The Biology and Culture of Tilapias

sex broodstock are overwintered in extremely dense conditions (50 kg
fish/m3 of water) at 15 to 17'~. This does not affect their reproductive
performance when put out to ponds. Presumably, the low winter temper-
atures suppress aggressive behavior.
Dr. Lovshin mentioned the possible advantages of sexing broodstock while
they are still immature and then keeping the sexes in separate ponds until
they are ready to spawn, as is done for carp breeding. This method gives
a high degree of control for handling pure stocks and for hybridization
work.
The best sex ratios to use for high broodstock performance were then
discussed. Experimental work in the Philippines has shown that a ratio of
one male to one female gives better results than one male to two or three
females for S. niloticus and S. aureus broodstock in plastic pools and aquaria.
This result is, however, in conflict with some other work, for example
.Campbell working in the Ivory Coast has found that high female:male
ratios of 5 or even 7 or more to one has given very intensive reproduction of
S. niloticus in shallow raceway-type ponds. The best sex ratio varies with
specific breeding behavior. For example, in S. melanotheron (T. heudelotii)
in the Ivory Coast, in which the male is the mouthbrooding parent, it is 1:l
in ponds.
In contrast to this, Dr. Lovshin's work in Brazil and the concept of 'male
pressure' suggests that even higher numbers of males may be advantageous.
Dr. Lovshin pointed out, however, that there is a difference between intra-
specific or pure strain crosses, in which sex ratios of about one male to
three females seemed to be generally accepted, and the interspecific hybrid
crosses where he had harvested more fingerlings from ponds with 'male
pressure': two males to one female. This may be due simply to the constant
availability of males to court any ripe female, but more work on sex ratios
in ponds and restricted environments is urgently needed.
It is not clear whether chemical stimuli play an important part in the
reproductive behavior of tilapias. In salmonids, males use chemical recognition
to select females that are ready to spawn. If such chemical stimuli are also
produced by tilapias, they could be major factors in mate selection and in
reducing male aggression against ripe females. Dr. Noakes pointed out that
there were fundamental differences between the salmonids and the lek
breeding tilapias and similar groups, like the centrarchids. In the salmonids,
the female excavates the nest and the male undoubtedly recognizes and
chooses his mate partly by chemical recognition. In the lek breeding tilapias
and centrarchids, however, it appears to be the female that makes the choice,
and of course, the male that excavates the breeding pit. For the centrarchids,
it has been shown that a male will attempt to court any fish or object qat
comes into his breeding territory. If this also applies to the tilapias, it argues
against chemical stimuli having importance over behavioral cues. Dr. Lowe-
McConnell added the observation that S. karomo in the field wander through
the breeding arenas and appear to inspect not only the males but also their
breeding pits before selecting a mate. This was observed in very clear water.
The problem of male aggression appears to be very different in restricted
environments, such as tanks and aquaria, from the situation in ponds. In the