EFFECT OF MAGNESIUM TO CALCIUM RATIO IN LOW SALINITY WATER ON GROWTH OF PACIFIC WHITE SHRIMP Litopenaeus vannamei

Ionic composition of water is a vital factor for shrimp growth and survival and can be more crucial than the salinity itself, since deficiencies in certain ions such as sodium (Na), potassium (K), calcium (Ca) and magnesium (Mg) lead to negative impacts on growth and survival of shrimp. Remarkable variability in ionic compositions of low salinity ground waters around the world emphasize the need to elucidate optimum ionic concentrations and ratios for shrimp production.  Magnesium has been identified as one of the key cations for the survival of Pacific white shrimp reared under low salinity conditions. However, in west Alabama water used for shrimp production, the availability of Mg is low relative to Ca which is the opposite of full strength or diluted sea water. Therefore, farmers are required to add an additional Mg source to remediate Mg:Ca ionic ratios (hereafter Mg:Ca ratio) in pond water which is costly.  Even after Mg is added to shrimp ponds in west Alabama the ionic ratios do not closely mirror the Mg:Ca ratio of seawater (Mg:Ca ratios of 0.5 - 1.0 or lower are quite common). A study was carried out to determine the effect of different Mg:Ca ratios in low salinity (3 g/L) water on growth and survival of Pacific white shrimp. A Low-cost salt mixture (LCSM) was used (comprised of Na, K, Mg, Ca concentrations of 298, 9, 17, and 39 mg/L, respectively in a 1-g/L solution, which is closely comparable to the major cations in 1-g/L dilute seawater) to formulate waters with different Mg:Ca  ratios (2.7:1, 2:1, 1.3:1, 0.7:1, 0.3:1, 0.2:1 and 0.1:1). Reconstituted sea salt (RSS) was used as a control treatment with an Mg:Ca ratio of 3.2, which is similar to sea water. Different Mg:Ca ratios were achieved by gradually decreasing the Mg level of LCSM, while keeping the Ca level constant. A growth trial with Pacific white shrimp (initial weight = 0.11 ± 0.01g and stocking density = 20 shrimp/tank) was carried out in 150 L plastic tanks, each equipped with a miniature fluidized bed bio-filter. Shrimp were fed four times per day using a standard ration over a 42-day growth trial. Results indicate a positive relationship between growth performance of shrimp and Mg:Ca ratios of low salinity water while no significant effect was observed on shrimp survival (Figure 1).