Supplementation of Chelated Magnesium to Diets of the Pacific White Shrimp, Litopenaeus vannamei, Reared in Low-salinity Waters of West Alabama LUKE A. ROY , D. ALLEN DAVIS 1 , AND TRI N. NGUYEN Department of Fisheries and Allied Aquacultures, Auburn University, Alabama 36849-5419 USA IMAD P ATRICK SAOUD Department of Biology, Bliss Street, American University of Beirut, Beirut, Lebanon Abstract Shrimp farmers using inland low-salinity waters in west Alabama have traditionally used agricultural fertilizers (K-MagÒ, muriate of potash) to raise pond water levels of potassium (K) and magnesium (Mg) to improve the rearing medium for Litopenaeus vannamei. Laboratory and farm trials were performed to investigate the potential of using dietary supplementation of Mg instead of costly agricultural fertilizers. A 5-wk laboratory trial was devised to test four diets with varying levels of Mg supplementation (0, 0.15, 0.30, and 0.60% Mg chelate) using a magnesium chelate (MgC)–amino acid complex. Juvenile shrimp were stocked into artificial low-salinity water (5 ppt) designed to contain low levels of Mg. A farm trial was also conducted to test the same diets under field conditions. Although both laboratory and on-farm trials revealed a trend for increased growth using the diet with the highest Mg supplement, results were not significantly different. The use of magnesium chelates as dietary supplements at levels higher than the requirement level to enhance survival, growth, and osmoregulatory capacity of shrimp reared in inland low-salinity well waters appeared to have limited potential. Until effective specialized diet formulations are produced, farmers should continue to supplement pond waters with fertilizers containing K and Mg. Farmers of the Pacific white shrimp, Litope- naeus vannamei, using inland low-salinity well waters (LSWWs) are faced with the challenge of rearing animals in less than ideal environ- ments. Depending on their source, inland waters available to culture shrimp can possess variable ionic compositions (K, Mg, and Ca levels) and salinity (Boyd and Thunjai 2003; Saoud et al. 2003). Despite the ability of L. vannamei to tol- erate a wide range of environmental salinities, farmers still have to address problems associ- ated with variation of ionic profiles among pond waters (Saoud et al. 2003) that can lead to poor growth and survival of shrimp (Davis et al. 2005; Roy et al. 2007a). Low-salinity problems can generally be solved through addition of specific ions (e.g., K and Mg) to culture water (McGraw and Scarpa 2003; Roy et al. 2007a). Davis et al. (2005) reported that under controlled laboratory conditions, K defi- ciencies of the water are remediable through potassium chloride supplementation. Commer- cial farmers using inland LSWW are mitigating this problem by increasing levels of K and Mg in their pond waters through addition of muriate of potash and/or K-MagÒ (McNevin et al. 2004). Unfortunately, adding large amounts of agricul- tural fertilizers to ponds can be costly. Treated water may be discarded during harvest or lost in overflow during the rainy season. Furthermore, ions can be complexed with the pond soil, thus reducing bioavailability to shrimp (Boyd 2007). For good culture conditions, the ionic profile of the low-salinity water must have appropriate levels and ratios of specific ions (Na : K, Mg : Ca, etc.), which should be similar to that of full-strength seawater that, for example, would be 28:1 for the Na : K ratio (Roy et al. 2007a). It has been reported that low-salinity water defi- cient in Mg resulted in higher respiration rates 1 Corresponding author. JOURNAL OF THE WORLD AQUACULTURE SOCIETY Vol. 40, No. 2 April, 2009 Ó Copyright by the World Aquaculture Society 2009 248