♂- 40 mm) in the present study area. Thus, study concludes that, although the water quality parameters are affecting their growth rates and relative densities.