declined (i.e., the numbers had risen). (This high, maintained catch suggests
to me that the biomasses to produce these fish must have been considerably
higher than recorded here.) This yield is very high for a natural water body,
comparable with unfertilized fish ponds.
Lake George was found to have a very high gross primary production
(82,400 k~/m~/~r) made up primarily of the bluegreen alga Microcystis,
consumed and digested by the tilapia. (Net primary production proved very
difficult to determine; respiration rates of microorganisms were very high
at the prevailing high temperatures.) The production of the main herbivores
(S. niloticus and S. leucostictus which also took some bottom debris, Haplo-
chromis nigripinnis and the cyclopoid cope od Thermocylops hyalinus)
was computed to be equivalent to 650 kJ/m! /yr (Burgis and Dunn 1978;
Burgis 1978), a mere 0.8% of the estimated net primary production. Burgis
and Dunn compared this figure with Prowse's (1972) finding that fish produc-
tion was 1.02 to 1.79% of net primary production in Malacca fish ponds
containing tilapia and grass carp. Thus, despite the sustained high fish yield,
the Lake George system appears to be inefficient as so little of the high
initial primary production is passed on to fish flesh. The reason for this is
not clear; it was suggested that perhaps the lake has too few herbivores; fish
numbers do not seem to be limited by food here, but possibly they are by
availability of suitable spawning grounds in this soft-bottomed lake.
Discussion: Factors Controlling Tilapia Numbers
in Natural ConditionsPhysico-chemical events, such as drying up of lakes (as has occurred
for Lake Rukwa, Lake Chilwa and much of Lake Chad) have decimated
tilapia populations from time to time. The role of deoxygenation, silt stress,
alkalinity stress, heat stress and effects of drying out the shallow spawning
grounds in the decline of the Lake Chilwa fishes were studied by Furse et
al. (1979). Deoxygenation when bottom water was brought to the surface
sometimes causes massive fish kills (as in Lake George). Increased alkalinity
may kill fishes in the soda lakes and lagoons. Winter temperature drops
also kill or stress the fish at high latitudes (as in Hong Kong, Israel). On
floodplains, the variations in flooding from year to year affect tilapia survival
and growth rates, as has become clear from scale studies backcalculating
growth in previous years (Dudley for Kafue fish, Duerre for Barotse fish).
But in large lakes biotic pressures may be relatively more important than
physicochemical ones in controlling tilapia numbers.
- GROWTH AND REPRODUCTION
Tilapia numbers are greatly affected by their ability to switch from
growth to reproduction in unfavorable conditions. Though there appear to
be some limitations (presumably genetic) to growth possibilities (for instance