M. E. Azim
Fish Culture and Fisheries Group, Wageningen Institute of Animal Sciences, Wageningen University,P.O. Box 338, 6700 AH Wageningen, The Netherlands.
M. C. J. Verdegem
Fish Culture and Fisheries Group, Wageningen Institute of Animal Sciences, Wageningen University,P.O. Box 338, 6700 AH Wageningen, The Netherlands.
M. M. Rahman
Department of Fisheries Management, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh.
M. A. Wahab
Department of Fisheries Management, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh.
A. A. Van Dam
Fish Culture and Fisheries Group, Wageningen Institute of Animal Sciences, Wageningen University,P.O. Box 338, 6700 AH Wageningen, The Netherlands.
M. C. M. Beveridge
Institute of Aquaculture, University of Stirling, Scotland FK9 4LA, UK.
Periphyton, Stable isotope ratio,Polyculture; Labeo rohita,Labeo calbasu,Catla catla
Fisheries Faculty Field Laboratory, Bangladesh Agricultural University (B AU), Mymensingh.
Variety and Species
A 90-day experiment was carried out between August and November 1999 in 10 earthen ponds at the pond facilities of the Fisheries Faculty Field Laboratory, Bangladesh Agricultural University (B AU), Mymensingh. All ponds are rectangular (7.5 m 10 m) with a maximum depth of 1.5 m. The ponds were individually supplied with ground water from an adjacent deep tube- well and fully exposed to prevailing sunlight . Pond embankments were covered with grass. The design was based on a previous experiment that resulted in an optimal stock ing ratio of 60% rohu and 40% catla in a periphyton- based system. Four stocking combinations were compared in the present study: 60% rohu plus 40% catla with a total stock ing density of 10000 ha-1 (treatment CR), CR plus 15% kalbaush (i.e. 1500 juveniles ha-1) (treatment C15), CR plus 3 0% kalbaush (treatment C30), and CR plus 45% kalbaush (treatment C45 ). Bamboo poles, used as substrates for periphyton, were plant edvertically into the pond. As a substrate control, CR without substrate was used (CR0 ). Two replicates of each treatment were assigned randomly to the ponds. Prior to the trial, ponds were renovated, aquatic vegetation removed and all larger aquatic animals eradicated by frequent netting. As substrates, bamboo poles (mean length = 2.0 m; mean diameter = 5.5 cm) were staked into the bottom mud, the upper portions extend ing above the water surface (Day 1). Substrates (nine poles m-2) were installed in a 40-m2 area, leaving a 2-m- wide perimeter free of poles in each pond. The poles added an effective submerged surface area of about 75 m 2 per pond. Ponds were treated with lime (CaO, 25 0 kg ha-1) and filled with water. On Day 7, semi-decomposed cow manure, urea and triple super phosphate (TSP) were applied at the rates of 4500, 150 , and 150 kg ha-1, respectively. Ponds were then fertilized fortnightly at the same rates during the entire experimental period. On Day 14, L. rohita (individ ual weight 4 – 5 g), C. catla(6 – 7 g) and L. calbas u(4 – 6 g) juveni les were relea sed into the ponds. Juveniles were collected from nearby hatcheries and stock ed in the afternoon, care being taken to gradually acclimatize the fish to pond conditions. Periphyton collection started on Day 20 and continued at fortnight ly intervals. From each pond, three bamboo poles were selected by random number tables. Two samples of periphyton were taken at three depths, 25, 50 and 75 cm along the length of the pole, starting at the water surface. After collection of the periphyton samples, the poles were returned to their original position and marked so that sampled poles were always excluded from subsequent samples. Half of the dried sample was transferred to a muffle furnace and a shed at 550ºC for 4 h and re-weighed. Dry mat ter (DM), ash-f ree dry mat ter (AFDM) and ash contents were determined by weight differences. The remaining portion of the sample from all dates was pooled (pond-wise) and used for the analysis of proximate composition, energy content and stable nitrogen and carbon isotopes in the laboratory of the Wageningen Institute of Animal Sciences (WIAS), Wageningen University and Research Centre (WUR), Netherlands. Nitrogen content was determined by Kjeldahl method. Fat content was determined b y Soxhlet apparatus. Energy content was determined by bomb calorimeter. From Day 14 onwards, the following variables were monitored daily (6 days per week) between 0900 and 1000 h: temperature, dissolved oxygen , Secchi depth and pH. Total alkalinity, chlorophyll a , total ammonia, nitrate nitrogen (NO3-N) and phosphate phosphorus (PO 4-P) were measured weekly in all ponds. Total alkalinity was determined by titrimetric method. Nutrient analyses were performed by HACH kit (model DR 2000). At the end of the experiment , all bamboo poles were removed, the ponds drained, and all fish were collected and weighed on a balance (Ohaus model C10 0R200; precision 0.1 g). Samples analyzed for stable isotope composition included all food sources within the ponds and the experimental fish. Plankton samples were collected monthly from the water column using a plankton net (45 Am) from all ponds. During stocking and harvesting, two individuals of each species per pond were selected randomly for stable isotope analysis. Plankton, periphyton and sediment samples were oven dried at 105ºC and fish samples at 70ºC for 24 h. Whole body biomass of fish was used for analysis. Stable nitrogen and carbon isotope compositions were determined using an Isotope Ratio Mass Spectrophotometer (Finnigan MAT Delta C) in the IRMS Laboratory of the WIAS, WUR, Netherlands. A one-way ANO VA was used for yield parameters. Survival of fish was analyzed using arcsine -transfo med data. Daily and weekly water quality parameters and periphyton biomass were compared by split-p lot ANOVA with treatments as the main factor and time as the subfactor using the SAS 6.12 program (SAS Institute, Cary, NC 27513, USA ). The pH values were transformed to hydrogen ion concentrations before statistical analysis. If a main effect was significant, the ANOVA was followed by Tukey’s test at P<0.05 level of significance.
Aquaculture 213 (2002) 131 – 149
Journal