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Oxygen Depletion

“All I need is the air that I breathe” may just be an old song by The Hollies, but if your fish are singing it to you, and you better listen!

We’re halfway through these blistering hot, dry days of July and August when ponds most often suffer oxygen depletion which causes fish populations to drop.

If your pond is experiencing fish die-off, you can almost bet it is from oxygen depletion, as a lack of dissolved oxygen is the most common cause of fish kills in ponds.

When trying to determine if your pond has sufficient aeration, it can be a simple as seeing your fish at the surface in the morning, gulping for air. Morning is the time when there is the lowest amount of dissolved oxygen, as photosynthesis (photo = light and synthesis = make) does not operate at night, so it will be most obvious in the morning if this condition exists. If you see your fish gasping for air, or if your fish are not consuming their pellet feed, the problem is acute and must be addressed immediately.

Gasping fish are a good indicator that oxygen is low, as oxygen is vital to fish, and integral to the ecological processes that keep water inhabitable to aquatic life – both fish and plants. And the factors that add oxygen to your pond water – wind, rain, and waves – they all help, but sometimes oxygen depletion threatens, and it is up to you to stop this thief in its tracks.

Weather is often a direct cause of depleted oxygen, but too hot of temperatures and low water levels are not the only culprits! Yes, oxygen depletion typically occurs when water levels are low, during drought conditions. But rainy and cloudy weather with reduced daylight means less photosynthesis, which means lower levels of oxygen production from plants, which in turn means less oxygen available to your fish on the following day. High winds can do it too, racing across shallow ponds, causing a mixing of the low oxygen water throughout the pond, even more toxic to your fish when dense blooms of algae are present. On the other hand, when there is a case of algae death, you’ll see a change in color from green to gray or brown, with a loss of oxidation within one to two days. As summer weather conditions considerably deviate from those beautiful, spring days with moderate temperatures where easy-going, single celled algae thrive – it’s time to put on your cape and guard your pond against the evils of oxygen depletion!

As August arrives and mid-summer thrives, water warms and so retains less oxygen than cooler water. Consider this: fish are cold-blooded, and they have a rise in metabolic rate when water temperatures rise, which in turn spurs the need for oxygen at the same time that less oxygen is available! So it is a double whammy right about now, as fish often are not getting their oxygen needs met!

This is even more likely when ponds are overstocked, that being too many pounds of fish per surface area of water. It is important to examine the ratio of pounds of fish to the available surface area of water. Knowing surface area is the key element in good pond management, affecting not only determination of adequate aeration for your fish but also impacting many other areas including fish stocking and harvest, herbicide, chemical, fertilization, and alkalinity applications. It is critical that the initial effort is made to correctly measure surface area. Should a pond be of irregular proportions, it can be plotted and broken into a variety of geometric shapes, such as circles, rectangles, triangles and ellipses, then simply apply the proper geometric formula(s) to compute surface area. You can get your formulas from most high school geometry books, or see the links below on measurement procedures. Measure out your pond en masse, or do it in pieces, by feet, then compute the surface area of your pond in acres. Tip: there are 43,560 square feet in an acre.

Once you determine your pond’s surface area, then you can use a rule of thumb that your total pounds of resident fish should not exceed 1,000 pounds per surface acre. Even close to this puts your pond in line to experience oxygen depletion. And don’t talk yourself out of this concern when you have a deep pond, as the water column may stratify, with the upper, warmer layers saturated with oxygen, and the deeper levels depleted of oxygen. This can stress fish and cause a drop in population.

Once you have a good estimate as to the surface area of your pond, you need to estimate your fish population. You know what you stocked and what you’ve taken out so far. To estimate the pounds of fish, simply catch a few fish and weigh them. Multiply your average weight by the estimated number of fish in your pond and you can now compare your pounds of finned friends to the available surface area with confidence. Alternatively, you can shoreline seine, but this is best done during June. If your resident fish population exceeds the recommended population, you have a perfect reason to go fishin’!

But before you’re sauntering off with your reel, you still need to get more oxygen into your pond. And you know, it’s not just your fish that need the oxygen, either! Oxygen is needed for aerobic digestion of algae, ammonia, and nitrates. Plus oxygen is vital in the bacterial decomposition of fish waste, debris and many types of organic matter. The whole pond ecosystem depends on oxygen. It’s time to swing into action!

Aeration is the best way to halt oxygen deprivation. To increase oxygen levels, pond water must be brought into contact with air.

If you have a motorboat, a quick and efficient fix is to back your trailer into shallow water or lodge your boat and allow the motor to run in place, thus aerating the water and saving your fish. You’re a hero!  The prop will function to aerate the water as long as it is stationary; if you are cruising around the pond, then the prop is pushing the boat, not the water, which results in substantially less oxygen absorption.

Another way to aerate is to use a pump, but remember to position the intake a good two to three feet below the surface. Alternately, you can pull water from near the surface and spray it back across the pond. There are many types of pumps and aeration systems to help you combat even the murkiest water and pond muck via submerged and surface equipment, but take care to consider the pros and cons of each approach, to see what best addresses your situation. For example, when extreme masses of anaerobic material are present, as toxic hydrogen sulfates may escape and poison your fish, you must continue with caution and knowledge to properly handle your particular situation.

Should you discover you have an emergency situation, you can put together a paddlewheel to run off the PTO of a farm tractor. The paddlewheel is used to break up the water into droplets which absorb oxygen from the air.

So the next time you hear The Hollies’ winsome tune, give a thought to your fish! Don’t wait until they’re gasping at the surface.


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