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 of water on the growth, survival, and physiology of Pacific white shrimp, Litopenaeus vannamei.
Due to the remarkable ability of Pacific white shrimp (Litopenaeus vannamei) to tolerate a wide range of salinities, it has become a candidate of choice for low salinity culture in different production systems located far away from the coast.
Inland shrimp production using low salinity groundwater (LSW) is a common practice in many countries throughout the world, including China, Thailand, Vietnam, Ecuador, Brazil, Mexico, the United States, Israel, Australia, and many other countries.
In the US farms in Florida, Alabama, and Texas are currently using saline well water with salinities of 1–15 g/L to produce shrimp, and some of these farms have been in production for >20 years (Roy et al., 2010).
Because of consistently low survival and production obtained in recent years by commercial shrimp producers in Alabama, and doubts regarding optimal concentrations of Mg2+ and Mg:Ca ratios, farmers are curious to know if additional Mg2+ supplementation to commercial ponds might improve growth, survival, and production of shrimp reared in low salinity water while reducing late-term mortality.
The current study evaluated the effect of Mg2+ supplementation on the performance of L. vannamei reared in earthen production ponds filled with LSW.
Materials and methods
This study was conducted on a privately owned commercial shrimp farm (Greene Prairie Aquafarm; Boligee, Alabama, USA) as two separate trials during the 2021 shrimp production season. The first was a commercial-scale pond trial, while the second was a tank study, both carried out on the same farm. The farm has 23 commercial shrimp production ponds of different sizes. Eight ponds were used for the study, ranging in size from 1.09 to 1.90 ha.
To raise aqueous Mg2+ concentrations in high Mg2+ ponds (n = 4; total area = 5.02 ha) to twice the normal amount used by commercial producers, additional Mg2+ concentrations of 55.05 ± 10.85 ppm were required. As a result, the total amount of MgCl2 tech used for that purpose was 15,429 kg (617 bags × 25 kg/ bag; 3075 kg/ha) with a total cost of $10,545 US Dollars (USD; $0.68/kg; $2101/ha).
Production and growth parameters: Pond groups were not significantly different in area, stocking density, weight of stocked PLs, or length of the culture period. No significant differences were detected in growth parameters between the pond groups.
The Johnson-Neyman procedure showed no significant differences in average body weight during weeks 1 to 14 between pond groups. Still, shrimp in high Mg2+ ponds were significantly heavier than those in low Mg2+ ponds during culture weeks 15 to 21 (Figure 1).
Even though shrimp harvested from high Mg2+ ponds had numerically higher final body weights (32.13 ± 5.39 g) than shrimp harvested from low Mg2+ ponds (26.13 ± 1.79 g), they were not statistically different (p = 0.079). This is likely due to pond-to-pond variation and the small sample size (4 ponds/group).
Based on the sample size calculation, the difference in final body weight between pond groups would be statistically significant if the sample size was nine ponds per group (effect size = 1.49, power = 0.84).
“Whole-body and hemolymph ionic profile: In each of the trial months, shrimp whole-body Mg2+ concentrations in high Mg2+ ponds were significantly higher than those in low Mg2+ ponds.”
Ponds with high Mg2+ concentrations had significantly higher whole-body Mg2+ concentrations than ponds with low Mg2+ concentrations. There was a significant difference (t(30) = 166.29, p < 0.0001) in the ratio between Mg2+ concentrations of shrimp whole-body to culture water between high Mg2+ ponds (mean ± SD: 52.67 ± 13.90; range: 33.98 to 86.37) and low Mg2+ ponds (129.02 ± 19.98; 95.88 to 164.57).
There were significant differences between pond groups for the other whole-body ionic profile concentrations and hemolymph ionic profile concentrations.
Production and growth parameters: Initial shrimp body weights were significantly greater in high Mg2+ tanks than in low Mg2+ tanks. Therefore, the effect of initial body weight on all other parameters was statistically evaluated and accounted for, whenever significant, by adding it as a covariate to the statistical model.
The six Mg2+ concentration-stocking density tank groups did not show that Mg2+ concentration or stocking density influenced final body weight, survival, feed conversion ratio (FCR), weekly weight gain, or percent weight gain.
“Shrimp in low Mg2+ tanks stocked with 20 shrimp/tank had a significantly higher thermal growth coefficient (TGC) than shrimp in high Mg2+ tanks stocked with 25 shrimp/ tank, while all other tank groups were not different.”
In addition, the final biomass of shrimp in high Mg2+ tanks stocked with 30 shrimp/tank was significantly higher than that in all other tank groups, and within low Mg2+ tanks, the final biomass of shrimp in tanks stocked with 30 shrimp/tank was higher than that in tanks stocked with 20 shrimp/tank.
Regardless of stocking density, shrimp cultured in high Mg2+ tanks had significantly lower TGC (Figure 2-C) and higher final biomass (Figure 2-E) than those in high Mg2+ tanks.
