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Effects of C/n Ratio and Substrate Addition on Natural Food Communities in Freshwater Prawn Monoculture Ponds

M. Asaduzzaman
Department of Fisheries Management, Faculty of Fisheries, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh.

M. M. Rahman
Aquatic Resource Science, Faculty of Fisheries, Kagoshima University, Shimoarata 4-50-20, Kagoshima 890-0056, Japan.

M. E. Azim
Department of Physical and Environmental Sciences, University of Toronto, 1265 Military Trail, Toronto, Onta rio, Canada M1C 1A4.

M. Ashraful Islam
Department of Fisheries Management, Faculty of Fisheries, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh.

M. A. Wahab
Department of Fisheries Management, Faculty of Fisheries, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh.

M. C. J. Verdegem
Fish Culture and Fisher ies Group, Department of Animal Sciences, Wageningen University, P.O. Box 338, 6700 AH Wageninge n, The Netherlands.

J. A. J. Verreth
Fish Culture and Fisher ies Group, Department of Animal Sciences, Wageningen University, P.O. Box 338, 6700 AH Wageninge n, The Netherlands.

Abstract/ Executive Summary:

An on-station trial was conducted to investigate the effects of three C/N ratios (10/1, 15/1 and 20/1) along with substrate presence or absence on natural food communities in freshwater prawn culture ponds. An experiment was carried out in 40 m 2 ponds stocked with a stocking density of 2 prawn juveniles (5.023 ± 0.02 g) m^-2. A locally formulated and prepared feed containing 30% crude protein with C/N ratio 10 was applied to all ponds. In order to raise the C/N ratio of the feed input to 15 and 20, tapioca starch was applied separately as a source of carbohydrate in addition to the artificial feed. Under substrate treatments, bamboo side shoots were posted vertically in pond bottoms resulting in 100% additional surface area as periphyton substrates. The treatments with different C/N ratios are referred to as“CN10”, “CN15” and “CN20”. Increasing the C/N ratio from 10 to 20 significantly increased the biovolume of phytoplankton, crustaceans and rotifers in the water column by 15%, 6% and 11%, respectively. The biovolume of periphyton was 50% higher in treatment CN20 compared to treatment CN10. Increasing the C/N ratio from 10 to 20 raised the biovolume of total heterotrophic bacteria (THB) in the water column (70%), sediment (36%) and periphyton (40%). The chironomids biovolume was also significantly higher (28%) in treatment CN20 compared to treatment CN10. The addition of substrates decreased the biovolume of water column plankton by 14% but the combined biovolume (plankton + periphyton) was almost double in substrate-added ponds. The biovolume of plankton, periphyton and THB increased significantly with culture time duration whereas the biovolume of benthic macroinvertebrates decreased significantly with culture time indicating that freshwater prawn grazed on them. A significant interaction between C/N ratios and substrate presence or absence was only observed for plankton biovolume in the water column. This study demonstrated that plankton, periphyton and microbial biofloc communities were underutilized by the freshwater prawn in treatment CN20. This leaves room for increasing the stocking density of prawn and/or inclusion of periphyton grazing fish species to improve nutrient utilization efficiency and overall sustainability.

Keywords: C/N ratio, Substrates addition, Freshwater prawn, Natural food community, Plankton, Periphyton, Heterotrophic bacteria, Benthic macroinvertebrates

Location: Fisheries Field Laboratory of the Faculty of Fisheries, Bangladesh Agricultural University (BAU), Mymensingh, Bangladesh

Start Date:

End Date:

Program Area: Variety and Species

Commodity: Prawn

Objectives:

To investigate the effects of three C/N ratios (10/1, 15/1 and 20/1) along with substrate presence or absence on natural food communities in freshwater prawn culture ponds.

Methodology:

