Shrimp size, density, growth rate and mortality

enter the first production stage and is reduced each time the shrimp are transferred to subsequent stages. The advantages of multi-stage production systems have been well identified by Cao and Jiang 1990, Fast 1991, Sandifer et al. 1991, Sturmer et al. 1991, Samocha and Lawrence 1992, Stern and Letellier 1992. In general, these authors feel that a multi-stage shrimp production system can improve stock inventory control and disease treatment, reduce predator damage and feed waste, alleviate the difficulty of monitoring survival rate, provide more efficient use of land, and potentially lead to increased numbers of crops per year. On the other hand, Stern and Letellier 1992 reported increased shrimp stress in a multi-stage production system due to the transfer of animals which may cause increased mortality. Let us assume that identical mechanical oxygenation and pumping efficiencies prevail and that biological efficiencies such as growth rate, mortality rate, feed conversion ratio, and final average weight of shrimp produced are constants that do not vary in the production systems under consideration. The annual operating cost of a shrimp production system is then approximately a function of the number of shrimp produced, since the number of shrimp targeted for production will dictate the number of postlarvae required, feed and electrical costs, and other expenses. The annual operating cost per kg of shrimp produced, therefore, should not vary with the number of stages in a production system. Thus, the financial return of a multi-stage shrimp production system, given the above assumptions, must be approximately a function of land and construction costs Chong, 1990. If we further assume that construction cost is proportional to the water surface area of the specific system, we can then compare the expected financial returns of shrimp production systems by comparing their land use efficiency. By adopting the above simplifying assumptions, the problem of optimizing a multi-stage shrimp production system is reduced to the following: Min j A pr j,i = i [N j,iA j,i] 1 where: A pr j,i is total water surface area of pondstanks in phase configuration j and phase i m 2 ; N j,i is number of pondstanks in phase configuration j and phase i integer c ; A j,i is water surface area of individual pondtank in phase configuration j and phase i m 2 .

2. Shrimp size, density, growth rate and mortality

Shrimp population density affects shrimp growth and mortality rates. Sandifer et al. 1988, Ray and Chien 1992, Wyban and Sweeney 1993 concluded that shrimp growth rate is inversely proportional to stocking density. High stocking density also increases the occurrence of disease Brock, 1992, and proper stocking density is one of the measures that can be used to reduce disease occurrence LeaMaster, 1992. Fulks and Main 1992 identified high stocking density as one of the non-pathogenic factors that contributed to the disastrous disease epidemic that reduced Taiwan from the world’s leading exporter of marine shrimp to a net importer. Given a set of environmental and water management practices, such as pond tank size, water flow rate and others, a production system can support a given number of shrimp of given size. Beyond an upper biomass limit, the water quality begins to degenerate, and as the biomass increases, so does the extent and frequency of disease and the reduction in growth rate. This upper biomass limit is often called the ‘critical standing crop’ or CSC. For any given production system and set of management practices, the CSC is a function of shrimp biomass in the system. For a single-stage production system, the stocking density can be set so that the CSC density will occur at harvest time. In other words, the initial stocking density is set so that the CSC will be reached when the shrimp are ready to be harvested. In this way, the system is below the CSC level and thereby operating below maximum capacity up until the day the shrimp are harvested. In an ideal shrimp production system, the stocking density would be set so that the CSC level is reached on the first day and the pondtank would expand continuously; the CSC level would be maintained since the continuous increase in shrimp biomass would be balanced by the continuous increase in water surface area and supporting inputs. This obviously cannot be done economically. In practice, the initial stocking density is set at below the CSC level and, as the shrimp grow and the biomass reaches the CSC level, the excess shrimp biomass is transferred to another pondtank so that the CSC level in any production pondtank is not exceeded.

3. Multi-stage production system design