Minerals for shrimp farmed at low salinity: balancing potassium, magnesium and calcium by salinity

Whiteleg shrimp can survive at very low salinity, so many areas farm them in low-salinity brackish water or groundwater, or during the rainy season when salinity drops sharply. The problem is that at low salinity, the ratio of the major minerals in the water is often skewed compared with seawater, particularly with a shortage of potassium (K) and magnesium (Mg). Shrimp may suffer scattered mortality, soft shell and slow growth even when alkalinity and pH are normal.

The article how to apply minerals for shrimp explained the timing and use of pond-applied and feed-mixed minerals. This article goes deeper into ionic balance when farming at low salinity: how much is enough and which minerals to prioritise.

Why are low-salinity ponds prone to mineral imbalance?

Potassium is the main cation inside cells and is needed to activate the Na⁺/K⁺-ATPase enzyme, which helps shrimp regulate osmotic pressure. Magnesium is the second most abundant cation in cells and a cofactor in many enzyme reactions; in crustaceans, the Na⁺/K⁺-ATPase enzyme does not work without Mg²⁺ (Global Seafood Alliance, 2019).

According to Professor Claude E. Boyd (Auburn University), ion concentrations in low-salinity water are not simply those of diluted seawater. They depend on the soil type, the geology the water flows through and the climate. So two ponds both at 5 ppt salinity can have very different potassium levels.

Two examples he gives:

• A pond in Ecuador at 8 ppt salinity had normal calcium, magnesium and sodium but potassium below 1 mg/L, whereas seawater diluted to 8 ppt should contain about 90 mg/L. Postlarvae could not survive until potassium was raised to 50 mg/L.
• A farm in Alabama (USA) at about 2 ppt salinity, with potassium around 6 mg/L, had heavy shrimp mortality in the first month. Raising potassium to 50 mg/L stopped the mortality.

Another article by Boyd (2003) records an Alabama farm at 2–4 ppt salinity with potassium and magnesium below 10 mg/L: in 2001 survival was only 19% and yield 595 kg/ha. In 2002, after adding salts to raise potassium to 35–40 mg/L and magnesium to about 20 mg/L, survival reached 67% and the average yield was 4,068 kg/ha.

Seawater mineral ratios as the "reference standard"

Normal seawater (about 34.5 ppt) contains on average: sodium 10,500 mg/L, magnesium 1,350 mg/L, calcium 400 mg/L, potassium 380 mg/L. By weight, the ratio Na : Mg : Ca : K ≈ 27 : 3 : 1 : 1. So in seawater, magnesium is about 3 times calcium, while potassium is roughly equal to calcium.

To estimate seawater-equivalent concentrations at your pond's salinity, Boyd provides multiplication factors:

Example in Boyd's article: at 2 ppt salinity, the seawater-equivalent level is about 22 mg/L potassium and 78 mg/L magnesium.

These are reference levels, not mandatory minimums. Boyd notes that the minimum concentrations of the major ions for shrimp have not been firmly established, and shrimp still live normally across a fairly wide range of ion levels.

Prioritise potassium first, magnesium second

According to Boyd (2018):

• Potassium is usually more important than magnesium. He suggests initially treating only potassium, since magnesium supplementation is usually expensive and the benefit is not necessarily proportionate.
• A potassium level of 20–30 mg/L is probably sufficient; to extend the interval between applications it can be raised to 50–75 mg/L.
• Check potassium monthly. Potassium and magnesium decline gradually after application because of adsorption by bottom soil, potassium being retained in clay minerals, dilution by rain and overflow. Ponds usually need 2–3 applications in a 100–160-day crop.

For magnesium, one Alabama farm at about 2.5 ppt salinity achieved good survival and yield by keeping magnesium at 10–30 mg/L, because raising it to the seawater-equivalent level (about 100 mg/L) is too expensive (Boyd, 2015).

Do not forget alkalinity and calcium

Alongside ionic balance, pond water needs bicarbonate above 60 mg/L (equivalent to total alkalinity above 50 mg/L CaCO₃) and calcium hardness at least equal to total alkalinity (Boyd, 2018). This is a minimum; intensive farms usually keep alkalinity at 120–180 mg/L CaCO₃ (150–200 mg/L in high-density lined ponds). Agricultural lime helps raise alkalinity and hardness but adds only small amounts of calcium and magnesium, so it is not used to correct ionic imbalance. To raise calcium, gypsum (CaSO₄·2H₂O) can be used: about 1.72 mg/L of gypsum raises calcium hardness by 1 mg/L. See also pH and alkalinity management.

Commonly used mineral salts

According to Boyd (2003), each 1 g of potassium chloride per 1 m³ of water raises potassium by about 0.5 mg/L. To raise potassium by 25 mg/L you need about 50 g of KCl per m³ of pond water. Dissolve it first, then broadcast it in front of the paddlewheel aerators so the salt disperses evenly.

4-step procedure for low-salinity ponds

1. Send a water sample for analysis of Ca, Mg, K, Na and alkalinity before stocking; do not guess from salinity.
2. Compare with seawater-equivalent levels using the factor table above, paying attention to the Mg being about 3 times Ca.
3. Supplement potassium first if it is deficient, then consider magnesium and calcium.
4. Recheck monthly and after heavy rain, and reapply when needed.

Besides minerals in the water, minerals mixed into feed also play a role. A trial in Ecuador in 5 ppt ponds showed that mineral-supplemented feed with a higher Mg:K ratio gave significantly higher yield and final weight than the control feed (Global Seafood Alliance, 2019). Davis and Saoud (2004) also reported that farms in Alabama that applied fertilisers rich in potassium and magnesium raised survival from about 25% to 56%.

Supporting Noben products

When choosing minerals for a low-salinity pond, farmers should check the Ca, Mg and K contents stated on the label carefully and compare them with the water analysis results. Noben mineral products, dosed according to the label:

• KALIMIX: shell-hardening mineral that builds a thick shell, designed for lined ponds.
• MIRAL N8: alkalinity-raising mineral that stabilises alkalinity and hardens the shell.
• MIX 79: alkalinity-raising mineral that builds a thick shell and helps prevent bent body and muscle necrosis (white muscle).
• CAPHOTE: feed-mixed trace and macro mineral that helps harden the shell and build a thick shell.

You can use the dose calculator to convert the label dose into the amount for each pond.

References

• Treating ionic imbalance in low-salinity shrimp ponds (translated from an article by C. E. Boyd) (in Vietnamese) – Department of Agriculture and Rural Development of Quang Ninh, 2018
• Revisiting ionic imbalance in low-salinity shrimp aquaculture – Global Seafood Alliance (C. E. Boyd), 2018
• Mineral salts correct ionic imbalances in culture water – Global Seafood Alliance (C. E. Boyd), 2003
• Calcium and magnesium use in aquaculture – Global Seafood Alliance (C. E. Boyd), 2015
• Nutritional considerations for Pacific white shrimp reared in inland low-salinity waters – Global Seafood Alliance (D. A. Davis, I. P. Saoud), 2004
• Improving the osmoregulatory capacity of Pacific white shrimp grown in low salinity – Global Seafood Alliance, 2019