Evaluation of aqueous magnesium concentration on performance of Pacific white shrimp (Litopenaeus vannamei) cultured in low salinity water of West Alabama, USA

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/m(2) 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 (similar to 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/m(2)) 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 pro-duction cycle compared to post-larvae, juveniles, or sub-adult shrimp.