Impact of high-density stocking and selective harvesting on yield and water productivity of deepwater rice-fish systems

We examine the productivity of deepwater rice-fish systems and management strategies that include high-density initial stocking and selective harvesting. All species of fish and prawns grow faster after 120 days of rearing, probably due to periodic selective harvesting that minimizes the competition for food and space, as well as physiological stress at reduced density. We observe a higher survival rate, a lower apparent feed conversion ratio (1.77) and higher fish yield (14.1%) in rice-fish culture with selective harvesting (T-1) than in rice-fish culture without selective harvesting (T-2). The highest paddy yield was recorded in T-1, primarily due to the higher number of panicles per m(2) (139.5) and the number of filled grains per panicle (111.5). The increase in paddy yield over rice mono-cropping was higher in T-1 (25%) than T-2 (16.9%). The smaller number of panicles (122.2/m(2)) and filled grains (98.5 per panicle) in rice mono-cropping was probably due to the absence of fish and prawns in the field as fish and prawns improve soil fertility, recover lost energy, and adjust energy flow by consuming plankton, weeds, insects and bacteria that compete with rice for nutrients. The highest rice equivalent yield (38.5 t ha(-1)), the output value-cultivation cost ratio (1.56) and net water productivity (Rs. 7.30/m(3)) in deepwater rice-fish culture was recorded when selective harvesting was practiced. This eco-friendly dual production system (rice and fish) and on-dyke horticulture, which generate near-term lucrative returns and generates employment opportunities, can be adopted and expanded in lowlands and waterlogged areas. (C) 2009 Elsevier B.V. All rights reserved.