TOWARDS ESTABLISHING ENVIRONMENTAL MINERAL GUIDELINES FOR LOW SALINITY SHRIMP CULTURE IN FLORIDA  

Research at HBOI in the 1990s indicated that Pacific white shrimp Penaeus vannamei could be cultured in near freshwater. Since that time, groups in Florida and the Southeast U.S. have undertaken low-salinity culture of marine shrimp; however, unexplained low survivals still persist. Early research focused on the minimum concentration of single mineral ions, but it was soon realized that ratios (e.g., sodium:potassium) were just as important. Most recommended ratios are based on ideal  ionic ratios found in full-strength seawater. Only a few ionic ratios have been examined and fewer reports indicate the acceptable ratio range. A series of short-term (5-7 days) survival experiments were conducted with P. vannamei postlarvae to establish ion concentration and ratio ranges for calcium (Ca), magnesium (Mg), and potassium (K) alone and in combination with each other at low-salinity (i.e., 2).  Experiments were conducted with 12-20 day old P. vannamei postlarvae (PLs). PLs were acclimated for three days to the control salinity of two prepared with Instant Ocean™. Following acclimation, 10 PLs (0.005-0.025 g/PL, dependent on experiment) were placed into plastic containers containing 4L of the appropriate experimental treatment solution (n=4 or 5 reps/trt) and supplied with light aeration. Shrimp were fed once per day beginning on day two. Survival was assessed at the end of each experiment.

Survival data from single ion experiments indicated minimum and maximum concentrations for Ca (6-12 &150-300 mg/L), Mg (18.7-37.5 & >600 mg/L), and K (6-12 & 48-72 mg/L). Survival data from the double and triple ion experiments revealed interactions, such as whether a high concentration of one ion could make up for a deficiency in another ion. For example, increasing the concentration of Mg (600 mg/L) increases survival when K is low (6 mg/L), but does not improve survival when Ca is low (6 mg/L). Increasing the concentration of Ca (150 mg/L) increases survival when either K or Mg is low (9 mg/L), but the benefit is lost at a Ca concentration of 300 mg/L. Increasing the concentration of K (38.5-77 mg/L) does not increase survival when either Mg or Ca is low (10, 6 mg/L). However, good survival (>80%) was seen in the following profiles that deviated with respect to the ideal: a) Semi-low (½ - ¾ of "ideal") ionic profile of one, two or all three ions; b) High Ca and high Mg (156/300 and 360/192 mg/L Ca/Mg), but higher Mg is preferable to higher Ca; and c) High Ca with low K and Mg (½ - ¾ of "ideal") as long as Ca does not exceed 150 mg/L.

These results suggest that the interplay of Ca, Mg, and K are more complicated then has been suggested.  Farmers should take a careful look at the ionic profile of their water, but there is a wide range from "ideal" concentrations and ratios that can result in good survival.  Further research on growth in non-ideal ionic ratio water is needed to support the reported survival data. This project was supported by the Florida Aquaculture Research Council and the HBOI Specialty License Plate program.