Ambligaster sirm is one of the important food fish resources and this is the dominant fish found in Sri Lankan coastal small pelagic fish landings. In the present study, a total of 691 Individuals of A. sirm obtained during the period from February to December, 2010 at the Chilaw fish landing center in the west coast of Sri Lanka, were analyzed to study the reproductive biology of A. sirm.
Fish were dissected open and the sex and maturity stage of the gonads were determined. Gonadosomatic Index (GSI) was used to determine the spawning season(s) Length- Weight relationships separately for males and females were also obtained. The observed average sex ratio within the study period was found to be significantly different from 1:1.
The estimated length-weight relationship for males was W=0.002685L3.438 (R2= 0.945), whereas the length-weight relationship for females was W=0.003L 3.386 (R2= 0.940). Maturity stages of gonads were basically divided into five categories based on macroscopic observations: virgin, developing, developed, gravid and spent. The estimated average GSI for each stage of females were 0.8593, 2.1095, 4.1606, 5.7694 and 2.7610 while the respective values for males were 0.6701, 1.3160, 3.6157, 5.5654 and 0.6552. The peaks of GSI values concluded the spawning season of A. sirm belongs to the period from May to July.