In the delicate balance of aquaculture, where even the smallest changes can determine success or failure, dissolved oxygen (DO) plays a quiet but essential role in supporting aquatic life. Picture a lively underwater world where fish, shrimp, and other creatures don’t rely on oxygen tanks, but instead depend on the oxygen naturally present in their environment. For those working in aquaculture, understanding dissolved oxygen—especially its concentration measured in parts per million (PPM)—is more than just science; it’s key to boosting productivity, caring for aquatic animals, and ensuring long-term success.

With the rising global appetite for seafood, aquaculture has become one of the fastest-growing food industries, providing more than half of the fish we consume worldwide. However, this growth also brings challenges—densely populated ponds, increased feeding, and changing environmental conditions can all impact oxygen levels, putting entire populations at risk. In this article, we’ll explore the importance of dissolved oxygen, its impact on aquatic health, the ideal PPM levels, and practical ways to monitor and maintain a healthy aquatic environment. Whether you're experienced or just beginning your journey in aquaculture, these insights will help you move from simply managing to truly thriving.

Why is Dissolved Oxygen Important for Fish?

Dissolved oxygen is to fish what air is to humans: an irreplaceable resource for survival. But its importance extends far beyond basic respiration—it influences every aspect of fish physiology, behavior, and productivity. Let’s break down its critical roles:

1. Respiration and Energy Production

Fish breathe by extracting dissolved oxygen from water through their gills. This oxygen is then used in cellular respiration to convert food into energy (adenosine triphosphate, ATP), fueling essential processes like growth, reproduction, and movement. Without sufficient DO, fish cannot metabolize food efficiently, leading to stunted growth and weakened immune systems. A study by the World Aquaculture Society found that fish in low-oxygen environments exhibit a 30–50% reduction in feed conversion efficiency, directly impacting farm profitability.

2. Immune Function and Disease Resistance

Oxygen deprivation stresses fish, triggering the release of cortisol—a hormone that suppresses immune function. Stressed fish are more vulnerable to pathogens like bacteria, viruses, and parasites. For example, warm-water species such as tilapia and catfish raised in DO levels below 5 PPM are three times more likely to develop bacterial gill disease (BGD), a common and often fatal condition (Aquaculture Research, 2021). Maintaining optimal DO is thus a proactive defense against costly disease outbreaks.

3. Reproduction and Larval Survival

Breeding fish require stable, high oxygen levels for successful spawning and larval development. Oxygen is critical for egg fertilization, embryo development, and the survival of fry (young fish), which have underdeveloped gills and higher oxygen demands relative to their body size. In salmon hatcheries, DO levels below 6 PPM during the larval stage can reduce survival rates by up to 40% (Salmon Aquaculture Technical Journal, 2020).

4. Water Quality Regulation

DO plays a dual role in maintaining water quality: it supports aerobic beneficial bacteria that break down organic waste (e.g., uneaten feed, feces) into harmless byproducts like carbon dioxide and nitrate. In low-oxygen conditions, anaerobic bacteria take over, producing toxic substances such as ammonia, hydrogen sulfide, and methane. These toxins further stress fish and create a vicious cycle of declining water quality. A 2019 study in Aquaculture magazine linked low DO to 70% of ammonia-related fish kills in freshwater ponds.

PPM for Fish: Optimal Dissolved Oxygen Levels

The ideal DO range varies by species, life stage, and water temperature. Generally:

Coldwater fish (e.g., trout): 6–8 PPM

Warmwater fish (e.g., tilapia, catfish): 5–7 PPM

Larvae/Juveniles: Require 1–2 PPM higher than adults due to higher metabolic rates.

Critical thresholds:

Below 3 PPM: Most fish show stress signs.

Below 2 PPM: Risk of mass mortality.

Seasonal adjustments are crucial. Warmer water holds less oxygen, so summer demands stricter monitoring.

How to Know if Low DO Levels Are Killing Fish？

Detecting oxygen depletion early saves lives. Watch for:

Behavioral Clues:

Fish gulping air at the surface.

Crowding near water inlets or aerators.

Reduced feeding activity.

Physical Signs:

Pale gills or erratic swimming.

Floating dead fish, especially at dawn (when DO is lowest).

Environmental Indicators:

Algal blooms (which deplete oxygen at night).

Foul odors from anaerobic bacteria.

What will Cause Low Dissolved Oxygen in Water?

Low dissolved oxygen (DO) in aquaculture systems is often caused by a combination of environmental, biological, and operational factors:

Rising Water Temperatures

Warmer water holds less oxygen. For example, water at 10°C can hold up to 11.27 mg/L of DO, but this drops sharply to 6.93 mg/L at 35°C. Summer heatwaves are a prime time for oxygen shortages.

Surge in Biological Oxygen Demand

• Overstocking: High fish densities increase oxygen consumption (accounting for 12–15% of total DO usage in crowded ponds).

• Organic Matter Breakdown: Decomposing feed, waste, and dead algae fuel microbial activity, which rapidly depletes oxygen.

Chemical Oxygen Consumption

• Inorganic Compounds: Oxygen is consumed during the oxidation of harmful substances like nitrites and hydrogen sulfide.

• Sediment Impact: Nutrient-rich sludge at pond bottoms can account for over 40% of oxygen depletion, surpassing even fish respiration.

Environmental & Physical Factors

• Low Atmospheric Pressure: Oxygen dissolves poorly at high altitudes or during stormy weather.

• Salinity: DO decreases by about 1% for every 100 mg/L increase in salinity. Seawater, for instance, naturally holds less oxygen than freshwater.

Ecological Imbalances

• Lack of Photosynthesis: Insufficient algae or aquatic plants reduces daytime oxygen production, especially during cloudy or rainy periods.

• Poor Water Circulation: Stagnant water limits oxygen exchange with the atmosphere, creating "dead zones."

How to Measure Dissolved Oxygen in Water?

Accurate measurement is most important. Popular tools include:

Portable DO Meters: Digital devices provide instant PPM readings. Calibrate monthly for accuracy.

Chemical Test Kits: Affordable for small-scale farms; involves color-matching reagents.

Continuous Online Dissolved Oxygen Sensors: Ideal for large operations, offering real-time data and alerts.

How to Increase Dissolved Oxygen for Fish?

Emergency Fixes:

Aerators: Install paddlewheels, diffusers, or venturi systems.

Water Exchange: Replace 10–20% of water with fresh, oxygen-rich sources.

Reduce Feeding: Temporarily halt feeding to lower organic load.

Long-Term Solutions:

Aeration Infrastructure: Invest in solar-powered aerators for energy efficiency.

Aquatic Plants: Introduce submerged plants like hornwort for daytime oxygen production.

Stocking Density: Adhere to species-specific guidelines (e.g., 1 kg fish per 50–100 L water).

Biofloc Systems: Enhance microbial balance to reduce waste buildup.

Conclusion

If you're working in aquaculture, you've probably heard that dissolved oxygen — or DO — is a big deal. And it really is! It's the most important thing to keep an eye on when it comes to water quality. Fish need oxygen to live and grow, just like we do, and they usually need DO levels somewhere between 5 and 6 parts per million (ppm) to stay healthy.  

But here's the thing — once DO drops below 3 ppm, fish start to feel the stress. If it keeps falling and goes below 2 ppm, things can get really serious, and they might not make it. That’s why checking DO levels regularly isn’t just a good idea — it’s absolutely essential for keeping fish and other water creatures safe and healthy.