M.A. Razzak
Department of Aquaculture, Bangladesh Agricultural University (BAU), Mymensingh 2202, Bangladesh
M. Frei
Department of Aquaculture Systems and Animal Nutrition, Institute for Animal Production in the Tropics and Subtropics, Universitat Hohenheim (480), D-70593 Stuttgart, Germany
M.A.M. Khan
Department of Entomology, BAU, Mymensingh 2202, Bangladesh
K. Becker
Department of Fisheries Management, BAU, Mymensingh 2202, Bangladesh
S. Dewan
Department of Fisheries Management, BAU, Mymensingh 2202, Bangladesh
Rice-fish culture, Insects, Zoobenthos, Weeds
Bangladesh Agricultural University, Mymensingh.
Farming System
Carp fish, Rice
Experimental site: The experiment was conducted at the Agronomy Field Laboratoty, Bangladesh Agricultural University, Mymensingh during the rainy season from June to November, 2005. The experimental site is under the Old Brahmaputra Flood Plain Agro-ecological Zone having non-calcareous dark grey soils of silt loam texture. The experiment was consisted with 12 plots, each comprising an area of 150 m2 . Small water channels (70 cm width and 30 cm depth) were made between the plots to supply water in the experimental plots. Each plot had an inlet and outlet in the dikes (height 60 cm, base width 50 cm and top width 40 cm) for regulation of water depth. Nylon nets fixed with bamboo poles were placed around each plot to prevent the entry of unwanted animals in the plot and escaping of stocked fish. Experimental design: The experiment was undertaken with four treatments each with three replications. The treatments were laid out in a randomized complete block design (RCBD). The treatments were: (1) rice-mono sex tilapia (Oreochromis niloticus), (2) rice-common carp (Cyprinus carpio), (3) rice-mono sex tilapia-common carp and (4) rice alone. Field management: The experimental plots were plowed two times using a power tiller. The weeds were removed and the land was then leveled by laddering. A small refuge pond was excavated in the middle of each plot, covering an area of 4 m 2 with 0.5m depth to provide shelter for fish during low water level and high temperature. A basal dose of fertilizer was applied one day before transplanting according to the recommended dose of BRRI (2004), i.e. 150 kg/ha of triple super phosphate (TSP) and 75 kg/ha of muriate of potash (MP). Urea was supplied according to the BRRI (2004), i.e. 220 kg/ha in the three control plots in three installments at 16, 30 and 55 days after transplanting (DAT) of rice seedlings with one-third of the total dose during each application. No pesticide was applied to the crop during the experimental period. The seedling of BR 11 were transplanted from a nursery into the experimental plots at 35 DAT in alternate row spacing of 35 cm and 15 cm. The plant to plant distance in the rows was 20 cm. The alternate row spacing provides enough space for easy movement of fish and to penetrate sunlight in the water between the rows which improves the growth of plankton for fish feed. Fish fingerlings were released at 18 DAT in the experimental plots at a density of 1 fish per m2. The mixing ratio of carp and tilapia in treatment three was 1:1. Fish were weighed by plot and released into the central refuge ponds. The average weight of the fish was 11.7 g. Feeding was started five days after stocking. Feed ingredients were purchased from the local market in the city of Mymensingh. The ingredients were thoroughly mixed and made into 4 mm pellets. The feed composition was 50% fish meal (Nutri Fish 65, Carolina By-Products Inc, Winchester, Virginia, USA), 44% wheat flour, 4% soybean oil and 2% mineral and vitamin premix (Eskavit Fish Premix, SK+F Bangladesh Ltd., Gazipur, Bangladesh). The proximate composition of feed on a dry matter (DM) basis was 41.2% crude protein, 12.4% crude lipid, and gross energy19.5 kJ/g. 6.4 g of feed per kg metabolic body mass per day (gkg-0.8/day) were provided at 2 x maintenance feeding. Feed was provided manually daily at 9 am. Feeding level was adjusted fortnightly based on the prospective fish biomass assuming a metabolic growth rate of 8 gkg-0.8/day. Water was supplied to the plots from the deep tube well and water level was raised gradually ranged from 15-25 cm with the growth of rice and fish. Collection and identification of arthropod samples: The insect samples were collected using a vacuum suction device, (Eco-Vac Insect sampler, Ecotech GmbH, Bonn Germany). Samples were taken six times at 35, 49, 63, 77, 91, 105 and 119 DAT. Sampling was done in the afternoon in all plots on all sampling dates. A subplot covering one fourth of the total area of each plot was assigned for insect collection. To reduce damage of the rice plants during the flowering stage, the sampling area was reduced at sampling date 91 and 105 DAT. During these sampling events, three samples each of 1 m 2 area were taken from each plot. All the collected insects were immediately killed and preserved in sampling containers containing 70% ethanol and stored until the laboratory analysis. The insect species were identified and counted by trained staff in the laboratory of the Department of Entomology, Bangladesh Agricultural University, Mymensingh. Zoobenthos and weed sampling: Benthic organisms have been collected at 45, 73 and 101 DAT using an 15 X 15 cm Ekman dredge sediment sampler. Three samples per plot were taken randomly and pooled into one composite sample. The soil samples were flushed through a 0.2 mm sieve and analyzed immediately. Benthic organisms were counted and identified using magnifying glass and microscope (where necessary). Weeds were collected manually from the whole plots at 30, 58 and 86 DAT. Collected weeds were washed and sun dried at least for 1 day and then the weight of dry weeds was taken. A representative sample was taken from the sun dried samples for determination of the dry matter. This was done by grinding the samples and drying them overnight in a laboratory oven at 105°C. Weed biomass was then expressed as dry matter/m2 . Data analysis: Mean values and standard deviations were calculated using MS Excel. Mean values were compared by performing one way analysis of variance (ANOVA), followed by and LSD test to detect statistically significant differences between the treatments at p <0.05. As arthropod data were not normally distributed, they were therefore log io transformed to comply with ANOVA assumptions. The untransformed mean values are presented in the tables. The software used for statistical analyses was Statistica, Version 5 for MS Windows (Stasoft, Tulsa, USA).
Bangladesh J. Fish. Res., 11(1), 2007: 19-28
Journal