BOD stands for Biochemical Oxygen Demand. Scientists use this laboratory test to measure how fast oxygen is consumed in water. High demand occurs when large amounts of rice straw or other dead vegetation is decomposing in a crawfish pond.
The bacteria and other organisms associated with decaying vegetation consume large amounts of oxygen and the warmer the water, the more rapidly oxygen is consumed. That is why oxygen levels in crawfish ponds often get critically low in the fall following flood-up.
Low oxygen levels will either slow the growth of young crawfish or kill them outright. This is why we recommend flushing water through the pond, especially early in the season when the weather is warm, to overcome the problem of high BOD and low oxygen. Cooler water reduces the BOD so low oxygen become less of a problem in winter.
Ponds with green, immature rice and few terrestrial grasses and weeds at flood-up will have the lowest BOD and the least oxygen problems. Rice stubble that has re-sprouted is not bad, but if a lot of straw remains from the rice harvest the BOD will be high and oxygen will be low.
Ponds with “volunteer” terrestrial vegetation like grasses and sedges usually have a high BOD and frequent low oxygen because most of these plants die when flooded. Ponds with rice stubble and weedy vegetation tend to have extremely low dissolved oxygen for days or weeks at a time. This can hurt crawfish production for the rest of the season.
Moving aerated water thru the pond is the best way of overcoming high BOD and long periods of low dissolved oxygen. Since water does not spread out evenly in a pond, it is best to drain a few inches of stale water out first and then add a few inches of aerated water to replace it. The larger the pumping capacity, the quicker this can be accomplished.
In contrast, adding water on the high side of the pond while water is overflowing at the drain only freshens up a small portion of the pond. Incoming water will follow a trap lane or tractor rut in the path of least resistance on its way to the drain.
After hurricanes Laura and Delta, many bayous and canals had high BOD and very low dissolved oxygen levels because of leaves, grass and organic debris that found its way into the canals. It is essential to pump this type of poor quality surface water with high BOD through an aeration screen.
Mixing water with air by passing it through an aeration screen will raise the dissolved oxygen to an acceptable level of 5 or 6 ppm. However, because of the high BOD of the surface water from all the organic debris it contains, the oxygen levels in the pond can rapidly drop back to critical levels in a day or two regardless of what type of vegetation is present.
If you flush your pond with canal water that has low oxygen and a high BOD, you may be spending a lot of money without getting much benefit.
In contrast, even though well water has no oxygen, its BOD is usually near zero. Aerating well water can raise oxygen levels to over 7 ppm, but more importantly, because of the well water’s low BOD it will retain the oxygen you add for a much longer time as it spreads out through the pond. Exactly how long it retains that oxygen will depend on the type and amount of vegetation and the water temperature.
Aeration screens don’t have to be pretty, but to be effective they have to break the water coming into the pond into tiny droplets to absorb oxygen from the air. Two or three levels of small mesh screen about 10 inches apart will work fine. Water that is pushed through a pipe and out a riser without being broken up into tiny droplets picks up very little oxygen.
Splashing water on tractor tires, cans, concrete piles, or old crawfish traps around the water discharge may only add 1 or 2 ppm of oxygen, so you are spending money on fuel or electricity but not getting much benefit from it.
Discharging water over a properly designed and constructed aeration screen can add 4 to 6 ppm oxygen, giving you the most benefit for your pumping dollar. The aeration screen can also be designed to filter out predatory fish and apple snails that make it thru the pump.
Most importantly, if you have no way to measure your oxygen levels most of these recommendations will be of little use. Remember, you can’t really manage dissolved oxygen if you can’t measure it.