The ionic profile of inland low salinity water sources can be dramatically different with variable concentrations of magnesium (Mg2+). Inland shrimp farmers in Alabama, USA, supplement Mg2+ at the start of each production cycle to reach concentrations >20 mg/L at 1 to 11 g/L salinities. However, this concentration may not be high enough for larger shrimp in the later phase of the production cycle. Therefore, two field trials were conducted on a commercial shrimp farm in west Alabama to evaluate the effects of Mg2+ concentration in low salinity water on growth, survival, and physiology of Pacific white shrimp, Litopenaeus vannamei. In the first study, additional Mg2+ was supplemented to four production ponds (28 mg/L Mg2+) and compared with four control ponds (12.8 mg/L Mg2+). A stocking density of 27 post-larvae/m2 was used in ponds, and shrimp were offered a commercial feed twice daily. After a 23-week pond trial, whole-body Mg2+ concentrations were higher in shrimp reared in Mg2+ treated ponds, while no significant differences in other study variables were observed between pond groups. After week 15 (shrimp weight > 22 g), weekly body weights of shrimp were significantly lower in the control group compared to the elevated Mg2+ treatment. This could be due to size-dependent deficiency in the availability of adenosine triphosphate (ATP) and Mg2+ to support growth of larger adult shrimp. The second experiment was conducted to evaluate performance of larger shrimp (∼13 g initial weight) reared in two on-levee flow-through tank systems. One tank system received water from a control pond with 12 mg/L Mg2+, while the second system was sourced with water from a production pond with a higher amount of Mg2+ (37 mg/L). In each system, twelve 800 L tanks were stocked with shrimp at three different densities (24, 29, and 35 shrimp/m2) to evaluate possible interactions between stocking density and Mg2+ concentration in water. Following an 8-week tank trial, no significant effect of Mg2+ concentration, stocking density, or their interaction was observed on growth, survival, or physiology of shrimp. Overall, these results confirm that even moderate elevations in Mg2+ in low salinity water are advantageous to adult shrimp (>22 g) in later stages of the production cycle compared to post-larvae, juveniles, or sub-adult shrimp.
