PD/A CRSP Nineteenth Annual Technical Report
Table of Contents
Cite as: [Author(s), 2002. Title.] In: K. McElwee, K. Lewis, M. Nidiffer, and P. Buitrago (Editors), Nineteenth Annual Technical Report. Pond Dynamics/Aquaculture CRSP, Oregon State University, Corvallis, Oregon, [pp. ___.]
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 7kg 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 co-culture of lotus and Nile tilapia technically and economically. Both systems can recycle nutrients effectively within ponds and are environmentally friendly culture systems.
Regular fertilization and feeding in fish ponds result in nutrients being deposited in pond mud. One hectare of old pond mud was reported to have the equivalent of 1.85 tons of urea and 2.30 tons of triple superphosphate (TSP; Shrestha and Lin, 1997) or 2.8 tons of urea and 3.0 tons of TSP (Yang and Hu, 1989). Pond muds are a major sink for phosphorus, and adsorption capacity is related to mineral composition and clay content of pond muds (Shrestha and Lin, 1996). Release of adsorbed-P to the water column is minimal, and phytoplankton are not as effective in utilizing adsorbed-P as rooted crops. Roots extended in interstitial water of soil provide a better opportunity to extract P from soil (Denny, 1972; Boyd, 1982; Smart and Barko, 1985), and hence, nutrient-rich mud removed from fish ponds has been widely used to fertilize rooted land crops such as mulberry (Hu and Yang, 1984), forage crops (Yang and Hu, 1989), and maize (Christensen, 1989). However, removing pond mud is labor intensive and its practicability is questionable (Edwards et al., 1986; Little and Muir, 1987).
Alternatively, aquatic macrophytes may utilize reserve nutrients in muds by either rotation between two crops or co-culture with fish. Although in actual practice fish and aquatic macrophytes are rarely raised together in the same system, the co-culture and rotated culture of lotus (Nelumbo nucifera) and fish have been practiced in China for many years. Hoffmann (1934, cited by Edwards, 1987) reported that a farmer reared fish in the same pond as lotus in China but with only 50% of the usual number of fish because they grew more slowly than when raised alone. The rotation of fish and aquatic macrophytes may give farmers two crops to market rather than one and could sustain them if a loss occurred in one of the two ventures (Edwards, 1987).
Lotus is an aquatic emergent plant that grows as tall as 1.5 meters. Lotus is an important and popular cash crop in many Asian countries. Lotus has multiple uses, for example, stems as fresh vegetables; rhizomes as fresh vegetables, canned food, dessert, and starch; seeds as dessert and medicine; flowers as religious ornaments; and several parts as raw materials to produce cosmetics. It is commonly planted in fields or ponds with nutrient-rich mud and has a growing season of three to five months for the Chinese rhizome variety that does not flower or produce, and five to eight months for the Thai variety. It can extract nutrients from old pond mud efficiently. 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: