Refinements in practical techniques have occurred and differences in detail between various hatcheries exist. Despite this, no fundamental change in non-recirculation hatchery technology (except for a general shift away from the use of greenwater) was reported in the literature between 1990 and 2008. However, the rearing of M. rosenbergii larvae in open earthen ponds was reported in Thailand (Tunsutapanich et al. 1996); this technique has been improved during the current decade (Thongrod et al. 2007). The use of earthen ponds for rearing the larvae of another commercially important freshwater prawn, M. nipponense, has also been reported (Wang & Qianhong 1999).
This method of rearing PL was described in detail by Tunsutapanich et al. (1997) and Thongrod et al. (2007). In the first study, four 1600 m2 earthen ponds were filled with water of 10p.p.t. salinity. Biological, chemical and physical treatments were applied to the ponds and continuous aeration was provided through tubing. No water exchange appears to have occurred during the cycle. Berried females (~25 individuals/kg or 40 g each) were introduced into cages placed in each pond in sufficient numbers to produce two initial larval stocking densities of 4 and 6 million larvae per pond (2.5-3.75/L). Naturally occurring plankton comprised the sole food available for the first week of the cycle (days 1-7). As the supply of natural food decreased, it was supplemented with Artemia nauplii (days 6-15), keeping the Artemia density above 10/L. These nauplii were partially (from day 12-15) and then totally (after day 15) replaced with (adult) Artemia biomass, which was fed until day 21. From this stage onwards, farm-made egg-custard diets (containing chicken egg, mussel flesh, squid, etc.) and 0.5 to 1.0 mm particulate diets were fed until the PL were harvested. A natural growth of sea grass (Ruppia maritima) was observed after the first week and was thought to help to maintain good water quality conditions. A modified 60 m seine net with a mesh opening size of approximately 2 to 3 mm was used for harvesting. The first metamorphosing animals were observed after 13 days at the lower density and 14 days at the higher. Survival rates were estimated to be around 68% at both stocking densities. Final harvests, achieved within 29 to 35 days, were 2.75 million PL at the lower stocking density and 4.05 million PL at the higher density.
Efforts to improve this technique reported by Thon-grod et al. (2007) have included the use of smaller ponds (400 m2), an increase in the number of aeration pipes to cover the whole pond bottom, and an increase in stocking density to 7.5/L. Productivity increased to 5 PL/L and total production attained 2 million PL per 400 m2 pond. This novel technology offers significant opportunities for increasing the supplies of PL, decreasing the volume of
Artemia cysts required, decreasing the consumption of water, shortening the larval cycle time (compared to con-crete/fibreglass tanks), and utilising less labour. In addition, this technique offers the possibility that even the relatively unsophisticated backyard hatcheries may be less necessary in future.
Research on recirculation systems has paid off and is being translated into commercial practice. Due to the need to develop sustainable aquaculture techniques that conserve vital water supplies and minimise effluent discharge, these methods are likely to be increasingly applied in the future.
Chinese papers on M. rosenbergii research are rapidly expanding but usually appear only in Chinese agricultural journals and fisheries magazines. Some ofthese papers refer to topics relevant to this chapter but provide only a title or an extremely brief abstract in English, so it is impossible to include information from them here. It is to be hoped that such research results will soon become available to a wider audience through publication, in English, in international aquaculture journals. Since China is now the major producer of farmed freshwater prawns (Chapters 1 and 17) it is certain that it has much to contribute to our scientific understanding of hatchery systems and management.
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