Effects of flooding dates and disposals of rice straw on crayfish, Procambarus clarkii (Girard), culture in rice fields

Abstract

To assess the effects of flooding dates and different disposals of rice straw on crayfish culture in rice fields, 18 earthen ponds were randomly assigned to six treatments in a 2 × 3 factorial arrangement; two flooding dates: early flooding (20 September 1978) and late flooding (15 October 1978), by three types of rice straw disposals: aerobic (baled), anaerobic (disked), partially aerobic (standing) decomposition of rice straw. The average crayfish production in early-flooded ponds (1183 kg/ha) did not differ significantly from that in late-flooded ponds (1127 kg/ha). Significant differences ( P < 0.05) in average crayfish production were found between standing ponds (1506 kg/ha) and disked ponds (803 kg/ha), and between baled ponds (1157 kg/ha) and disked ponds. The average crayfish production in standing ponds was not significantly higher than in baled ponds. Von Bertalanffy's growth model revealed that all crayfish in early-flooed ponds attained an average maximum length ( l ∞) and Brody's growth coefficient ( k) of 92.2 mm and 0.055, respectively, whereas all crayfish in late-flooded ponds had an l ∞ and k of 82.7 mm and 0.090. Crayfish in disked ponds showed the poorest growth of all treatments. Late-flooded ponds had a higher population density than early-flooded ponds. Among rice straw disposals, the population density was highest in baled ponds, followed by standing and disked ponds. The harvestable crayfish sizes (total length > 75 mm) in decreasing order were: 19.4 g, from standing ponds; 18.3 g, from baled ponds; and 17.0 g, from disked ponds. Rice straw decomposed fastest in baled ponds, followed by standing ponds and disked ponds, with average weight losses of 77.1%, 66.6%, and 48.9%, respectively, after 5 months of decomposition. The C : N ratio of rice straw at flooding was 57. It took 4 months for the C : N ratio to drop below 17 for rice straw in baled and standing ponds. The average C : N ratio of rice straw in disked ponds was 23 after 5 months of decomposition. Eighteen weeks after flooding, early-flooded ponds had a higher periphyton biomass (337 g/m 2) than late-flooded ponds (216 g/m 2). The periphyton biomass was highest in standing ponds (358 g/m 2), followed by baled ponds (333 g/m 2) and disked ponds (307 g/m 2).