Status of Rainbow Trout ( Oncorhynchus mykiss-Waldaum 1792) Population after the Cessation of Stocking in Waterways of Horton Plains, Sri Lanka

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.