11 Introduction The stock of Macrobrachium rosenbergii (de Man), presumably ap- pearing from the adjoining rivers, con- stitutes a very lucrative seasonal fishery in Vembanad Lake (South India). The differential migration of male and female populations of this species into the lake has already been reported (Raman, 1967; Kurup et al., 1992), while its population structure and morphotypic composition were also delineated (Harikrishnan and Kurup, 1997). However, no attempt has so far been made to analyse the morphotypic composition of male and fe- male populations appearing at different Indian J. Fish., 50(1) : 11-19, Jan.-Mar., 2003 regions of the lake and also to bring out the temporal changes that the stock un- dergoes commensurating with transfor- mation and developmental pathway. The present account encompasses details of temporal and spatial variations in the availability of male and female morphotypes and their transitional stages in the lake. Materials and methods. The exploited population of M.rosenbergii from 13 zones of Vembanad Lake and confluent rivers (Fig.1.) was examined from March 94 to February 96, during monthly fishery survey cruises undertaken with the help of M.B. King- Temporal and spatial distribution of male and female morphotypes of Macrobrachium rosenbergii (de Man) in Vembanad Lake M. HARIKRISHNAN AND B. MADHUSOODANA KURUP School of Industrial Fisheries, Cochin University of Science and Technology, Cochin 682 016, India ABSTRACT The exploited stock of Macrobrachium rosenbergii in Vembanad Lake were examined thoroughly during 1994-‘96 period in order to find out temporal and spatial variations, if any, in the availability of male and female morphotypic forms. Blue clawed morphotypes of both sexes outnumbered the orange clawed ones during both years, while small males and small females constituted only an insignificant proportion during 1994-‘95 (0.13 and 0.15% respectively). With regard to temporal availability, a glaring dominance of orange clawed morphotypes during January to August and a subsequent predominance of blue clawed morphotypes from September to December were also encountered. In general, the predominance of blue clawed morphotypes were quite discern- ible in all zones, however, orange clawed morphotypes of both sexes could also be observed in significant percentages in upstream and riverine areas in con- trast to downstream areas.