Basic aerator design and the types of aerators used in pond aquaculture have become somewhat standardized. Paddle wheel aerators of relatively similar design are used in most large ponds (>1 acre), and vertical pump aerators are commonly used in small ponds.
Although the choice and design of aerators have become somewhat standardized, the ways aerators are used vary widely, probably because there has been little systematic economic evaluation of aeration practices in large commercial ponds.
Although many aeration experiments have been conducted in small ponds, it is difficult to relate the results of those experiments to conditions in large ponds because oxygen dynamics, mixing characteristics, fish behaviour, and other factors vary so much with pond size.
Because there have been few appropriate aeration trials, farmers base their aeration practices on the availability of labour and capital, their individual production goals, and the perceived effectiveness of various practices.
The variety of aeration practices used in pond aquaculture is represented at one extreme by the continuous use of intensive aeration (up to 10 hp/acre) in marine shrimp ponds and, at the other extreme, by infrequent, “emergency aeration” with 0.5 to 1 hp/acre in some fish culture ponds.
In marine shrimp ponds, aerators are also used for constant water circulation, which in many respects is as important as the oxygenation process in the culture of this bottom-dwelling animal.
Aeration practices used on most commercial catfish farms lie between these two extremes.
The evolution of aeration on catfish farms:
Before about 1980, catfish stocking and feeding rates were low and there were few problems with dissolved oxygen, except during unusual events such as sudden phytoplankton die-offs or prolonged periods of cloudy weather during the summer.
As farmers sought to increase production by stocking fish at greater densities and increasing the feeding rates, episodes of critically low dissolved oxygen concentrations became more frequent.
In the early stages of intensified production, oxygen problems remained relatively rare and catfish farmers used existing farm equipment (such as irrigation lift pumps) or aerators fabricated in farm machine shops.
These early aerators used a readily available power source—a tractor PTO. Tractor-powered aerators could be moved around the farm to any pond that required supplemental aeration. Quite often, the first sign of a problem would be fish struggling to obtain oxygen at the pond surface.
Farmers would then quickly place an aerator in the pond and fish would congregate in the small area of oxygenated water near the aerator. As farmers continued to increase their catfish stocking and feeding rates in an effort to grow more fish, most ponds needed aeration on summer nights.
Tractors proved too expensive and difficult to maintain for everyday use. Also, tractor-powered paddle wheels are not particularly efficient because only a portion of the energy from the tractor engine is transferred to the aerator.
Floating paddlewheel aerators powered by electric motors were more efficient, and by the mid 1980s they had become the most common type used on catfish farms.
How much to aerate:
In commercial catfish ponds, aeration is commonly about 1.5 to 2 hp/acre in each pond. For example, two 10-hp electric paddle wheel aerators may be used in a 10- to 15- acre pond. There is, however, a strong trend in the catfish industry to increase aeration intensity to 2 to 3 hp/acre (three 10-hp aerators in a 10- to 15-acre pond).
Many farmers also maintain a few tractor-powered aerators (one for every four or five ponds is common) for emergency situations where high oxygen transfer rates and aerator mobility are more important than aeration efficiency.
Mechanical aeration at 1.5 to 2 hp/acre meets only a fraction of the total oxygen demand of all organisms in the pond. In a typical 15-acre catfish pond, the total oxygen consumed in respiration by fish, plankton and sediment during the summer may range from 100 to more than 200 pounds of oxygen per hour.
Most of the oxygen demand is accounted for by plankton and sediment respiration rather than by fish.
When operated in water with a dissolved oxygen concentration of about 2 mg/L, good paddle wheel aerators transfer about 1.5 to 2.5 pounds O2/hp.hour under field conditions.
Thus, an energy input of 40 to 130 hp (about 2.5 to 8.5 hp/acre) would be required to meet the total respiratory demands of fish, plankton and sediment and maintain dissolved oxygen concentrations of 2 mg/L.
Clearly, mechanical aeration at 1 to 2 hp/acre will not improve the dissolved oxygen concentration in the entire pond, but is used only to provide a small refuge of aerated water near the aerator.
When dissolved oxygen concentrations are low, fish congregate in that area and remain there until oxygen conditions improve throughout the pond. The practice of aerating only a portion of the pond and only when concentrations fall to critically low levels has the disadvantage of routinely exposing fish to suboptimal concentrations of dissolved oxygen.
Despite this drawback, this is the most common aeration practice in commercial catfish ponds and is the only practice that has proved to be economically rational. Maintaining dissolved oxygen concentration above a critical threshold throughout the pond has not been shown to be economically justifiable using currently available aeration technology.
Author:
Craig Tucker
