Abstract:
River Nilwala located in the Southern part of Sri Lanka, with a catchment area of
960 km', consists of a complex tributary system draining through human
settlements and natural forest areas. Considering the enormous natural and
anthropogenic effects on these tributaries, the fish fauna of selected tributaries
representing different altitudes and habitat types along the river course from
Ahalakanda to Malimbada, was studied from September 1999 to May 2000 at eight
selected sites.
Physico-chemical such as temperature, conductivity, DO, BOD, pH, salinity, water
depth, flow rate, NO3 and PO4-3 concentrations were determined monthly. The
percentage composition of the substrate type of each site was determined. A
checklist of the fish fauna was prepared. The relationships between the species
distribution to water quality parameters and the substrate type have been
established.
A total of 42 fish species were recorded from all sites. Out of these, 4 species
namely, Amblypharyngodon melettinus, Esomus thermoicos, Eleotris fusca,
Pseudosphromenus cupanus were not reported from Nilwala basin in previous
studies. The sites located on upper tributaries were of high standard water quality
and were associated with substrates consisted of boulders, pebbles, gravel (> 60%)
and both these criteria were highly correlated to the species composition. Changing
water quality of intermediate sites did not show any significant correlation with
species composition but associated with specific substrates. As river flows to
lowland areas the substrate is replaced by silt. Immediate changes of the water
quality from upper tributaries to lowland areas were reflected by the downstream
species composition, which is different from that of upland tributaries.
