POPULATION CHARACTERISTICS AND EXPLOITATION OF THE IMPORTANT MARINE PRAWNS OF AMBALAPUZHA, KERALA

The prawn fishery of Ambalapuzha is carried out by trawl nets from October to Nfay, and by the indigenous gear from June to September when the extensive mud-bank formations occur in this region. The fishing ground extends from 4 to 40 metres depth zone, the shallow water (4 to 10 metres) zone being fished by the non-mechanised boats and the 10-40 metres depth zone by the mechanised vessels. The fishery is supported by three species of penaeid prawns, namely, Metapenaeus dobsoni (Miers), Parapenaeopsis stylifera (H. Milne Edwards) and Penaeus indicus H. Milne Edwards. The population of all these species is predominantly composed of females, although in M. dobsoni and P. stylifera males are found in higlwr proportion than females in the lower size groups. The preponderance of females in the population and the disparity in the sex ratios in different size groups have been attributed to the reproductive activities and differential growth rate of the sexes. The males and females of M. dobsoni, P. stylifera wid P. indicus respectively attain a length of 97|115; 91|98 and 158|138 mm at the end of first year of life and 122|138; 117|123 and 189|181 mm at the end of second year, and each of the species lives for about 2 years. Highest difference in the growth rate between the sexes is observed in P. indicus and the lowest in P. stylifera. The bulk of the commercial fishery of M. dobsoni and P. stylifera is supported by the 7-12 month old prawns and that of P. indicus by the 7-18 month old prawns. Majority of the spawning population in the foomer two species is composed of O year old females and in the latter species by the one-year old females. The estimated annual total mortality rate in the males of M. dobsoni and P. stylifera is found to be about 4. while in the females of these species as well as in the males of P. indicus it is about 3. The lowest mortality rate is obtained for the females of P. indicus.