Abstract:
Poecilia reticulata (Guppy) was first introduced in the 1930’s from Central America to dry zone
and Western province of Sri Lanka as a bio control agent for mosquito larvae and since has
widely distributed across North Western, Western and Southern provinces, living in a variety of
aquatic habitats such as streams, marshes, paddy fields as well as ditches in urban areas and has
an ability to survive in polluted water and tolerate extreme environmental fluctuations. This
species is the one of the most abundant in the Sri Jayewardenepura canal system. The present
study was carried out to investigate the ecological factors affecting the distribution and
abundance of P. reticulata population in Sri Jayewardenepura canal system. Fish and water
sampling were carried out from 6 sampling sites once a month, (from January to December
2016). Water quality parameters investigated were pH, Dissolved Oxygen (DO), Biochemical
Oxygen Demand (BOD), electrical conductivity, temperature, hardness, alkalinity, nitrate and
phosphate. Water flow rates and water depths of all sampling sites were also recorded. P.
reticulata was found to be the most abundant species in all sites, higher populations occupying
slow moving shallow water. Environmental parameters observed revealed the fact that all the
sites have been subjected to moderate or high pollution and most conspicuous were the DO and
BOD levels. Mean DO values ranged between (0.83-2.33 mg/l) which were lower than the
recommended level for aquatic life (minimum of 3 mg/l) in all sites, which have reached levels
that are potentially harmful for fish. Similarly, a maximum BOD value of 4mg/l is recommended
as the suitable level for aquatic life but the recorded values at selected sites were considerably
high (3.86-8.32 mg/l) indicating organic pollution and an abundance of oxygen-requiring microorganisms
in the water. Polluted water quality of sites were further confirmed by high values for
Nitrate (3.85-10.455 mg/l), Phosphate (1.90-3.19 mg/l), Electrical Conductivity (224-892
μs/cm), Hardness (80.0-127.92 mg/l) and Alkalinity (70.67-179.17 mg/l). However, there was no
effect of polluted water quality on the distribution and abundance of P. reticulata as all sites
recorded high densities of (125.0±35.7/m2). However there was a marked difference in the adult
sizes of fish, where both the adult males and females were smaller (Female-35 mm, Male-25
mm) than previous records (Female-60 mm, Male-40 mm) for the study area. This may be an
indication that these adverse environmental conditions are affecting their growth rates.
