Use of molasses as carbon source in limited discharge nursery and grow-out systems for Litopenaeus vannamei

Increased environmental regulations and loss of crops due to viral disease outbreaks have created a demand for productive, pathogen-free, efficient and sustainable shrimp production practices. These methods, for the most part, call for raising shrimp under limited, minimal, water exchange with careful monitoring of water quality. Two studies were conducted using the Pacific white shrimp Litopenaeus vannamei to evaluate the effect of carbon supplementation on selected water quality indicators and shrimp performance in the nursery and the grow-out phases using the limited water discharge practice with molasses as carbon addition. A preliminary nursery study was conducted in four 40 m(3) greenhouse-enclosed raceways stocked with 12-day old postlarvae at a density of 1800 m(-3) and were fed 45% crude protein feed. Molasses (24% carbon, w/w) was used for carbon supplementation to provide 6 g of carbon for each I g of total ammonia nitrogen (TAN) as measured in the water, and started 32 d after stocking, shortly after the TAN level has risen over 3 mg L-1. The data suggest that molasses addition resulted in rapid reduction of TAN and nitrite-nitrogen and could be used to prevent buildup of these compounds. The grow-out study was carried out using twenty 7.8 m(3) tanks stocked with juvenile shrimp at a density of 81 m(-3). Shrimp were fed a commercial diet (30% crude protein, Rangen Inc., Buhl, ID). Feed rations were determined based on assumed growth of 1 g week(-1), FCR of 1.5 and weekly mortality rate of 0.5%. Molasses additions, at five levels, were started 26 days post stocking, to provide 0%, 50%, 100%, and 150% of the theoretical carbon level needed for bacteria to assimilate the TAN provided in the feed. The fifth treatment was based on the actual level of TAN in the tanks and was adjusted daily on as needed basis. The data suggest that under the condition of this study, addition of molasses did not result in a significant effect on water quality or on shrimp performance. (c) 2006 Elsevier B.V. All rights reserved.