The experiment had a 3 × 2 factorial design with three levels of C:N ratio (10, 15 and 20) and two levels of substrate (with and without substrates). Treatments with different C/N ratio are referred to as “CN10”, “CN15” and “CN20”. Treatments were executed in triplicate and assigned randomly between ponds. The experiment was carried out at the Fisheries Field Laboratory of the Faculty of Fisheries, Bangladesh Agricultural University (BAU), Mymensingh, Bangladesh for a period of 120 days. A 81 × 8.9 m earthen pond was drained completely and partitioned by galvanized iron sheets into 18 small ponds of 40 m² each with an average water depth of 1 m. The ponds were rain-fed and fully exposed to prevailing sunlight and used before for research. Before starting the experiment, ponds were manually cleaned of aquatic vegetation. All unwanted fishes were eradicated by rotenone application at the rate of 100 g pond^-1. Lime (CaCO₃ was applied to all ponds at the rate of 250 kg ha^-1 (Day 1). On Day 2, ponds were filled with water from the nearby deep tube-well. On Day 4, 15 bambookanchi (side shoots of bamboo) per m 2 water surface area, with a mean diameter of 2.8 cm were posted vertically into the bottom mud in substrate treatment ponds, excluding a 0.5 m wide perimeter . This resulted in an additional area of 40 m² for periphyton development equaling about 100% of the pond surface area. On Day 5, all ponds were fertilized with semi decomposed cattle manure (3000 kg ha^-1), urea (100 kg ha^-1) and triple super phosphate (100 kg ha^-1). After fertilization, the ponds were left for 10 days to allow plankton development in the water column and periphyt on growth on substrates, and were subsequently stocked. Juveniles of M. rosenbergii (5.023 ± 0.02 g) purchased from a nearby commercial hatchery were stocked in the ponds at a density of 2 juveniles m^-2. A locally formulated and prepared pellet feed (2 mm) containing 30% protein with C/N ratio close to 10 was applied. The daily feeding rate was 5% body weight at the start of experiment, and declined gradually to 3% body weight at the end of the culture period with assuming 80% survival of total stock in each pond. Plankton samples were collected monthly by pooling 101 of water from five different locations in each pond and passing them through a 45 μm mesh plankton net. The concentrated samples were preserved in small plastic bottles with 5% buffered formalin. Qualitative and quantitative estimations of plankton were done using a Sedgewick- Rafter (S –R) cell containing 1000 1-mm³ cells. From each pond, three bamboo kanchi were selected randomly and 2×2 cm² samples of periphyton were taken at each of three depths (25, 50 and 75 cm below from the water surface) per pole on a monthly basis starting after 7 days of substrate installation. Periphyton samples from different depths and different bamboo kanchi were pooled and preserved in a labeled plastic vial containing 5% buffered formalin. After vigorous shaking, a 1 ml sub-sample was transferred in an S–R cell and the periphyton number was estimated in 10 randomly selected cells under a binocular microscope (Swift, M-4000). Total bacterial load of pond water, sediment and periphyton were determined on monthly basis between 09:00 and 10:00 h. All samples were collected from 5 different locations, mixed homogenously and collected with sterile glass bottles for bringing to the Bacteriological Laboratory, Department of Microbiology and Hygiene, Faculty of Veterinary Science, BAU, Mymensingh, Bangladesh. The benthic macroinvertebrates samples were collected monthly with an Ekman grab (area: 225 cm²). In each pond, bottom mud samples were collected from 3 different locations, which were then combined into a composite sample. Benthic macroinvertebrates were collected after filtering sediments through a 250 μm mesh sieve and pre served in a plastic vial containing a 10% buffered formalin. The biovolumes of plankton, periphyton and benthic macroinvertebrates were calculated according to Rahman et al. (2006). The biovolumes of heterotrophic bacteria were calculated using the value of Nakano and Kawabata (2000) . The biovolumes of plankton , periphyton, THB and benthic macroinvertebrates were analyzed by repeated measures ANOVA with addition of substrate and C/N ratio as main factors and time as the sub-factor. The data were checked for normality, and percentage and ratio data were arcsine transformed. All ANOVA were performed using SAS 6.21 program (SAS Institute, Cary, NC 27513, USA). If a main effect was significant, the ANOVA was followed by Tukey's Test at P<0.05 level of significance.

Source of Information: Aquaculture 306 (2010) 127– 136

Article Link (Online Journal): doi: 10.1016/j.aquaculture.2010.05.035

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

In conclusion, increasing C/N ratio increased the biovolume of plankton, periphyton, heterotrophic bacteria and benthic macroinvertebrate. However, the availability of pond communities in the present research seemed to be underutilized by the freshwater prawn. This suggests further investigation on the possibility of decreasing artificial feeding rate or increasing in stocking density of prawn. In this system, the biomass of plankton and periphyton seemed to be totally unutilized by the freshwater prawn. Therefore, inclusion of both plankton and periphyton grazing fish species like tilapia can further increase the production, system environment and overall sustainability in C/N-CP ponds and is subject of further research.

Knowledge Management: Journal