Shrimp rearing: stocking density, growth, impact on sediment, waste output and their relationships studied through the nitrogen budget in rearing ponds

Abstract

A quantitative and qualitative study of solid and dissolved waste output was carried out in relation to stocking density, through the study of nitrogen budgets in Penaeus stylirostris rearing ponds. Stocking densities ranged from 1 to 30 shrimp m −2. The nitrogen budget was determined, taking into account the different forms of nitrogen in the water (particulate, dissolved inorganic, dissolved organic), in the sediment, in the feed pellets, and in shrimps. Relationships were established between growout, stocking density, quantities of waste generated in ponds, characteristics of outflow and of sediment. Shrimp reared at different stocking densities showed different growth performances. At the end of the rearing cycle, shrimp reared at 1 m −2 and at 30 m −2 weighed 39.2±4.3 g and 16.8±2.4 g respectively. The quantity of wastes generated was proportional to the stocking density. Rearing one kg shrimp generated 60.8 and 157.2 g waste-N (dissolved+solid), at 1 m −2 and 30 m −2 respectively. The accumulation of waste-N in the sediment increased with stocking density. At the lowest stocking density, the sediment acted as a `supplier' and not as an accumulator of nitrogen. The nitrogen budget shows that up to 38.4% of the nitrogen entering the ponds in the inflow and in feed pellets, may accumulate in the sediment. The calculated outputs due to diffusion to the atmosphere and to denitrification account for 9.7 to 32.4% of the total. An increase in the level of the pond floors during the rearing period, by from 0.50±0.28 to 4.54±0.73 cm, appears to have been related neither to input and sedimentation of solids (seston or feed pellets), nor to hydraulic erosion of the pond banks, but to an erosion produced by the swimming activity of the shrimps.