Nitrogen Budget and Water Quality Management in Larviculture Ponds of the Swimming Crab

ABSTRACT The objectives of the present study are to determine the cycle and budget of nitrogen in larviculture ponds of the swimming crab Portunus rrituberculatus and to evaluate their impact on water quality and survival rate. Field surveys were carried out at the Hiroshima Prefecture Fish Farming Center from 1990 to 1993. Among nitrogenous compounds. concentrations of NH -N and particulate organic nitrogen (PON) showed peaks just before the mass exchange ofpond water, when the main component of sedimentary material changed from phytoplanktonic organic matter to nonphytoplanktonic organic matter. As a result of N budget analysis, it was made clear that the main factors enhancing N concentration in the pond are the input of phytoplankton culture medium and feed in Phase I, while only feed contributed in Phase II. High survival rate is supported both by low input and low concentration of N, especially of NH -N, when high conversion rate of feed-N to crab larvae-N was attained. Then, variation !n the individual nitrogenous compound per unit time per pond (gNd-’ pond -‘) revealed that feed, sedimentation and water discharge are the main factors affecting the variation of NH -N. Input of concomitant NH -N in “green water” (culture of green algae) also signifka&ly contributed to the increasesf NH -N concentration in the pond. As practical measures for improving the culture system, reduction of phytoplankton load and feeding load to the level which causes NH -N concentration below 34 pg at 1-l are proposed. These measures will improve not only tffe survival rate and the cost of crab seed production but also the water quality of pond water. Thus. the environmental impact of effluent water will be significantly minimized.