Effects of Stocking Density Fluctuations on Benthic Habitat Conditions in Freshwater Aquaculture Systems

Stocking density is one of the major managerial tools in pond aquaculture; however, the actual density of organisms is not static throughout a crop due to death, harvesting, and restocking, and this variability could impact on the environment of the sediments inhabited by the benthic community. In this research project, it was determined how nominal stocking density and the actual mortality-related fluctuation in stocking density impact the sediment and bottom water in an earthen pond aquaculture system. The re-analyzed a publicly accessible experimental data set where green mud crabs (Scylla paramamosain) were stocked at 1, 1.5, 2 and 3 crab m⁻² in replicated tanks over 101 days. Survival decreased exponentially with increasing density (r² = 0.98), total production dropped 31% with stocking rates tripling, and feed conversion efficiency went up from 5.2 to 21.7. Nitrite concentration of sediments increased 52% over the range of minimum and maximum density and correlated with feed loading (r = 0.96), while organic matter and sediment oxygen demand remained equal at different densities, and organic matter exceeded the normal range of 1–3% for all densities. Oxygen levels in the bottom water decreased from 4.2 to 2.8 mg/L throughout the culture period. These data suggest that loading with feed and dead fish due to density is what changes the nitrogen status in sediments, not organic matter.