Regardless of the Mg2+ concentration, tanks stocked with 30 shrimp/ tank had significantly higher final biomass than those stocked with 20 or 25 shrimp/tank (Figure 2-F).
Whole-body and hemolymph ionic profile
Whole-body Mg2+ concentrations of shrimp in all high Mg2+ tanks were higher than those in low Mg2+ tanks. Higher whole-body Mg:Ca ratios were found in shrimp reared in high Mg2+ tanks stocked with 25 shrimp/ tank compared to those in low Mg2+ tanks stocked with 20 shrimp/tank.
Shrimp stocked in tanks with 25 or 30 shrimp/tank in the low Mg2+ system had Na:K ratios significantly higher than those in the high Mg2+ system.
Hemolymph in shrimp from high Mg2+ tanks stocked with 25 or 30 shrimp/tank had significantly higher Mg2+ concentration and Mg:Ca ratio than hemolymph from shrimp in low Mg2+ tanks stocked with 20 shrimp/ tank.
Inland low salinity aquifers that provide water for Pacific white shrimp in earthen ponds in west Alabama have variable Mg2+ concentrations that are extremely deficient on most farms.
Therefore, farmers need to add magnesium salt (typically K-Mag®, a commercial grade potassium magnesium sulfate) to raise the Mg2+ concentration in pond water. However, specific Mg2+ requirements are still not well established. Hence, concentration in seawater diluted proportionately to a given salinity is assumed to be the safest reference value to attain optimal growth and survival of L. vannamei (Boyd, 2018).
During the study, no differences were detected in performance parameters or physiological variables of shrimp between the control (Mg2+ = 12.9 ± 4.0 mg/L) and Mg2+ treatment (Mg2+ = 28.1 ± 22.8 mg/L), except for significantly higher whole-body Mg2+ concentrations in shrimp reared in elevated Mg2+ concentrations.
“The reference Mg2+ concentration in seawater at the tested salinity (2.1 g/L) is 82.2 mg/L.”
Hence, Mg2+ concentrations attained in control and treatment culture water in this trial were both suboptimal, being lower than 50% of what Mg2+ should be at the salinity in which the experiment was carried out.
The growth performance of shrimp observed during the current study agrees with observations from Galkanda-Arachchige et al. (2021), which revealed that elevated Mg2+ concentrations in ponds were not high enough to yield significantly higher growth in shrimp compared to the control since Mg2+ concentrations are below the optimal range.
In parallel, insignificant differences in the final weights of Pacific white shrimp reared in low salinity (4 g/L) waters containing various concentrations of Mg2+ were documented by Zacarias et al. (2019). This is assumed to be due to no Mg2+ deficiency in test treatments (167 to 205 mg/L) compared to concentrations of Mg2+ in diluted seawater (~156 mg/L) at the respective salinities.
Findings from the 8-week on-levee tank experiment confirmed that there were no significant main effects or interactions of Mg2+ concentration (12 and 37 mg/L) or stocking density (24, 29, and 35 shrimp/m2 ) on growth performance, survival, FCR, hemolymph osmolality, and osmoregulatory capacity of shrimp.
Though it is not statistically significant, there was a numerically lower growth of shrimp in the control group with lower Mg2+ concentrations. This could be due to the extra energy expenditure needed to maintain osmoregulation or a size-dependent deficiency in Mg2+ bioavailability to support the molting mechanisms of adult shrimp in low salinity waters.
“In conclusion, the stocking densities that we tested did not negatively impact the growth performance of shrimp reared at suboptimal Mg2+ concentration. In addition, the benefits of Mg2+ supplementation in low salinity shrimp production systems with suboptimal Mg2+ concentrations have been confirmed.”
Together, this suggests that commercial shrimp producers in west Alabama using inland low salinity waters will likely continue to face challenges late in the production cycle due to low Mg2+ concentrations in production ponds and should closely monitor aqueous Mg2+, particularly late in the production cycle.
This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “EVALUATION OF AQUEOUS MAGNESIUM CONCENTRATION ON PERFORMANCE OF PACIFIC WHITE SHRIMP (LITOPENAEUS VANNAMEI) CULTURED IN LOW SALINITY WATER OF WEST ALABAMA, USA” developed by: Hernandez, D. – Alabama Fish Farming Center and Auburn University, Abdelrahman, H. – Cairo University, Alabama Fish Farming Center, Galkanda, H. -Arachchige – Wayamba University of Sri Lanka, Kelly, A. – Alabama Fish Farming Center, Butts, I. – Auburn University, Davis, D. -Auburn University, Beck, B. – US Department of Agriculture, Roy, L. – Alabama Fish Farming Center and Auburn University.
The original article, including tables and figures, was published on DECEMBER,, 2022, through AQUACULTURE.
The full version can be accessed online through this link: https://doi.org/10.1016/j.aquaculture.2022.739133