Wagner C. Valenti, William H. Daniels, Michael B. New and Eudes S. Correia
The original work of Takuji Fujimura and his team on larval culture in Hawaii in the 1950s (Chapter 1) was based on the 'greenwater' principle. It involved the deliberate encouragement of phytoplankton development (mainly Chlorella spp.) in the rearing water (New 1990) and was thought to improve water quality and increase survival to metamorphosis. However, high pH (9.5 and above) and algal blooms or crashes often causedlarval mortality (Hudon etal. 1989). Today, very few prawn hatcheries use the greenwater system because commercial experience has shown that 'clearwater' systems are easier to manage and are thus more efficient. These include both open (flow-through) and closed (recirculating) systems. As clearwater systems have evolved, they have been reviewed by New (1990, 1995) and described in detail in several manuals, including those by New & Singholka (1985), Valenti (1985), Cavalcanti et al. (1986), Chowdhury et al. (1993), Valenti (1998) and New (2002).
Flow-through hatchery systems are based on regular water exchange to reduce toxic substances which accumulate in the rearing water. These originate from the metabolism of larvae and Artemia nauplii, and bacterial decomposition of waste food, faeces and dead organisms. Large amounts of brackishwater are therefore required for this type of hatchery.
Closed or recirculation hatchery systems have been developed to minimise water losses and maximise water quality for raising larvae. Minimal water usage is very important for inland hatcheries, where water must be transported long distances or artificial seawater is used. Recirculation allows continuous or semi-continuous processing of water to remove nitrogen and solid wastes and disinfection for disease management. The use of recirculation systems in Macrobrachium rosenbergii hatcheries has several advantages over open systems. These include more stable water quality; lower water consumption; heat conservation; lower labour requirements; control or prevention of pollutants, parasites, predators and competitors; and enhanced feasibility of establishing inland hatcheries.
This chapter summarises and supplements the information presented in the former manuals, with the results of recently published research and with the chapter authors' personal experiences. Readers of the current chapter will quickly recognise that there are many different, sometimes conflicting, ways of managing prawn hatcheries.
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