Influence of water volume, surface area, and water replacement rate on weight gain of juvenile freshwater prawns, Macrobrachium rosenbergii

Abstract

The effects of combinations of water volume, bottom surface area, and water replacement rate on weight gain of juvenile freshwater prawns, Macrobrachium rosenbergii, were evaluated in two experiments. Prawns were fed a semi-purified diet and individually cultured in clear plastic containers, which were circular and slightly tapered. Each container received an independent water supply in a flow-through system. This design was used to evaluate weight gain in the absence of potentially confounding effects of tactile, chemical and visual stimuli. The first experiment was a 2×2×2 factorial design consisting of two levels of each of the three factors, water volume (1375 and 700 ml), bottom surface area (70.9 and 35.4 cm 2), and water replacement rate (86 and 44 min). After 60 days, weight gain of juvenile prawns (initially 221.8 to 227.0 mg) was significantly greater (232%) under conditions of both greater water volume and bottom surface area. Higher water replacement did not compensate for lower water volume. There was significantly more weight gain (149%) with greater bottom surface area at the lower rate of water replacement. However, at the higher rate of water replacement, greater bottom surface area did not result in a significant increase in weight gain. The second experiment was designed to determine at what point the reduction in growth identified in the first experiment became operative. It consisted of two treatments in which weight gain at low water volume and bottom surface area (750 ml and 78.5 cm 2) was compared to that at high water volume and bottom surface area (1300 ml and 147.4 cm 2) at equal rates of water replacement. Mean stocking weight of the prawns was 7.9 mg and the duration of the experiment was 143 days. Reductions in weight gain began to occur when a critical biomass density of approximately 500 mg/l was attained as reflected in significant differences in weight-frequency distributions between treatments at 90 days and thereafter. The response also reflects a previously identified relationship between maximum density and body size for aquatic animals. A culture volume that allows for maximum, density-independent growth of animals for all treatments is needed for experiments in which growth (weight gain) of crustaceans is used as a response variable to compare performance of different diets. Knowledge of the biomass density that causes growth reduction can be practically applied to establish selective harvest or stock manipulation strategies whereby overall production per unit of time or space can be increased. Alternatively, knowledge of this density threshold can be used to reduce growth rates and thereby assist in distributing production through time.