Abstract:
This study was conducted over a five-month period at Rawan Oya, a 13.5km long
left bank tributary of the River Mahaweli. Data on species composition and
mircohabitat conditions were collected from 30 random sampling sites, representing
10 pools, 10 riffles and 10 runs. A single observation was made at each site, where a
10m-length of the stream was blocked with drag nets to get the values for abundance.
Fishes were captured using the cast nets and scoop nets, identified and released
back at the site of capture. The species richness, relative abundance and the
microhabitat conditions were recorded for each habitat type. Physico-chemical
parameters of water such as the stream velocity, temperature, pH, water depth,
conductivity, Hardness, DO and BOD5
were determined monthly at pre-selected
sampling sites. A questionnaire survey was done among people in adjacent villagers,
who are selected randomly. A total of 15 fish species belonging to 8 families were
recorded of which 2 are endemic and 4 are exotic. The subsequent data analysis
revealed that the Shannon Wiener diversity index and the Magalef Index are
significantly high in pools. ANOVA test showed significant variation among the 3
habitat types in terms of the microhabitat features measured. Further, the Pearson
Correlation test demonstrated that the different fish species have different
microhabitat preferences and different habitat associations. Runs seemed to be the
most suitable habitats for most of the fish species lotic habitat with the highest
species richness compared with the other two habitat types (12 species). Riffles
harbored the poorest in species composition (2 species). Pools had an intermediate
number of species (9). Puntius bimaculatus, Hypostomus sp and Tor khudree only
occurred in pools while Devario malabaricus, Glossogobius giuris,
Lepidocephalichthys thermalis, Mystus vittatus, Rasbora daniconius and
Xiphophorus maculatus are confined to runs of the stream with no species confined
to riffles. Garra ceylonensis and Schistura notostigma occupy all three kinds of
habitat types but with higher abundances in riffles. Among the fish found only in
pools and runs, Puntius filamentosus were recorded in significantly higher numbers
in pools than that in runs in contrast to the alien species such as Poecilia reticulata
and Oreochromis mossambicus that showed a higher preference for runs.
