Evaluation of Fish Production Using Organic and Inorganic Fertilizer::Application to Grass Carp Polyculture

摘要

Efficient usage of fertilizers in organic or inorganic forms show conflicting results in terms of net fish production under various climatic conditions. Manures applied to polyculture ponds as organic fertilizer, require a process of decomposition before the nutritional contents are released, assimilated, and utilized by plankton. On the other hand, inorganic (chemical) fertilizers are granular and concentrated with primary nutrients. Major nutrients such as nitrogen (N) and phosphorus (P) readily dissolve in water in ionic forms of nitrate, ammonium and orthophosphate. The differences in fertilizer solubility (nutrient release rate) and decomposition processes (organic fertilization) make the water quality and biological response distinctively different in inorganic fertilizer applied ponds. Previous research has indicated that one grass carp, Ctenopharyngodon idella, can support the growth of three silver carp. This means the excreta of herbivorous fish can be utilized to fertilize the water and produce plankton for filter-feeding fish to consume. Due to an inconsistent supply of grass foliage, farmers in North Vietnam found it difficult to include an adequate percentage of grass carp to support polyculture operation without supplementary fertilization to maintain adequate levels of natural foods in the pond. This supplementary fertilization, in particular manure application, often impacted water quality, which negatively affected health of grass carp. In the present study, an attempt was made to evaluate the production of grass carp polyculture when N:P were maintained through supplementary organic fertilization. An assessment of organic and inorganic fertilizer application on fish production was also undertaken and compared with grass carp polyculture performance. Fish biomass change and fertilizer treatments had significant interaction (P = 0.001). The fish production was found to be significantly different for organic fertilizer, grass carp + organic fertilizer and inorganic fertilizer treatments. The net fish production was highest (5.6 plus or minus 0.03 tonnes/ hectare) in grass carp + organic fertilizer treatment followed by organic fertilizer (4.0 plus or minus 0.06 tonnes/hectare). The inorganic fertilizer treatment yielded the least with 3.0 plus or minus 0.05 tonnes/hectare. The control ponds with no fertilizer was applied produced only 0.29 plus or minus 0.01 tonnes/hectare.