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

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.

Results

Pond trial

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.

Tank trial

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.

Discussion

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