The bioassay section of this study provides, for the first time, estimations of lethal and growth reducing concentrations for the major production-limiting water quality variables for P. monodon. These critical concentrations were assessed using comparable experimental facilities and procedures. Unlike the majority of previous studies, continuous-flow rather than static bioassays were generally used and growth reducing concentrations were established following growth experiments rather than using estimates of acutely lethal concentrations and an 'application ' factor. The interactive effects of a number of variables were also assessed which provided relevant information for actual culture situations.
Prawn farming ponds are complex ecosystems which farmers attempt to manage by stimulating and maintaining rapid prawn growth. This study quantified the effects of some of the major management strategies, not only on prawn survival and growth but also on other aspects of the pond ecosystem including bacteria, meio- and macrobenthos, algae and water quality. The effects of the pond management strategies examined on prawn growth and aspects of the pond ecosystem are summarised in Table 4. 1. All the management strategies assessed affected chlorophyll a concentrations and many affected other water quality variables. The effects of management strategies on these variables would have been even more pronounced if each model pond had not been managed on an individual basis to sustain algal blooms and maintain water quality.
A useful experimental system for pond trials was developed to assess the effects of major pond management strategies on survival, growth and food conversion efficiency for prawns, as well as many other aspects of the pond ecosystem.
The results and recommendations in this report should allow prawn farmers to manage their ponds on a less empirical basis, reduce the likelihood of water quality crises and help farmers to deal with them when they do occur and ultimately increase the production of cultured prawns.
Acutely toxic and growth-reducing concentrations of ammonia, dissolved oxygen (DO) and pH were determined for juvenile penaeids. These results were then used during 8-1 2 week experiments where the effects of different water exchange rates, fertilisation regimes, stocking densities, pond preparation periods and feeding rates on prawn production indices and water quality were assessed.
For P. monodon, weight gain decreased exponentially with increased stocking density in the range 5 to 40 prawns m·2, whereas biomass gain and food conversion ratio increased and feeding activity and food conversion efficiency decreased. A longer period of pond preparation resulted in a 20 % increase in growth for P. monodon. Results indicated that a 50 % reduction in commercially used feeding rates can be achieved without affecting production of P. monodon in sub-tropical conditions. The dynamics of bacteria, algae, meio- and macrofauna and prawn feeding behaviour, in relation to pond management practices, are discussed.
These results provide a basis for objective farm pond management. They should assist farmers to manage prawn farming ponds on a less empirical basis, reduce the likelihood of water quality crisis and increase pond production.
