Fish, chitin, and chitosan production 281Stocking density
In addition to the limitations imposed by the available oxygen, the density of
fish population can increase to the point of growth inhibition. Overcrowding can
result in a reduction in growth rate due to stress and competition for food,
oxygen and space, and in poor food utilization caused by wastage. It is generally
known that there is a limit to the density at which fish can be stocked in ponds,
for beyond a certain point, the advantage of growing a larger population of fish
is cancelled by the slower growth of the fish, in spite of an excess of food.
Figure 6.10 shows some experimental results of mean weight of fish being
cultured at different stocking densities (SD) in earthen ponds, but at a constant
organic loading of 150 kg COD/(ha-day). Each pond had a dimension of 20 × 10
× 1 m^3 (length × width × depth). These data clearly show the effects of SD on
the mean fish weight. After 6 months of operation, the highest mean fish weight
of 118 g was observed in the pond having a SD of 1 fish/m^2 , while the lowest
mean fish weight of 27 g occurred in the pond with a SD of 20 fish/m^2.
The data from Figure 6.10 were used in plotting Figure 6.11 to show the
effects of SD on the mean fish weight and total fish yield. It can be seen that
although a lower SD could give a higher individual fish weight, it resulted in a
lower total fish yield; the opposite took place in case of a high SD. This
information implies that fish to be used as human food should be reared at a low
SD to obtain table-size fish. Fish to be used as animal feed can be reared at a
high SD to maximize the total fish yield; these fish, small in size, may be used
directly or processed further prior to being used as animal feed.
Hydrogen sulfide (H 2 S)
Un-ionized H 2 S, mainly from anaerobic decomposition of bottom sludge, is
extremely toxic to fish at concentrations that may occur in natural waters.
Results from various bioassay studies suggest that any detectable concentration
of H 2 S should be considered detrimental to fish production (Boyd 1979).
Sulfide formation often occurs in anaerobic and facultative waste
stabilization ponds due to the reduction of sulfate (SO 4 2-) under anaerobic
conditions:
Anaerobic
SO 4 2-+ organic matter S2- + H 2 O + CO 2 (6.2)
bacteria