of the young and extent of flooding are positively correlated, suggesting
that high flooding would increase growth and survival (Dudley 1974). Later
studies, however (Dudley 1979), indicated that year-classes are larger follow-
ing low-flood years. These Sarotherodon will not spawn if the water is too
deep, but Dudley considered that high year-classes following low-flood
years might be due to the Sarotherodon entering a stressed condition and
breeding at a smaller size in dry, low-flood years. In support of this view he
found ripe females of S. andersonii of 18 cm TL in 'dry' years, compared
with a minimum breeding size of 26 cm TL in high-flood years (Dudley
1979).
The relative growth rates of young and older fish may differ in different
lakes. For example, T. rendalli's first year growth appeared to be slower in
Lake Kariba than on the Kafue floodplain, but the Lake Kariba population
nearly caught up in weight in the fourth year and in length in the fifth
year (Kapetsky 1974).
For S. mossambicus in Lake Sibaya, South Africa, growth rates were
lower and fell off more rapidly (Bruton and Allanson 1974) than for this
species in Plover Cove Reservoir, Hong Kong (Hodgkiss and Man 197713)
/(Figure 4). The precocious breeding in Lake Sibaya has been explained in
terms of food quality (Bowen, this volume), the adults being in poor con-
dition as the food available to the deeper-living adults is less nutritious than
that available to the juveniles living in the warm shallows. Female growth
rates fell off more rapidly than male growth rates in both places. In Lake
Sibaya, females may breed at one year (8 to 10 cm SL), males when one or
two years old (12 cm SL); maximum age was 7 to 8 years, the largest males
23/29 cm SLITL, the largest females 18/22 cm (Bruton 1979).
Tilapia Production In, and Yield From, Natural WatersFor natural waters we have to distinguish clearly between the biological
production, i.e., the total elaboration of fish tissue during a given time
interval (generally taken as a year), and the yield, i.e.,the harvestable part of
the production. The catch, sometimes loosely called 'fish production,'
is really a yield. Ecologists considering energy transfers from one trophic
level to another are concerned with total production (all the fish tissue
produced in the time interval, whether or not it survives to the end of that
time), whereas fishery biologists are more interested in the available produc-
tion (the amount of fish tissue surviving at the end of that time). In natural
waters the yield (catch) is generally only a small proportion of the available
production, depending on the selectivity and efficiency of the fishing method
(both the gear used and how much the fish move about). Also, due to the
many causes of mortality in natural waters, the available production may be
much less than the total production (Table 2 and Figure 5 of data from the
Kafue floodplain show this well). In ponds, where fish are cropped by
draining the pond, the yield reflects the available production much more
closely, and, if predation and diseases are controlled, available production
may be much nearer to total production than it can be in natural waters.
Analyses of catch statistics are used for determining whether a fishery