The effects of size grading and length of nursery period on growth and population structure of freshwater prawns stocked in temperate zone ponds with added substrates

In temperate regions, the relatively short growing season increases the need for development of technologies to maximize growth and production rates for freshwater prawns. Studies have shown that new technologies, such as adding artificial substrates, increasing feeding rates, using advanced nursed juveniles, and grading of juveniles prior to stocking, can increase production without sacrificing average weights. While these technologies have been evaluated in isolation, it may be possible to further increase production by combining these technologies. This study evaluated the impacts of size grading of nursed juveniles prior to pond stocking and the effects of extending the nursery period (from 61 to 133 days) when combined with added substrate in temperate ponds. Three treatments were evaluated: 61-day ungraded juveniles (current regional technology; controls), 61-day graded juveniles, and 133-day graded juveniles. Individual mean stocking weights for the three treatments were 0.67, 0.72, and 3.1 g, respectively. All ponds were stocked at 59,280 ha(-1) and were provided with artificial substrate in the form of polyethylene "safety fence" oriented vertically at a rate sufficient to increase available surface area in the ponds by 50%. Compared to control ponds (61-day ungraded), grading of 61-day juveniles significantly increased (P < 0.05) total production (2549 and 2945 kg ha(-1), respectively), production/size index (PSI), and daily yield. Extending the nursery period (and thereby increasing the average stocking weight) did not significantly improve (P > 0.05) total production or average individual weight. Feed conversion ratios were significantly higher for 133-day juveniles compared to controls (2.8 vs. 2.3). The failure of animals weighing over 350% more at stocking to increase average weights or total production at harvest appears to be due to earlier sexual maturation and the resulting cessation of growth. (C) 2003 Elsevier Science B.V. All rights reserved.