Lotus-fish Culture in Ponds: Recycling of Pond Mud Nutrients

An experiment was conducted in nine 200-m2 fertilized earthen ponds at the Asian Institute of Technology, Thailand, from January to September 2000. This experiment was designed to assess the recovery of pond mud nutrient by lotus (Nelumbo nucifera), to assess pond mud characteristics after lotus-fish co-culture, and to compare fish growth with and without lotus integration. There were three treatments in triplicate: A) lotus-tilapia together; B) tilapia alone; and C) lotus alone. Seedlings (0.39 ± 0.09 kg) of Thai lotus variety were transplanted to ponds of treatments A and C at a density of 25 seedlings pond-1, while sex-reversed all-male Nile tilapia (Oreochromis niloticus) fingerlings (8.6 to 10.3 g) were stocked at 2 fish m-2 in ponds of treatments A and B when the water depth had been increased to 50 cm due to increasing lotus height. Ponds stocked with tilapia (treatments A and B) were fertilized weekly with urea and triple superphosphate (TSP) at a rate of 28 kg nitrogen and 7 kg phosphorus ha-1 wk-1 after tilapia stocking. There was no fertilization in ponds of treatment C. Lotus co-cultured with tilapia or cultured alone in ponds was able to effectively take up nutrients from old pond mud (about 300 kg N and 43 kg P ha-1 yr-1) and resulted in the reduction of nutrients in mud by about 2.4 t N and 1 t P ha-1 yr-1. There were no significant differences in lotus growth performance between treatments A and C, while Nile tilapia cultured alone grew significantly better than when co-cultured with lotus. The partial budget analysis indicates that lotus cultured alone generated the highest net return, and lotus contributed the largest portion of net income in lotus-tilapia co-culture. The present experiment has demonstrated the effectiveness of nutrient removal from old pond mud by lotus and the feasibility of rotation and coculture of lotus and Nile tilapia technically and economically. Both systems can recycle nutrients effectively within ponds and are environmentally friendly culture systems. NINETEENTH ANNUAL TECHNICAL REPORT 60 Parameters Treatment A (Lotus-Tilapia) Treatment B (Tilapia Alone) STOCKING Density (fish m) 2 2 Total No. of Fish 400 400 Mean Weight (g fish) 9.2 ± 0.4 9.6 ± 0.4 Total Weight (kg) 3.7 ± 0.2 3.9 ± 0.2 HARVEST Mean Weight (g fish) 58.4 ± 3.1 a 117.5 ± 6.4 b Total Weight (kg) 10.0 ± 1.3 a 38.5 ± 1.2 b Survival Rate (%) 42.6 ± 3.7 a 82.3 ± 3.5 b WEIGHT GAIN Mean Weight Gain (g fish) 49.1 ± 3.5 a 107.8 ± 6.1 b Daily Weight Gain (g fish d) 0.26 ± 0.02 a 0.57 ± 0.03 b Total Weight Gain (kg) 6.3 ± 1.4 a 34.6 ± 1.2 b Net Yield (kg ha yr) 608.7 ± 134.8 a 3,344.6 ± 113.4 b Gross Yield (kg ha yr) 964.3 ± 123.3 a 3,717.0 ± 113.2 b Water levels of ponds can be increased as lotus grows. Fish can be stocked when water levels reach 30 cm and harvested four to five months after lotus is planted. Additionally, lotus shoots provide substrate for growth of epiphytic algae that are consumed by tilapia (Bowen, 1982; Lowe-McConnell, 1982; Shrestha and Knud-Hansen, 1994). The purposes of this study were to: 1) Assess the pond mud nutrient recovery by lotus plants; 2) Assess pond mud characteristics after lotus-fish coculture; and 3) Compare fish growth with and without lotus integration. METHODS AND MATERIALS The experiment was conducted using a randomized complete block design in nine 200-m2 earthen ponds at the Asian Institute of Technology (AIT), Thailand, from January to September 2000. There were three treatments in triplicate: A) lotus-tilapia co-culture, B) tilapia alone, and C) lotus alone. All ponds were used for intensive fish culture with commercial pelleted feed prior to this experiment. The ponds were dried for one month and filled with water to 10 cm deep one day prior to lotus transplanting. Seedlings of Thai lotus variety, purchased from a local farm, were transplanted to ponds of the lotus-tilapia and lotus alone treatments (A and C) at a density of 25 seedlings pond-1 on 22 January 2000. The average length and weight of the transplanted lotus seedlings were 1 m and 0.39 ± 0.09 kg, respectively. After lotus seedlings were transplanted, water was added weekly to all ponds and water depth increased as the height of lotus increased. Sex-reversed allmale Nile tilapia (Oreochromis niloticus) fingerlings (8.6 to 10.3 g in size), obtained from AIT Hatchery, were stocked at 2 fish m-2 