The effect of size-grading juvenile Macrobrachium rosenbergii prior to stocking on their population structure and production in polyculture : II. Dividing the population into three fractions

Abstract

Juvenile prawns (ca. 1.1 g) were size-graded into upper (32%), middle (45%) and lower fractions (23%). The graded fractions and the graded and recombined control were stocked into 400-m 2 earthen ponds, at a density of 2/m 2. The prawns were grown for 97 days in polyculture with tilapia, common carp, silver carp and grass carp. The proportion of females was higher than that of males in all of the groups except the lower fraction in which males predominated. The fractions differed in the proportions of the male and female morphotypes. The upper fraction had a low proportion (8%) of small males (SM) and a high proportion (22%) of blue claw males (BC). The lower fraction showed the reverse trend, with a high proportion (50%) of SM and a low proportion (3%) of BC. The middle fraction was intermediate and similar to the control. The proportions of mature females in the upper and lower fractions were 63 and 6% respectively. The middle fraction was intermediate (21%) but lower than the control group (45%). Survival in the grow-out phase was ca. 88% in all the groups. The three graded groups differed significantly in mean weight, with the largest prawns occurring in the upper fraction. Since the price of prawns is size-dependent, the net income from the upper fraction was almost nine times that of the lower fraction. The weighted mean income from the three fractions did not differ from that of the control. The large differences in yield and net income among the graded fractions, as well as the lack of increase in the net income compared with the control, are explained by early male morphotype determination.