Abstract:
The rainbow trout (Oncorhynchus mykiss) was originally introduced to Sri Lanka as
a sport fish in 1882, then it was called Salmo gairdneri and reported to be surviving
in few numbers only in the Horton Plaints, although, the regular stocking ceased in
1989. This study is concentrated into determine the status of the present population.
The study consisted of regular sampling period during April 1998 to March 1999
and occasional investigations in 2000 and 2004. The Rainbow trout, size ranging
from 1.0 to 39.0 em, mature population isjust about 1000 in the Horton plaints and
they exhibited size dependent distribution over different stream habitats. For
breeding they migrate to first order streams where fry and fingering can be found as
well. Spawning period is from late December to early March in a year when the
temperature was lowest. High egg mortality between 40% and 78% was observed
due to poor water quality. Average length of fish caught in each station indicated
downstream increase. Surface water temperature and pH had positive relationship
while dissolved oxygen content and total dissolved solid showed negative
relationship with downstream direction. The slit load in water is high in higher
order streams. The populations examined were devoid in headwater streams other
than those ofBelihul Oya.
