Effects of two commercially available low-protein diets (21% and 31%) on water and sediment quality, and on the production of Litopenaeus vannamei in an outdoor tank system with limited water discharge

Effects of feeding Litopenaeus vannamei with commercial low-protein diets on shrimp performance and on water and sediment quality were studied under high shrimp density with limited water discharge. Twelve tanks (each 10.5 m(2)) were stocked with juvenile shrimp (average weight, 1.69 g) at a density of 40/m(2). Shrimp in six tanks were fed a 21% protein diet, and shrimp in the other six tanks were fed a 31% protein diet. Selected water quality parameters were monitored over a 94-day period. Shrimp growth, survival, feed conversion ratio (FCR) and selected sediment quality parameters were evaluated at harvest. Shrimp survival was significantly higher (P = 0.041) in the 31% treatment (96.2%) than in the 21% treatment (90.6%). Mean final weight of the shrimp in the 31% treatment (14.04 g) also was significantly higher (P < 0.001) than in the 21% treatment (12.17 g). The FCR of the shrimp in the 31% treatment (1.75) was significantly lower (P>0.001) than in the 21% treatment (2.15). The gross production of the 31% treatment (540 g/m(2)) was 22% higher than that of the 21% protein diet (441 g/m(2)). No significant differences were found between the 21 and 31% treatments in the daily-measured water quality parameters (dissolved oxygen, temperature, pH, Secchi readings, and salinity) and the weekly-measured parameters (total ammonia-N, nitrite-N, nitrate-N, reactive and total phosphorus, 5-day carbonaceous biochemical oxygen demand (cBOD(5)), chemical oxygen demand (COD), total suspended solids and volatile suspended solids (VSS)). Differences in sediment quality parameters (volume of sediment, COD, cBOD(5) and VSS) between treatments were not statistically significant. This work shows that the 31% protein diet provided better production results than the 21% protein diet, yet it did not significantly deteriorate the water or sediment quality. (C) 2001 Elsevier Science B.V. All rights reserved.