Water quality, dissolved oxygen, dying carp,

With the current weather how it is, I urge you all to read this. But, before you start reading this, I must stress that the following are not my words. All credit goes to Bernice Brewster who posted this on a Fishery Management page. The information is so important and accurate, I urge you to spread it around anyone with an interest in the well being of their fish. 

There have been a lot of questions about dissolved oxygen of late, so I though I would post just some of the factors which affect the solubility of this vital gas.  Happy Reading! Factors impacting on the solubility of oxygen are as follows:

1) Temperature – the solubility of oxygen decreases as the water temperature increases. This means in the summer months, when the fish are most active and their oxygen demand is greatest, the lake water physically holds less oxygen.

2) Stock density –in heavily stocked lakes the dissolved oxygen concentration is at a premium, especially when the water temperature is highest in the summer. A large population of fish in a lake can cause the dissolved oxygen concentration to fall to critically low levels, causing the fishing to slow down or even stop.

3) Feeding fish – in the summer months when fish are feeding heavily, especially many carp waters where large amounts of bait are available, this may cause the oxygen concentration to drop significantly. The digestion of food consumes large volumes of oxygen, when large numbers of fish are feeding their oxygen demand increases, affecting the amount of available dissolved oxygen.

4) Algae and aquatic vegetation or pond weeds - during the hours of daylight, all plants, including algae and aquatic weeds produce sugars using the energy from sunlight and carbon dioxide, which in the aquatic environment is dissolved in the water, a process known as ‘photosynthesis’. Oxygen is released as a waste by-product of photosynthesis and which in the aquatic environment results in the dissolved oxygen concentration increasing significantly through the daytime and may exceed 130% saturation. Once it gets dark, photosynthesis ceases and respiration in algae and aquatic weeds dominates which means they consume oxygen and carbon dioxide is produced as a waste gas. Most importantly, all aquatic plants are more efficient at extracting oxygen from the water overnight, the result of which is the oxygen concentration drops away and is at its lowest just before dawn. The classic signs of overnight oxygen depletion are either the biggest carp are found dead in the morning, or there is a significant fish mortality overnight. This rather indicates that aquatic weeds or algae are bad for a fishing lake, but this is not the case, the plants play an important role in adding vital oxygen to any lake the important factor is to ensure the weed growth is not prolific but contained through cutting if necessary.

5) Oxygen supersaturation- in the summer months, particularly during periods of intense sunlight, when algae or submergent plants are producing oxygen gas as a waste by-product, the dissolved oxygen concentration in the water can be excessive. Providing the dissolved oxygen concentration is below 300% saturation, this is unlikely to have any detrimental effect on adult fish in a lake. Fry or fingerlings may not tolerate such a high level of oxygen and may suffer from ‘gas bubble disease’ where the gas drops out of solution in the blood and is seen as gas embolisms (bubbles) in the fins and eyes. Adult fish can cope with an elevated dissolved oxygen concentration but if it exceeds 300% saturation may also develop gas bubble disease. If the oxygen concentration in a lake is approaching 300%, heavily aerate the water, if possible allowing the return water to hit a solid object, which will allow the oxygen to vent as gas.

6) Wind shadow – trees and densely planted shrubs planted around the margins create what is known as a ‘wind-shadow’. The assumption is that wind hitting a group of trees will pass straight through the branches whereas when wind meets a bank of trees and shrubs they form a complete barrier and the wind does not drop back onto the lake for a distance which is roughly 10 times their height.

7) Successive overcast/dull days – when the weather is overcast over a period of days, the low light intensity means that aquatic plants cannot photosynthesize efficiently, respiration dominates and as a result over four or five days the oxygen concentration in the water declines, often to an unacceptably low level for the fish.

8) Low air pressure – this climatic condition often accompanies dull, wet weather. When an area of low pressure sweeps through, as it passes over water, it literally sucks out the dissolved oxygen. The best description of the effect of low pressure is that it’s akin to opening a bottle or can of fizzy drink, with the accompanying rush of escaping gas.

9) Autumn leaf fall – when leaves fall into the water they have an adverse effect on the oxygen concentration, possibly due to bacterial activity on the dead leaves and or, chemical reactions with tannins and other residues which are found in leaves.

10) Long term ice cover – ice obviously seals the water and prevents essential oxygen from dissolving into the water. When ice is thin enough to melt during the daytime it represents little hazard, but we have had winters where a thick layer of ice forms over the surface for periods of days. The problem with this ice cap is the dissolved oxygen can become depleted and the fish die.
It is usually presumed the initial sign of low oxygen is fish seen gasping or piping at the water surface, but fish will only show this behavior when they are in severe distress and are within minutes of dying. The first indication of low oxygen is when fish behave as though it’s winter, they stop feeding and become very lethargic – mechanisms designed to conserve oxygen and energy.

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