May 16, 2017 0 Comments

We're not perfect, we have all been there and its an area that we need to be in touch with daily during the summer if you want to make the most of the growing season!

So a little bit about Dissolved Oxygen then...

Maintaining a healthy level of oxygen is key to keeping your fish feeding, so its important to regularly take oxygen readings, by doing so you will quickly learn the contributors to changes in DO2, making it easier for you to anticipate a problem so you can act before any disaster.

Cool water holds oxygen much more effectively than warmer water, so as water temperatures increase, the waters ability to hold DO2 is decreased. Coupled with the rise in oxygen demand from aquatic life within the water, theres no wonder summer oxygen levels fluctuate so much, which is why its so important to check oxygen regularly. 

We always measure oxygen in mg/l rather than %saturation, this gives us a more comparable reading. What happens when oxygen is low?

  • Above 4mg/l is healthy
  • Below 3.5mg/l carp will reduce feeding
  • Below 3mg/l carp will almost stop feeding completely
  • Below 1.5mg/l you will begin notice fish gasping for air at the surface 

So by the time you see physical signs of distress, the oxygen has probably been too low for a long time, this is why its important to know when oxygen is on its way down because feeding when oxygen is low will add to the problem as the feed isn't going to be eaten and the feed waste will contribute to the decline in dissolved oxygen.

Oxygen is transferred through surface diffusion on a lake (another reason why water surface area is more important than depth) but once the water is at 100% saturation, no more oxygen can diffuse through the surface. However weed and algae produce oxygen as they photosynthesise, which lifts the dissolved oxygen level above 100%, this is how water becomes supersaturated. Obviously as photosynthesis can only occur during daylight hours, this is the period when oxygen will be increasing dramatically, although the respiration process during the night will cause oxygen to decline during the hours of darkness. So your highest reading of dissolved oxygen will be at the end of the day when the lake has had a full day of sunlight, and your lowest reading will be at first light after the lake has been through a night of weed/algae respiration before the sunlight then allows photosynthesis to lift the oxygen level once again. Its a good idea to take a reading just before first light occasionally, so that you know what level of oxygen you have when it is at its lowest point to give you peace of mind because taking a reading after just 1-2hours of sunlight can be the difference of as much as 1mg/l. 

This is another reason why weedy waters are not ideal for growing fish, because DO2 will be very high during the daylight hours and very low during the night, this inconsistency is not what you want when growing fish! 

Bacterial action during the organic breakdown process of silt, weed, leaf-fall etc will also contribute to a depletion in oxygen. Which is why its a good idea to apply chalk during the winter to accelerate the breakdown of silt before the summer when the biological oxygen demand (BOD) of the water is much higher!

Warm stormy weather is classic oxygen zapping weather, we had a few stormy days in the middle of the summer in 2016 which caused lots of problems in fisheries all over, and even some of our ponds suffered. So its always a good idea to have aeration on standby, and cut back on the feeding if oxygen takes a dive!

Summer is obviously the time to be on the ball with the oxygen meter, especially post-spawning with the added oxygen demand from the growing fry! This is why we take stock management so seriously, removing offspring and unwanted fish gives the best chance to resident fish that you want to grow! 

Oxygen monitoring equipment

There are a number of oxygen reading probes on the market, and we see plenty of variations, but there is only 1 that I could recommend. Cheap probes don't last and they often need constant calibration, whereas the probe that we use is turned on and accurately reading oxygen with the touch of a button, I've had the same oxygen meter for 5 years, without an problems or battery changes! Without a doubt its a must have tool for any fish farmer or fishery owner! Contact for more information or order yours from us now!



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