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

Abstract

Juvenile prawns were size-graded into an upper (one third of the population) and a lower (two thirds) fraction prior to stocking at a density of 2 per m 2 into earthen ponds for a grow-out period of 105 days. Three graded and recombined control groups were included in this study. One control was of the same origin as the graded groups. The other two control groups consisted of larger and smaller prawns, similar in size to the graded upper and lower fractions. The effects of grading on morphotype differentiation, population structure and yield characteristics were analysed. At the end of the trial, the proportion of females was slightly higher than that of males in all of the groups. The males in the graded upper fraction consisted almost entirely of orange and blue claw males. This group had a 25% greater mean weight and yield than the controls. The graded lower fraction and the controls had a similar male morphotype composition consisting of 35–40% small males and approximately 60% orange and blue claw males. The graded lower fraction had a 10% lower mean weight and yield than the control. The three control groups were similar in mean weight and yield. Yields of the weighted combined upper and lower fractions and the controls were similar. The results of this study indicate that the relative position of juvenile prawns within the population is more important than absolute size in determining further growth rate and yield.