Effects of stocking different fractions of size graded juvenile prawns on production and population structure during a temperature-limited growout period

Size grading of juvenile prawns prior to pond stocking is used to disrupt negative social interactions. Animals graded off the upper end of the size range can outperform ungraded animals by 20-50%. However, reports differ on the performance of the lower grade fraction. Some studies indicate that lower grade prawns can out perform ungraded populations within a 120-140-day growing season while other studies indicate that >140 growing days are required. The objective of this study was to compare the growth of this lower grade fraction with the upper grade fraction and an ungraded fraction of the same population within a temperature-constrained growing season of 100-120 days. Prawn juveniles that had been nursed 60 days were separated into three groups. Approximately 50% of the population was maintained ungraded with the full range of size variation. The other 50% was separated into two fractions (large = upper grade; small = lower grade) using a #13 bar grader (0.5-cm spacing). Ponds were randomly assigned to receive one of the three treatments (ungraded controls, upper grade, and lower grade) with average stocking weights of 0.5 +/- 0.3, 0.8 +/- 0.4, and 0.2 +/- 0.1 g ((x) over bar +/- S.D.), respectively. There were three replicate 0.04-ha ponds per treatment. All ponds were stocked at 59,280 ha(-1). Artificial substrate was added to ponds at a rate to increase available surface area by 50%. After 104 days, there was no significant difference (P>0.05) in survival of prawns from the three fractions (overall average 88%). Total production, marketable production (>20, >30 g) and average individual weight was significantly greater (P<0.05) in prawns from the upper grade fraction. At harvest, there was no significant difference (P>0.05) in total production or average individual weight of prawns stocked from the ungraded or lower graded fractions. Impacts of grading procedures on population structures were much more pronounced in females than in males with upper graded animals having a significantly higher (Pless than or equal to0.05) percentage of sexually mature reproductive females. Compared to ungraded juveniles, projected net returns were lower for lower graded prawns, higher for upper graded, and higher for combined lower and upper under all model farm scenarios. In summary, stocking of the upper graded fraction increased total production, average weights, and marketable production. There was no difference in lower graded and ungraded animal production variables. (C) 2004 Elsevier B.V. All rights reserved.