The Importance of Dissolved Oxygen in Aquaculture

Dissolved oxygen (DO) is arguably the most critical water quality parameter in aquaculture. It is essential for the respiration, growth, and overall health of farmed aquatic species. Maintaining an optimal DO level is not just beneficial—it is a fundamental requirement for a successful and sustainable operation.

What is the Suitable Level?

For most commonly farmed fish and crustaceans (such as carp, tilapia, catfish, and shrimp), the optimal dissolved oxygen concentration is above 5 mg/L (milligrams per liter).

Ideal Range: 5 mg/L to saturation (which is typically around 7-9 mg/L depending on temperature and salinity). Within this range, aquatic animals feed actively, convert food efficiently, and exhibit strong growth and disease resistance.

Stress Level: When DO falls between 3 mg/L and 5 mg/L, fish become stressed. They may reduce their feeding, leading to slower growth and increased susceptibility to illness.

Critical Level: A DO level below 2-3 mg/L is lethal for most species over a short period. At this point, fish will gasp for air at the water surface (a behavior called "piping"), and mass mortality can occur rapidly.

It is important to note that some high-value species, like trout and salmon which inhabit cold, fast-flowing waters, require even higher levels, typically above 6 mg/L.

Consequences of Low Dissolved Oxygen

Reduced Growth and Feed Conversion: Fish eat less and use energy less efficiently when oxygen is low.

Increased Stress and Disease Susceptibility: Low DO weakens the immune system.

Fish Mortality: This is the most direct and devastating consequence of oxygen depletion.

Water Quality Deterioration: Low oxygen can lead to the production of toxic substances like ammonia and hydrogen sulfide by anaerobic bacteria.

How to Maintain Adequate DO Levels

Aquaculture managers must actively monitor and manage DO. Key strategies include:

Aeration: Using paddlewheel aerators, diffusers, or blowers to mechanically inject oxygen into the water is the most common method.

Water Exchange: Replacing oxygen-depleted water with fresh, oxygen-rich water.

Managing Stocking Density: Avoiding overstocking ponds to reduce the biological oxygen demand.

Managing Feed: Removing uneaten feed and avoiding overfeeding to prevent its decomposition, which consumes large amounts of oxygen.

In conclusion, vigilant monitoring and management of dissolved oxygen, keeping it consistently above 5 mg/L, is a non-negotiable practice in aquaculture. It is the cornerstone of animal welfare, operational efficiency, and ultimately, the profitability of the farm.