Dissolved Oxygen and how it can fluctuate.

Welcome back to the second blog of this year. I hope you enjoyed the brief story of my background, and hopefully it has in a small way inspired other people to work hard and follow their passion. We live in a more competitive world than ever, and it is so important to stand out in a crowd of applicants. Try and see every opportunity, no matter how small as a potential link to another door opening, which may guide you in the direction you are aiming for long term.


If I look back at my journey over the last 10 years, it can be broken down into several opportunities that I grabbed with both hands and gave 100%. Often, they involved little pay and long hours, but ultimately led me to where I am now. Whatever motivates you to get up in the morning and push forward in life, focus on this and in time you will achieve your goals.

Enough talk about me, let’s get down to some fish farming!!!


If there is one question that I get asked more than any other I would say it is about oxygen levels. The most important tool on the farm during the warmer months is our oxygen meter. With 80 ponds to manage, it is vital that we have the ability to monitor the DO (Dissolved Oxygen) levels and adjust water flow, aeration and feed rates accordingly.


Having an oxygen meter is great, but it is important to be consistent, and understanding when to take readings is vital to fully monitor what is happening in the water daily. To understand this in more detail you must first look at what is affecting the oxygen levels. Many clients have said to me in the past “our oxygen levels will be fine, we only have a few fish”!! But it’s not simply a case of stocking levels. You must factor in algae blooms, presence of submerged weed and marginal plants, light penetration, nutrient loading, air pressures, water temperature and water turn over. It is all these factors combined, that are fluctuating daily which will affect the oxygen levels. So yes, monitoring the oxygen is great, but also monitoring what is happening in and around the pond is key.

Changes in pond colour and reduction in feed (waste food) are some of the early signs to look out for when monitoring oxygen levels. If the levels continue to drop this will eventually lead to fish sitting in water flow or around aeration, as they become more stressed and struggle to diffuse oxygen. The key is to understand what is happening before it reaches this critical stage, as a stressed fish is more likely to suffer with disease and parasites.

Air pressure and water temperature

Air pressure is often talked about in angling. “a big low pressure is coming in” often meaning the fishing is going to be productive. Perhaps this is linked with strong winds helping to move the fish around, or maybe the fish just feel more comfortable feeding when the air pressure is lower. In a farming scenario we watch the DO levels very closely when low pressure is forecast. With less air pressure forcing the oxygen into the water the levels can decrease rapidly. This can then be accelerated if the algae start to die off, increasing the BOD, (Biological Organic Demand).

High pressure generally results in high DO levels, but as mentioned before other factors can affect this.

Water temperature –

To understand how water temperature can affect DO levels you need to first look at how oxygen molecules behave in water. At cooler water temperatures the oxygen molecules are packed in tightly with little movement, therefor a higher concentration can be found. As the water warms the molecules start to move at a faster rate meaning less space for the oxygen, resulting in a lower saturation rate.

When to take your oxygen readings

As already discussed, regular monitoring of oxygen levels is key to managing a pond or lake. However, equally as important is when you take your measurements. Once you understand the diurnal (daily) rhythm of a pond, you can then manage the oxygen levels and record accurate fluctuations of oxygen.


During daylight hours photosynthesis will be taking place. This will include the algae, marginal plants and submerged weeds that will produce oxygen, increasing the levels throughout the day. The opposite is true during the night as photosynthesis stops, but respiration continues. For this reason, we always take our first reading between 6am and 9am in the morning. This is the time when the DO levels will be at their lowest and we can make the decision to feed or increase aeration. If we find ponds below 5mg/l in the morning we will reduce the feed ration for the day and take a second reading in the afternoon to check that the levels have increased sufficiently. DO levels will often jump from 5mg/l to 10mg/l during this time and we will introduce the remaining food ration.

As you can see from the information above, there are many factors to consider when monitoring DO levels in your pond or lake. Even with our experience we still get caught out with ponds that are fine one day and extremely low the next. The key is consistency in what you are doing, and you should avoid any disasters.

Thanks for joining us again.

Best fishes.