Abstract
To determine the effects of integration of fish and prawn in rice field on the productivity, nutrient flow and abundance of benthos, insects and weeds was evaluated through three field experiments at the Bangladesh Agricultural University, Mymensingh from July, 2007 to November, 2008 with a view to develop a sustainable rice-fish or rice-prawn culture system for the nutritional security and economic benefit of the rural people. The experiments were conducted in a randomized complete block design (RCBD) with five treatments and three replications for each treatment. The treatments were: rice combined with fish with regular urea fertilization, treatment RF-urea, rice combined with prawn with regular urea fertilization, treatment RP-urea, rice combined with fish with supplementary feeding, treatment RF-feed, rice combined with prawn with supplementary feeding, treatment RP-feed and treatment Rice only was kept as control i.e., without fish and prawn. The fish fingerlings and prawn juveniles were released in the experimental plots at a density of 1 fish m2 and 2 prawns m2 respectively. Fish species such as Cyprinus carpio and Oreochromis niloticus were stocked at a ratio of 1:1. Rice- variety BR 11 was selected for first and third experiments and BR 29 was selected for second experiment. Monitoring of physico-chemical parameters of water and yield data of rice, fish and prawn were taken in all experiments. Physico-chemical parameters of water were found within the suitable ranges for fish and prawn culture. The presence of fish decreased morning and afternoon dissolved oxygen concentrations in all experiments. Chlorophyll-a, phosphorus and NH4+ and NO3 concentrations. showed significantly (p<0.05) higher levels in the rice-fish treatments compared to the rice-prawn and Rice only. With respect to aquatic weeds, the amounts of biomass of it recorded in the treatments with fish were significantly lower than the biomass obtained in the treatments without fish. Similar to aquatic weeds total number of benthic organisms was considerably low in the treatments with fish than the treatment without fish. Fish significantly reduced the number of total insects and dipteran sampled by a vacuum suction device in rice fields. Between the two fish treatments the higher gross and net yield were obtained in RF-feed where supplementary feeding was done and the lower gross and net yield were recorded in RF-urea where urea was applied in all experiments. Between the two prawn treatments the higher gross and net were obtained in RP-feed where supplementary feeding was done and lower gross and net yield were recorded in RP-urea where urea was applied. No significant difference was observed among the treatments regarding rice grain and rice straw. Protein content in RF-urea for tilapia was significantly higher than the protein content in RF-feed for tilapia. Lipid content is increased by supplying artificial feed. Between the two fish treatments the significantly higher lipid contents was found in the treatment RF-feed 40.0% for carp and 20.6% for tilapia. Carp had higher lipid contents than tilapia in all treatments. Between the two prawn treatments the significantly highest lipid contents was found in the treatment RP-feed than the treatment RP-urea. Ash contents decreased with increasing supply of feed. Tilapia had higher ash contents than carp in all treatments. Between the two prawn treatments the significantly highest ash contents was found in the treatment RP-urea than the treatment RP-feed. Highest N contents in grain and straw were found in treatment RF-urea with 1.68% and 0.68% respectively. The nitrogen content of fish and prawn is directly coupled to the protein content. Nitrogen content in RF-urea (9.5%) for tilapia was significantly higher than the N content in RF-feed (8.9%). The fertilized treatments RF-urea, RP-urea and Rice only were isonitrogenic and received 102.3 kg N ha1 each. Amounts of N supplied as feed were lower with 11.2 kg N ha1 in RF-feed and 27.3 kg N ha1 in RP-feed. Fish N output was the highest in treatment RF-feed followed by RF-urea. Prawn N output was significantly higher in the treatment RP-feed than the treatment RP-urea. The highest grain N output recorded in the treatment RP-urea followed by other treatments. Straw N output was higher in the treatment RF-urea than the other treatments. The treatment Rice only and RP-urea had a highly negative N balance while RF-urea was very low negative N balance and the two feed supplemented treatments were positive. According to the findings of the present study, it may be concluded that the introduction of fish and prawn in rice fields has profound impacts on the availability of nutrients in the water and soil which ultimately increases the yield of rice grain and straw and at the same time provides an additional yield of tilapia, common carp and prawn from the same land.