in the lotus-tilapia and tilapia-alone treatments (A and B) on 9 March 2000, when water depth reached 50 cm. Water depth was increased continuously up to 1 m with the growth of lotus, and it was maintained at 1 m throughout the rest of the experimental period by adding water weekly to replace evaporation and seepage losses. Ponds stocked with tilapia (treatments A and B) were fertilized weekly with urea and TSP at a rate of 28 kg nitrogen (N) and 7 kg phosphorus (P) ha-1 wk-1 after tilapia stocking. There was no fertilization in ponds of the lotus-alone treatment (treatment C). During the experiment there was no fish sampling and no removal of dead lotus parts such as dead leaves from ponds. Matured lotus pods with seeds were harvested periodically and air-dried to separate seeds (with husk). On 14 September 2000, all ponds were drained. Tilapia were harvested after 189 days of culture, while different parts of lotus (flower, pod, leaf, stem, and root) were harvested separately (after 236 days of cultivation). Integrated water samples were taken biweekly from the entire water column near the center of each pond at about 0900 h for analyses of pH, alkalinity, total ammonium nitrogen (TAN), nitrite-N, nitrate-N, total Kjeldahl nitrogen (TKN), soluble reactive phosphorus (SRP), total phosphorus (TP), chlorophyll a, total suspended solids (TSS), and total volatile solids (TVS) (APHA et al., 1985; Egna et al., 1987). Water temperature and dissolved oxygen (DO) were also measured at the time of collecting water samples with a YSI model 54 oxygen meter (Yellow Springs Instruments, Yellow Springs, Ohio). The nutrient budgets for nitrogen and total phosphorus in ponds during the experimental period were calculated based on inputs from water, stocked tilapia fingerlings, transplanted lotus seedlings, fertilizers, and soil as well as on losses in harvested tilapia and lotus, discharge water, and mud. Mud samples were collected with 5-cm-diameter plastic tubes from the top 10 cm of pond bottom before lotus introduction and after fish and lotus harvest. Total nitrogen (TN) and TP in samples of mud and different parts of lotus and tilapia at the beginning and end of the experiment were analyzed using the methods described by Yoshida et al. (1976). Data were analyzed statistically by analysis of variance and ttest (Steele and Torrie, 1980) using SPSS (version 7.0) statistical Table 1. Growth performance of Nile tilapia cultured alone and integrated with lotus for 189 days in 200-m2 ponds. Mean values with different superscript letters in the same row were significantly different (P < 0.05). Table 2. Growth performance of lotus cultivated alone and integrated with Nile tilapia for 236 days in 200-m2 ponds. Mean values with different superscript letters in the same row were significantly different (P < 0.05). Parameters Treatment A (Lotus-Tilapia) Treatment C (Lotus Alone) TRANSPLANTING Density (seedlings pond) 25 25 Total Biomass (kg pond) 9.90 ± 0.10 9.38 ± 0.48 HARVEST Leaf (kg pond) 429.93 ± 37.07 450.19 ± 36.03 Pod (kg pond) 11.50 ± 0.74 14.25 ± 1.61 Flower (kg pond) 5.10 ± 2.86 7.16 ± 1.63 Root and Stem (kg pond) 159.13 ± 14.92 177.33 ± 6.34 Total Biomass (kg pond) 605.67 ± 32.89 648.92 ± 28.10 BIOMASS GAIN (kg pond) 595.77 ± 32.87 639.55 ± 28.17 NET YIELD (t ha yr) 46.07 ± 2.54 49.46 ± 2.18 GROSS YIELD (t ha yr) 46.84 ± 2.54 50.18 ± 2.17 NEW AQUACULTURE SYSTEMS/NEW SPECIES RESEARCH 61 software package (SPSS Inc., Chicago, Illinois). Differences were considered significant at an alpha level of 0.05. All means were given with ± 1 standard error (SE). A partial budget analysis was conducted to determine economic returns of lotus-tilapia integrated culture, tilapia alone, and lotus alone (Shang, 1990). The analysis was based on farmgate prices in Thailand for harvested tilapia and lotus products (seeds and flowers) and on current local market prices for all other items expressed in US dollars (US$1 = 40 baht). Farmgate price of Nile tilapia varied with size: $0.250 kg-1 for size 50 to 100 g and $0.375 kg-1 for size 100 to 200 g. Farm-gate prices of lotus seeds and flowers were $0.75 kg-1 and $0.125 piece-1, respectively. Market prices of sex-reversed all-male Nile tilapia fingerlings ($0.0125 piece-1), lotus seedlings ($0.125 piece-1), urea ($0.1875 kg-1), and TSP ($0.3125 kg-1) were used. The calculation for cost of working capital was based on an annual interest rate of 8%.