Salinity basics — ppt, refractometers, hydrometers
Salinity is measured in parts per thousand (ppt). Refractometers are accurate to ±1 ppt and read reliably from 0–100 ppt — the standard field tool. Hydrometers drift with temperature; correct with a temperature reference chart or replace with a refractometer.
Species-safe ranges
- Vannamei: 0.5–40 ppt tolerated; 10–25 ppt optimal.
- Monodon: 5–35 ppt; 15–25 ppt optimal.
- Tilapia: 0–15 ppt; grows fastest at 0–5 ppt (freshwater).
- Milkfish: broad euryhaline; 15–35 ppt in grow-out ponds.
PL acclimation protocol
Match hatchery salinity to pond salinity by drip acclimation over 4–6 hours in a shaded acclimation tank. Aim for ≤2 ppt drop per 30 minutes. Include a temperature acclimation step (≤1 °C per 15 minutes) alongside salinity.
Monsoon dilution and rainfall response
A 100 mm rainfall event drops pond salinity by 3–5 ppt in the surface layer within hours. Turn on aeration to mix before stratification sets in. Post-monsoon PL stocking should match observed pond salinity, not the label salinity from the pre-monsoon plan.
Topping up: seawater vs freshwater
In coastal ponds, top up with filtered seawater to counter dilution. Inland low-salinity farms use bore water or municipal water; test the source for TDS, calcium and magnesium before use.
Daily monitoring routine
Measure salinity daily at mid-morning (10:00) at 30 cm depth. Log with pH and temperature — the three move together after weather events and reveal what the pond is doing before the animals show signs.