Ammonia Crisis in a Mid-Cycle Shrimp Pond
At day 45, a 1.0 ha HDPE-lined Litopenaeus vannamei pond stocked at 120 PL/m² (18 ppt, 28–31 °C) shows a dawn dissolved oxygen of 3.2 mg/L despite 12 HP/ha of paddlewheel aeration, pH 7.8 at dawn rising to 8.6 by 15:00, total ammonia nitrogen 4.5 mg/L, nitrite-N 0.8 mg/L and alkalinity 90 mg/L as CaCO3. Feeding is 180 kg/day split into four meals. Water exchange has been cut to 2%/day because the source creek is low and turbid. On the second morning, several hundred moribund shrimp are found along the pond edge, feeding response at the trays has dropped sharply, and Vibrio counts from hepatopancreas swabs on TCBS are 10^4 CFU/g. The farmer proposes spreading 300 kg of agricultural lime and adding molasses to 'fix the ammonia'.
1. Assess the relative contribution of oxygen, ammonia and pH to the mortality, supporting your judgement with a calculation.
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Ammonia is the acute driver, and pH is what makes it acute.
Calculate the un-ionised fraction. At 28 °C and 18 ppt the pKa for ammonia is close to 9.3. At the afternoon pH of 8.6: fraction NH3 = 1 / (1 + 10^(pKa − pH)) = 1 / (1 + 10^0.7) = 1/6.01 ≈ 0.17. So NH3-N ≈ 4.5 × 0.17 ≈ 0.75 mg/L, roughly an order of magnitude above the ~0.05–0.1 mg/L at which penaeid growth and survival are impaired. At the dawn pH of 7.8 the same TAN gives only ~0.03 mg/L NH3-N, which is why the shrimp look worst in mid-afternoon and why the farmer's 'dawn' reading is misleading.
Oxygen is a compounding stress, not the primary cause. At 3.2 mg/L the shrimp are below the ~4 mg/L needed for comfortable feeding, which explains the tray response, but L. vannamei tolerate this for hours, not enough on its own to kill hundreds.
Nitrite at 0.8 mg/L is secondary but adds methaemoglobin-type stress, and the 10^4 CFU/g Vibrio finding indicates opportunistic invasion of already-stressed animals rather than a primary pathogen.
The danger in the lime proposal is direct: 300 kg of agricultural lime in a 1 ha pond at 1 m depth (300 mg/L) will push pH up, shifting more of the same TAN into the toxic NH3 form and converting a bad afternoon into a lethal one.
2. Set out the actions you would take in the first 48 hours, in priority order, and justify the sequence.
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Hour 0–6: stop adding stress.
- Cut feed by 70–100% for 48 h. Feed is the source of the nitrogen (180 kg/day at ~30 g TAN per kg feed is roughly 5.4 kg TAN/day entering a pond that cannot dilute it) and uneaten feed drives oxygen demand. Removing it addresses both ammonia and DO simultaneously.
- Do not lime. Do not add molasses yet. Lime raises pH and increases NH3. Molasses is a carbon source whose bacterial bloom consumes oxygen; applying it at 3.2 mg/L dawn DO risks a crash before it helps.
- Maximise aeration. Run all paddlewheels plus 6 HP/ha of emergency blowers, positioned to keep the whole pond moving; the moribund shrimp at the edge are in the least-circulated water, and edge mortality is a circulation signature.
Hour 6–24: dilute and stabilise. 4. Exchange 20–30% of pond volume, but only with source water matched in salinity and temperature; a cold, fresh flush at 18 ppt causes moulting, which is when shrimp are most ammonia-sensitive. 5. Test pH and TAN again at 14:00 and 02:00, not once a day. The 14:00 pH is the number that decides whether the situation is improving.
Day 2–3: restore the pond's capacity. 6. Add a nitrifying/probiotic blend and a small alkalinity supplement (to hold 100–120 mg/L) once DO is above 4 mg/L. 7. Re-check Vibrio; if counts remain ≥10^4 CFU/g with continuing mortality, consider a prescribed treatment only after water quality is stable, since antibiotics work poorly in a pond with 4.5 mg/L TAN.
Sequence rationale: the fastest lever on toxicity is stopping the nitrogen input and lowering pH by dilution, not chemical additives.
3. Design the 7-day feeding and monitoring plan that decides when normal feeding resumes.
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Feeding ladder (180 kg/day baseline).
- Days 1–2: 0–20% (30 kg/day), broadcast only into aerated zones.
- Days 3–4: 40% if dawn DO ≥4 mg/L and the 14:00 NH3-N is below 0.1 mg/L.
- Days 5–6: 60–70% if tray checks show feed consumed within 2 h on three consecutive checks.
- Day 7: return to 100% only if all criteria below hold for 48 h.
Monitoring, twice daily (02:00–06:00 and 14:00–15:00): DO, pH, temperature; TAN and nitrite every other day; alkalinity every three days; Vibrio swabs on days 1, 3 and 7; mortality counted at the same four edge stations each morning so the trend is comparable; and a 50-shrimp sample scored for gill fouling and shell lesions.
Quantitative release criteria:
- Dawn DO ≥4 mg/L for three consecutive days with current aeration.
- NH3-N (calculated, not TAN) ≤0.1 mg/L at the 14:00 pH.
- Mortality back to background (<20 shrimp/day for a 30 m edge count).
- Feeding trays cleared within 2 h.
Guardrails. If pH exceeds 8.8 or NH3-N exceeds 0.2 mg/L, step feeding back 20% and increase exchange. If dawn DO drops below 3 mg/L, revert to day-1 status regardless of other indicators. Log every change with time and reason - the farmer's instinct will be to resume full feeding as soon as shrimp appear at the trays, and ammonia toxicity lags appetite by about 24 hours.