AVERAGE
WATER DEPTH
The recommended
average water depth is 1 meter for non-aerated static water ponds. The water
depth in the pond should be at least 80 cm at the inlet and no more than 1.2m
at the outlet.
The
Maximum Water Depth for static water pond production is
determined by the distance sunlight can penetrate into the water column. This
is because photosynthesis can occur in the water column up to twice the depth
the sun’s rays can penetrate. In ponds, sunlight can generally penetrate to a
depth of about 30-80cm depending on the levels of water turbidity. The oxygen
generated during the process of photosynthesis then dissolves into this section
of the water.
In addition, the
sun’s rays warm u p the water. Hence, the upper water column in ponds has the
best conditions for fish production. For this reason, a maximum water depth of
1.2m id recommended. Beyond this depth, pond waters start becoming devoid of
oxygen and stay cooler. Ponds thermally stratify from about 1.2m water depth
downwards. This means that the temperature and quality of the top 1.2m of water
will be distinctly different from that of the water below 1.2m deep. When the
pond is deeper than 2 meters, the total volume of bottom zero-oxygen water is
greater than that of the water containing oxygen. Incidences of fish kills
consequently, become more likely when there is a change in weather (for
instance when it rains, it is windy, it becomes cold, etc). This is because,
the cool rain causes the oxygen rich water at the surface to go down and in the
process, the oxygen-deficient water from the bottom is pushed up. The mixing of
the oxygen-deficient water from the bottom with the rest of the pond water
results into an overall total oxygen depletion. When this happens, the farmer
observes that fish which showed no signs of stress the previous day, or a few
hours ago, are suddenly all dead and floating on top of the pond. Even though
catfish above 100 g can survive water deficient in oxygen for a while by
breathing atmospheric air, there is also the additional risk of the mixing of
hydrogen sulfide and other toxic chemicals that may be present in the deep,
zero-oxygen layer. Hydrogen sulphide is lethal to fish, even in extremely small
amounts. See chapter 7 for more details on water quality requirements for
catfish production.
Therefore, there is no added advantage in
having a pond with a water depth greater than 1.2m. The deeper the pond is, the
more expensive it is to construct, and the riskier it becomes to manage water
quality because stratification becomes more likely – unless one has equipment
to mechanically mix and aerate the water. Such equipment is expensive and
requires a reliable source of power. In addition, for the current farm gate
price offered for table sized catfish in Nigeria, it does not yet make economic
sense to invest in mechanical aerators for grow-out ponds.
The
Minimum Water Depth in a pond should be not les than 60 cm.
When the pond water depth is less than 60 cm:
a. The
pond’s carrying capacity is reduced considerable. For catfish ponds, the total
volume is very important because the water dilutes the catfish waters. Because
catfish can breathe air after they have surpassed the fingerling stage (i.e.
from + 100g), waste build-up becomes the first limiting factor. Shallow ponds
have less water volume than deep ponds and therefore have a lower carrying
capacity. This means fewer kilograms of fish can be harvested from the same
pond area. Having the recommended depth therefore, increase potential to
harvest more kilograms of fish from the same pond, making it more productive.
b. It
is easy for wading birds, such as the marabou stork and heron to enter the pond,
scare predate upon the fish. Such birds can only enter ponds they can step down
in.
c. Aquatic
weeds are more likely to grow in shallow ponds. Excess weeds in the pond
interfere with seining and reduce the levels of dissolved oxygen available for
fish production. Consequently, FCRs are likely to increase and pond yields to
decline.
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