What is dissolved oxygen and why does it matter for fish farms?

Dissolved oxygen (DO) is the amount of oxygen gas that has dissolved into water and is available for aquatic organisms to breathe. Fish and shrimp extract it across their gills in the same way that mammals extract oxygen from air through their lungs. The critical difference: water holds far less oxygen than air — and how much it can hold depends heavily on temperature, salinity, and biological oxygen demand in the system.

In a well-managed aquaculture system, DO is the primary variable separating profitable production from losses. Everything else — feed quality, stock health, water chemistry — matters less if DO drops below threshold, because the consequences are immediate: stressed fish convert feed poorly, become susceptible to disease, and at sustained low levels, die.

How DO is measured

Dissolved oxygen is measured in milligrams per litre (mg/L), sometimes expressed as percentage saturation (the percentage of the maximum possible DO at that temperature and salinity). At 20°C in freshwater, fully saturated water holds approximately 9.1 mg/L. At 30°C — a typical UAE summer water temperature — that drops to approximately 7.5 mg/L. At 35°C, it drops to 7.0 mg/L. Before you've added any fish, feed load, or algae to consume oxygen, you're already starting 20% below the baseline of a temperate system.

Modern DO sensors use optical (luminescence-based) measurement rather than the older electrochemical (Clark cell) method. Optical sensors are more stable, require less calibration, and are not consumed by the measurement process. OxyNano deployments use Aqualabo multiparameter probes that measure DO alongside temperature, salinity, pH, and ORP — providing the full picture needed to understand what's driving oxygen levels in a given system.

Critical DO thresholds by species

Different species have different tolerance ranges. The numbers below represent industry-standard operational thresholds — not absolute survival limits, which are typically lower. Operating at or below these thresholds causes suboptimal outcomes that compound over weeks and months.

These thresholds assume normal temperature ranges. In a UAE summer system at 30–35°C, the physiological oxygen demand of the fish is higher (metabolic rates increase with temperature), while the water's oxygen-holding capacity is lower. The margin between "optimal" and "stress" effectively shrinks — and a system that manages DO adequately in November may be chronically under-oxygenated in August without any change in management.

What happens when DO drops

Below-threshold DO triggers a cascade of physiological and commercial consequences, most of which are measurable. The first visible signs are behavioural: fish move toward the surface or inlet where DO is highest, feed intake drops, and responsiveness to feeding slows. These are early-warning indicators. If DO remains suppressed, the impacts become quantifiable in your production data.

The commercial consequence most operators focus on is FCR — the kilograms of feed required to produce one kilogram of fish weight gain. FCR is the primary efficiency metric in aquaculture economics. Studies consistently show that FCR deteriorates by 0.3–0.8 points for every 1 mg/L of sustained DO deficit below optimal. On a 50-tonne annual production system, a 0.5-point FCR increase represents a significant feed cost increase with no additional output.

Disease susceptibility is the second major consequence. Low DO stresses the immune system in fish and creates anaerobic conditions in sediment and biofilm where pathogens proliferate. Vibrio bacteria — a major cause of mortality in marine aquaculture — thrive in low-DO, high-organic-load conditions. Treating disease outbreaks is expensive; creating the conditions that cause them is preventable.

The UAE-specific problem: overnight DO crashes

In systems with significant algae or phytoplankton loads — which is most outdoor ponds in the UAE — dissolved oxygen follows a diurnal cycle. Photosynthesis produces oxygen during daylight hours, often pushing DO above 12 mg/L in the afternoon. After sunset, photosynthesis stops but biological oxygen demand continues — fish respiration, microbial activity, sediment decomposition — and DO drops continuously through the night. The lowest point is typically between 3 and 5 AM, often three to four hours before the farm team arrives for the morning feed check.

This overnight crash window is when most acute fish mortality events occur in UAE aquaculture. Operators who don't have continuous DO monitoring often discover a mortality event in the morning and attribute it to disease, water quality, or feed issues — without knowing that DO dropped to 1.5 mg/L for two hours before dawn. Conventional paddle wheel aerators provide some protection but consume significant power running all night when demand is uncertain. They also treat only the surface, while the highest oxygen demand is at depth where fish and sediment compete for available DO.

Nanobubble DO management for aquaculture

Nanobubble systems address the UAE aquaculture DO problem on three levels. First, at 85–90% oxygen transfer efficiency, they deliver significantly more DO per unit of energy than conventional aerators — critical when running all night. Second, nanobubbles distribute through the full water column, treating the depth where overnight DO deficits are most severe. Third, ozone nanobubbles reduce the bacterial and organic load in the sediment layer that drives overnight oxygen consumption — addressing the root cause rather than just compensating for it.

On systems we've instrumented with continuous DO logging, nanobubble supplementation consistently reduces overnight DO crash amplitude — the difference between the late-afternoon peak and the pre-dawn minimum — by 40–60%. That difference is the margin between a crisis and a controlled system.