The need to aerate varies seasonally because water temperature affects the rates of respiration and photosynthesis. Problems with low dissolved oxygen concentrations are rare when water temperature is consistently below 60 °F (15 °C).
Problems are common when water temperature is above 80 °F (27 °C). Based on average water temperatures, aeration in northwest Mississippi catfish ponds is uncommon from mid- November through February and frequent from mid-May through mid-September. Local climates and unseasonable temperatures will, however, alter the need to aerate ponds.
Episodes of low dissolved oxygen concentration usually occur at night during the summer. Most catfish farmers manage each pond individually by measuring the dissolved oxygen concentration at intervals throughout the night and aerating when it falls below a level considered critical by the individual farmer (usually 3 to 5 mg/L).
Oxygen is measured manually (see SRAC publication 4601) or, far less commonly, with continuous monitoring systems in each pond. Aeration continues until past dawn when measurements of dissolved oxygen indicate that phytoplankton photosynthesis is producing oxygen.
Commercial catfish ponds aerated on this “as-needed” basis require 500 to 1,000 hours of supplemental aeration cumulatively during the summer growing season. This corresponds to an average of about 3 to 6 hours per day per pond.
Note, however, that the duration of aeration varies greatly among ponds on a given day. Some ponds may need no aeration, while others require continuous aeration throughout the day.
The length of time aeration is used also depends on weather conditions. For example, during periods of warm, cloudy weather most ponds may need continuous aeration for several days. Some catfish farmers begin aerating all ponds on the farm at some set time early each night rather than basing aeration on dissolved oxygen levels.
This practice reduces the labor required for frequent monitoring, but increases energy costs by starting aeration in some ponds well before it is needed.
Little research has been conducted on this aspect of pond aeration. If water mixing is important, aerators should be placed where they will enhance pond circulation patterns.
For example, water circulation in large, rectangular ponds is optimized by placing paddle wheel aerators off the bank near the middle of the long axis of the pond to direct currents across the short axis .
This produces two or more (depending on how many aerators are used) water circulation cells in the pond. The worst placement is in a corner, with currents directed diagonally across the pond. When several aerators are placed in a pond, they can be located where the current of each aerator enhances the flow produced by the others aerators.
For example, an aerator can be placed in each corner of the pond to direct currents parallel to the banks in the same clockwise or counterclockwise direction. One drawback to this arrangement is that aerator currents tend to erode the pond bottom around the pond margins and deposit the eroded material in the middle of the pond.
The resulting central mound of loose sediment can make it difficult to harvest the aquaculture crop by seining. Also, running electrical cables to the corners of every pond on a large farm will be expensive.
Water mixing is not the primary goal of using aerators in catfish ponds, so optimizing water circulation patterns is not the most important consideration in aerator placement. Catfish become conditioned to moving near the aerator when dissolved oxygen concentrations are low, and permanently installed electric aerators should be located mainly for the convenience of the farmer. Aerators should be placed near a graveled, all-weather road for easy access during operation and maintenance.
They also should be near the main power source to reduce the length of power lines. If more than one permanent aerator is to be installed in each pond, many farmers prefer to install them near each other (all in one end of the pond, for example) rather than to space them widely along the pond bank.
If several aerators are located far apart and one of them fails at a critical time, fish will be forced to swim a long distance through oxygen deficient waters to find a haven near the remaining aerators.
Putting all the aerators in one end of the pond also reduces the cost of the electrical supply and makes routine maintenance easier. Portable aerators, such as tractor PTO-driven paddle wheels, should either be placed in the same location each time they are used so that fish become accustomed to the location or placed where oxygen-stressed fish have congregated.
For example, when the dissolved oxygen concentration varies from one end of the pond to the other, most fish will be in the end where the concentration is highest and the aerator should be placed at that end.
If a portable aerator is used to supplement an existing aerator or to replace a malfunctioning permanent aerator, it should be placed near the other aerator so that fish are not forced to swim a long distance to find the new aerator.
Author:
Craig Tucker
