Species-specific DO requirements, Weiss equation, overnight DO sag calculation and aeration sizing for salmonid and coarse fisheries.
Un-ionised ammonia toxicity, Henderson-Hasselbalch calculation, EA salmonid and cyprinid standards, and ammonia management protocols for managed fisheries.
Emergency aeration protocols for acute fishery DO events — paddlewheel deployment, liquid oxygen injection, EA notification and post-event review.
DO management, ammonia control and emergency aeration for fishery and aquaculture waterbodies — species requirements, EA standards and emergency response.
Dissolved oxygen management for fish farms, RAS, flow-through raceways and pond aquaculture. Species-specific DO targets, paddlewheel aerators, pure oxygen systems, emergency response and telemetry.
Dissolved oxygen concentration in a fishery waterbody is governed by the balance between oxygen supply (surface re-aeration, photosynthesis, aeration equipment) and oxygen demand (respiration of fish, invertebrates, bacteria, and algae; sediment oxygen demand). In warm, productive lakes and ponds during summer, this balance can become critically negative overnight — when photosynthesis ceases at dusk, the combined oxygen demand of algal respiration, BOD from decomposing organic matter, and sediment uptake can drive DO from 10 mg/L (afternoon supersaturation) to below 2 mg/L by dawn.
The Weiss (1970) equation gives saturation DO at standard pressure: DO_sat (mg/L) = exp(−139.34411 + 157570.1/T − 66423080/T² + 12438000000/T³ − 862194900000/T⁴) where T is temperature in Kelvin. At 15 °C, saturation is 10.1 mg/L; at 25 °C it falls to 8.3 mg/L; at 30 °C to 7.6 mg/L. Warm summer temperatures thus simultaneously lower saturation DO, increase fish metabolic demand, accelerate algal and bacterial respiration, and reduce the rate of surface re-aeration — a quadruple jeopardy for high-stocking-density fisheries.
Overnight DO sag calculation: ΔDO (mg/L) = (SOD × 24 + BOD₅ × 0.6) × depth⁻¹ × (1 − f_aer) where SOD is sediment oxygen demand (g O₂/m²/day), BOD₅ is water column biological oxygen demand (mg/L), depth is mean water depth (m), and f_aer is the fraction of demand met by aeration equipment. This simplified model gives a first estimate of overnight DO depression; alarm setpoints should be set conservatively.
| Temperature (°C) | DO Saturation (mg/L) at 1 atm | Salmonid Alarm (< 7 mg/L) | Cyprinid Alarm (< 5 mg/L) | Kill Risk (< 2 mg/L) |
|---|---|---|---|---|
| 10 | 11.3 | 38% saturation | 44% saturation | 18% saturation |
| 15 | 10.1 | 69% saturation | 50% saturation | 20% saturation |
| 20 | 9.1 | 77% saturation | 55% saturation | 22% saturation |
| 25 | 8.3 | 84% saturation | 60% saturation | 24% saturation |
| 30 | 7.6 | 92% saturation | 66% saturation | 26% saturation |
Deploy calibrated optical DO probes at mid-depth and at 0.5 m depth (worst-case surface layer). Datalogger with GSM alarm: SMS to duty fishery manager when DO < 5 mg/L (salmonid) or < 4 mg/L (coarse fish). Log data at 15-minute intervals. Calibrate monthly using air-saturated water or Winkler titration.
Review 2–3 years of temperature and DO data to identify the highest-risk period (typically July–August in UK). Map the relationship between water temperature, Chl-a, and overnight DO depression. Identify whether the primary driver is sediment oxygen demand, algal BOD, or high fish loading. This determines the primary intervention.
Size diffused-air or paddlewheel system for steady-state DO support: Q_O₂ (kg/day) = SOD × area + BOD × volume / depth − surface re-aeration. For fishery ponds < 1 ha and < 2 m depth, paddlewheel aerators at 1 kW/ha are often sufficient for summer support. Diffused-air is preferred for deeper lakes where surface mixing is less effective.
In July–August, run aerators continuously from 22:00 to 07:00 as standard. If Chl-a > 50 µg/L or temperature > 22°C, run continuously 24 hours. Pre-position emergency paddlewheel at the highest-stocking-density zone. Confirm DO > 6 mg/L at 07:00 before feeding fish in the morning.
Dense algal blooms cause extreme DO diurnal swing: supersaturation (> 200%) in afternoon, severe depression (< 2 mg/L) overnight. Address root cause (phosphorus input reduction), not just the DO symptom. Short-term: reduce feeding rate during bloom; increase aeration overnight; monitor for UIA toxicity which increases with the pH swings associated with dense blooms.
After any DO event where fish showed distress: suspend feeding for 48 hours; allow DO to recover to > 6 mg/L; assess fish for lesions (Aeromonas secondary infection is common post-hypoxia); report suspected mortality above de minimis to EA (Environmental Permitting (England and Wales) Regulations 2016 reporting threshold: death of wild fish); take water samples for post-event chemistry.
UIA toxicity increases as DO-driven pH swings alter the NH₃/NH₄⁺ equilibrium — closely linked to DO management.
Read MoreEquipment, deployment protocols, and EA notification procedures for acute fish kill prevention.
Read MorePure-O₂ systems, paddlewheels, and fine-bubble diffusers for managed fish production systems.
Read MoreDO management, ammonia control and emergency aeration for fishery and aquaculture waterbodies — species requirements, EA standards and emergency response.
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