Dear Friends:

Julian Chara [julian@cipav.org.co] asked
> I have a comment about table 6, in which you made a comparison of the
> economic benefit of the two systems:
> I understand that you did not include cost of aeration since it is common
> to both systems. However I would like your comments about the
> importance of aeration, considering the costs and benefits obtained.
>
> Q.1. Aren't these costs significative? i.e. initial investment and operation costs.
>
> Q.2. Considering the results and water quality parameters obtained,
> would you recommend the use of aerators for a similar stocking density?

I was at Saint Horge Farm in Manizales in Oct. last year. I knew a few
aerators are installed in fishponds in Colombia. Aeration has been one of
important factors to increase fish production in China since the 1970s,
especially if your production target is over 7.5 t/ha. The cost of aeration
depends, but it may be about 5% of the total cost of fish farming. Let me
give you a little more information about aeration.

--
Mechanical aeration
Li Kangmin

Aerators are mechanical devices that increase the rate at which oxygen
enters water. They are comparatively effective devices to improve water
quality, prevent fish gasping and increase fish yields if the production
target is above 7.5 t/ha.

There are two basic techniques for aerating pond water:
(1) water is splashed into the air or
(2) bubbles of air are released into the water.

Splasher aerators can be divided into vertical pump aerator with an
impeller, pump-sprayer aerator, like the photo of this paper
http://www.ias.unu.edu/proceedings/icibs/ic-mfa/li/3.jpg and a paddle wheel
aerator, which splashes water into the air as the paddle wheel rotates.

The functions of aerators are to increase dissolved oxygen, to stir pond
water, and to aerate the harmful gas in pond water. Under different
conditions one or two functions are dominant at a time.

1. Increasing dissolved oxygen
The effect of increasing dissolved oxygen is the inverse of the saturation
degree of dissolved oxygen in pond water. Therefore, an aerator will bring
the effect into full play when the DO is lower in the night or at dawn.
Also the effect of increasing dissolved oxygen is related to the power and
loading. When fish gasp at mid night, opening aerator can increase DO from
1.4 mg/l to 2.6 mg/l after 3 hours.

2. Stirring water
In ponds, heat enters at the surface and the surface water heats faster
than deeper waters. In temperate regions, large ponds may stratify in the
spring and remain stratified until fall. In small, shallow ponds in
temperate regions and in tropical ponds, stratification often exhibits a
daily pattern. If a 3 kilo watt aerator operates for one hour, it can mix
the epilimnion (uniformly warm water) and the hypolimnion (uniformly cold
water) of the 0.6 ha pond with 2.5-2.7 m , water depth and the DO will be
evenly distributed.

3. Aerating harmful gas
Aerator can accelerate the volatilization of harmful gases such as H2S, NH3.

The rational use of aerators has some advantages. It can help utilize water
body fully. It can mix water temperature, and it will prevent or save fish
from gasping or surfacing. It will enable us to increase the stocking
density, to stabilize water quality, to be able to increase feeding, to
raise metabolism of fish, and to be able to prevent fish diseases. In
comparison, the fish yield of a fishpond with aerators is 13.8-14.4% higher than the control.