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

Abstract

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.