The giant freshwater prawn, Macrobrachium rosenbergii (de Man), is on highly esteemed food of the Thai people. In the past, this particular type of prawn was very abundant in the rivers and lagoons of Thailand, but now there is significant reduction due to water pollution (Menesvesta and Piyatirativokol, 1980). In addition, commercial fishing of the prawn without proper management and regulation led to its depletion.
The increasing demand on prawn consumption, coupled with the short supply, has resulted in many efforts to raise the prawn in a controlled environment, such as an aquaculture system.
The development of grow out production procedures for prawns in general and Macrobrachium rosenbergii in particular relies almost enterily on the monoculture strategy (Shang, 1981 in Cohen and Ra'anan, 1983). Even Thailand in general, the giant prawn monoculture is prefered instead of polyculture. Among the reasons for this is the hight economic benefits, as seen by the hight price of prawns much higher than fish.
In monoculture system of the giant prawn rearing, stocking prawn at high densities have led to the following problems: First, interms of ecological balance (Cohen et al., 1983), where the prawns appear to concentrate on the bottom of pond only, while the water column is devoid of aquaculture organisms. The result is an absence of controlling influence on the ecological process of the pond. Under these conditions, two extreme algae blooms have been known to damage the prawn yield: either unicellular (e.g. Anabaena) or a multicellular (Cladophora sp.) both leading to anoxic conditions. Second, according Cohen and Ra'anan (1983) the benthic nature of the organisms has resulted in only the bottom of the pond being utilised then by limiting the prawn yield potential of the whole pond. Even under monoculture, applied feeds may serve more as food sources for natural productivity than do so as prawn feed (Malecha et al., 1981).
Blooming algae over long periods of time has resulted in ecological unbalance in monoculture ponds. Not all of these may be beneficial (Seymour, 1980). Excessive phytoplankton growth is a problem, because it may negatively effect water quality in fish ponds in at least three ways (Smith, 1985). First, it can lead to chronic night time oxygen deficits, since algae are the major consumers in the pond as well as the producers of detritus that is broken down by oxygen consuming bacteria (Boyd, 1973; Boyd et al., 1978; Schroeder, 1975). Second, dense algae blooms collapse peridically; subsequent decomposition of the dead cause costly fish kills due to anoxia (Seymour, 1980) and due to toxic ammonia level. Excess ammonia can kill fish even in ponds that are constantly aerated (Shilo and Rimon, 1982). Finally, dense algae blooms can produce chemicals than ruin the flavor of fish (Lovvel and Sackey, 1973; Brown and Boyd, 1982).
The purpose of the present study is to: (1) determine the effect of by using paddle wheel aeration in prawn monoculture on the development of associated natural food organisms, and (2) determine algae blooms under the conditions.
The experiment was conducted at the farms of freshwater prawn joint facility in Banglen District, Nakhon Pathom Province, Thailand. The project site is located approximately 90 kms west of the National Inland Fisheries Institute.
