The Biology and Culture of Tilapias

338 '
non-fe&ing period.
Many plants and animals adjust their life-history strategy to maximize
the intrinsic growth rate of the population ('r'). The 'decision' when to
produce gametes and breed involves a tradeoff with respect to any indivi-
dual's contribution to the population. By delaying breeding, the indivi-
dual increases its size and energy stores for the future whereas by breed-
ing early, it reduces its exposure to mortality risks and shortens the generation
time.
This theoretical framework can be fitted to all the tilapia examples
given. For example, in Lake George, there is a classical response of earlier
breeding with exploitation (fishing mortality). In Lake Sibaya, S. mossam-
bicus, when they reach about 8 cm, are exposed to heavy predation from
birds and Clarias if they remain in the rich feeding grounds inshore and here
they breed at a small size. The theoretical framework, however, does not tell
us the mechanism for the switch to breeding. The mechanism is probably
some sort of energy constraint rather than a simple behavioral or ecological
factor such as competition for breeding sites or mates.
The theoretical framework does, however, tell us where to look for
the mechanism or at least for ways to manipulate it withoot having to
unravel all the detailed behavioral, endocrinological and other physiological
interactions. The basic theories of population dynamics and natural selection
tell us to look for correlations between environmental changes and reproduc-
tive output.
Dr. Jalabert proposed a model of a balance beam between somatic growth
and gonad development with genetic and environmental factors controlling
its swing. It was generally agreed that the difficulty, expense and complexity
of endocrinological and other physiological research made it unlikely that
more research in these fields would bring rapid payoffs for aquaculture in
the form of techniques to manipulate the switch. The best approaches were
identified as behavioral studies, environmental manipulation and work on
the genetics of maturation age and size. It was felt, however, that some
useful techniques for culturists could emerge from attempts to identify and
synthesize pheromones or other waterborne factors. There was further
discussion of the existing experimental evidence and the best possible clues
for environmental studies.
The question of available oxygen was debated and it was pointed out
that 1 kg of small-sized tilapias (say individual weight about 10 g) require
about twice as much oxygen as 1 kg of, say, 200 g fish. It is possible, there-
fore, that in a pond situation with densely crowded small fish, the available
oxygen for digestiveloxidative processes becomes so reduced that metabolic
pathways are affected. Perhaps, as a result of this, nutrients begin to be
channelled into gonadal products. Also, as the gill surface area of fish limits
their ability to take up oxygen, the relative growth of this to the overall size
of the fish could be involved in the 'switch' in environments with variable
oxygen availability (see Pauly 1981Editors).
' There are two factors that could encourage a fish to continue to channel
)
all its assimilated nutrients into somatic growth rather than gonadal products,
unlimited food supply and a low risk of mortality. In culture, the questions
of food supply and quality are relatively easy to investigate. The risk or