Chelated Potassium and Arginine Supplementation in Diets of Pacific White Shrimp Reared in Low-Salinity Waters of West Alabama

—Dietary supplements have been proposed as a potential remediation strategy to counteract mineral deficiencies in low-salinity well waters (LSWW) used for shrimp culture in Alabama. Existing strategies (i.e., application of fertilizers such as K-mag and muriate of potash) are costly to farmers attempting to raise levels of potassium (Kþ) in their ponds. Previous laboratory studies using dietary supplements of chelated Kþ increased growth but not survival of Pacific white shrimp Litopenaeus vannamei reared in lowsalinity water. To determine whether this approach is applicable in the field, two growth trials were conducted in flow-through outdoor tanks that used water from farm ponds. The first trial (6 weeks) was conducted simultaneously at two separate low-salinity farms in west Alabama and evaluated two practical diets, one with a chelated Kþ supplement and one without. One of these farms supplements Kþ to its water and is considered a low-stress environment (LSE), while the other farm does not supplement its water and is considered a highstress environment (HSE). Results indicated that dietary supplementation of Kþ (1.0%) in the absence of an appropriate ionic profile (i.e., the HSE) failed to enhance growth or survival of Pacific white shrimp. Another 9-week growth trial was conducted at the LSE farm. Shrimp were offered either a practical basal diet or the basal diet supplemented with 0.5% chelated Kþ, 1.0% chelated Kþ, or 0.4% arginine. Although no statistically significant effects of chelated Kþ supplementation were observed, results of this experiment and two other experiments showed an increase in growth associated with chelated Kþ supplementation, indicating a marginal benefit. Present results do not justify Kþ supplementation of diets for Pacific white shrimp reared in inland LSWW. Better shrimp survival and growth are observed when Kþ is added to culture waters to mitigate imbalances in Na:K and Cl:K ratios. Inland shrimp farming in low-salinity well waters (LSWW) is a growing industry in the United States and various other countries around the world (Boyd and Thunjai 2003; Saoud et al. 2003; McNevin et al. 2004). Inland aquaculture of marine species is desirable for many reasons, notably biosecurity, reduced property prices, and availability of saline waters that are unsuitable for traditional agriculture (Hopkins et al. 1996). However, previous research showed that inland well waters were generally very low in salinity and had ionic compositions that were significantly different from those of seawater (Boyd and Thunjai 2003; Saoud et al. 2003). Moreover, potassium (Kþ) was deficient in the majority of these waters (Saoud et al. 2003; McGraw and Scarpa 2003; McNevin et al. 2004). Low-salinity problems can be solved in many cases through addition of specific ions to culture water. Under controlled laboratory settings, Kþ deficiencies of the water were shown to be remediable through the addition of KCl to culture waters (Davis et al. 2005). Commercial aquaculturists using inland LSWW are mitigating the problem by increasing the Kþ levels in pond waters through addition of muriate of potash, KMagt, or both (McNevin et al. 2004). Unfortunately, adding large amounts of salt or Kþ compounds to ponds is costly, especially since treated water may be discarded during harvest or lost in overflow during the rainy season. Nutritional supplements that improve the osmoregulatory capacity of shrimp have been proposed as another method of mitigating low-salinity water (Gong et al. 2004; Roy et al. 2006). Arginine could be one such supplement since it is easily phosphorylated as a high-energy derivative controlling cell content of ATP (Schoffeniels 1970), which is necessary for driving the Naþ–Kþ ATPase pump during osmoregulation (Lucu and Towle 2003). Gong et al. (2004) and Saoud and Davis (2005) proposed dietary supplementation of Kþ and energy compounds to remedy low concentrations in culture waters. Addition of Kþ in the form of chelated Kþ to diets of shrimp reared in lowsalinity waters in the laboratory improved growth (Roy et al. 2007). In this experiment, we evaluated the effects of supplementation with arginine and Kþ (chelated to an amino acid complex) in practical diets * Corresponding author: [email protected] 1 Present address: Department of Biology, American University of Beirut, Beirut, Lebanon. Received June 21, 2006; accepted December 8, 2006 Published online June 28, 2007 265 North American Journal of Aquaculture 69:265–270, 2007 Copyright by the American Fisheries Society 2007 DOI: 10.1577/A06-045.1 [Article]