The Importance of Dissolved Oxygen in Fish Culture


 By James Rakocy
 Research Aquaculturist
 C.V.I. Agicultural Experiment Station

 A recent marketing study on St. Croix has indicated that a
locally grown freshwater fish called tilapia compares favorably with
saltwater fish and, if available for sale, would be purchased by a
large number of consumers. At the College of the Virgin Islands
Agriculture Experiment Station, these food fish are being raised in
cages in freshwater ponds in an effort to develop procedures for
commercial production. The results thus far indicate that tilapia
farming appears to be a potentially profitable enterprise that would
benefit the Virgin Islands by creating employment and reducing
dependence on imported fish.

 Fish culture involves some basic differences that are not
encountered in other types of animal production. One very impor-
tant difference is that fish must extract the oxygen they need for
growth and maintenance from water rather than air. Oxygen is
abundant in air, comprising 21%, but is scarce in water. Oxygen
that dissolves into water constitutes about 0.001% of the water
and yet oxygen is just as vital to fish as it is to land animals.
A tilapia farmer in the Virgin Islands should therefore understand
the principles that govern dissolved oxygen in ponds and follow
procedures that will ensure the maintenance of adequate levels
of dissolved oxygen for maximum fish production.

 Fish prefer to live in water that is "saturated" with oxygen.
 Water is described as being saturated when it contains all of the


dissolved oxygen that it can theoretically hold. Water containing
less or more than the theoretical concentration is said to be under-
saturated or supersaturated with oxygen, respectively. The amount
of oxygen in water at saturation depends on the water temperature
and other factors. For example, at 700F the concentration of
dissolved oxygen at saturation is 9.0 milligrams of oxygen per
liter of water (mg/liter). As the water temperature increases to
900F, the dissolved oxygen concentration decreases to 7.4 mg/liter
at saturation. Therefore, less dissolved oxygen is available to fish
during hot weather.

 Tilapia are very hardy fish that can tolerate low dissolved
 oxygen levels, but they feed most vigorously and grow most
 rapidly when dissolved oxygen concentrations do not average
 less than 5 or 6 mg/liter (about 66% saturation). At lower dis-
 solved oxygen levels, feeding activity and growth decrease. At a
 dissolved oxygen concentration of 1 mg/liter, tilapia become
 stressed and rise to the surface to obtain additional oxygen in the
 thin surface film. Tilapia can live this way for several hours, but as
 the dissolved oxygen level approaches 0 mg/liter, they will even-
 tually die.

 In fish culture ponds, oxygen levels fluctuate greatly during
 the course of a day (Figure 1). Dissolved oxygen concentrations,
 which are lowest at one hour after sunrise, increase during the
 daytime until late afternoon and then decrease steadily throughout
 the night. In tilapia ponds, concentrations will vary from less than
 2 mg/liter at 7:00 AM. to greater than 20 mg/liter at 4:00 P.M.
 Oxygen is produced during the day by microscopic plants called


14 -


12 -


10 -


 12 6 p.m. 12
Figure 1. Daily Dissolved Oxygen Cycle in Fish Culture Ponds.


6 a.m.


6 a.m.


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