would need very precise criteria to assess small variations in the intensity of gametogenesis
to investigate this. I remember for instance the contradictions between Hyder (1970a)
and Siddiqui (1977b) concerning the seasonality of S. leucostictus. Such contradictions
probably indicate the lack of precise criteria.HEPHER: The works I know show that, for fish in general, day length affects vitello-
genesis more than the other stages of oogenesis, but since you didn't mention this effect
at all, may I assume that it does not exist in tilapias?JALABERT: I don't know. We have to make controlled environment experiments to
investigate this. The same is true for the effects of salinity on reproduction and growth.
The evidence seems to be that in tilapias, responses to environmental changes are very
variable indeed and may differ even within the same species. We conducted some recent
experiments with trout to try to select for early and late reproduction to spread avail-
ability of eggs. We observed a shift in the timing of reproduction after only one genera-
tion. For tilapias, therefore, we can expect different responses for different species and
strains.
NASH: We cannot generalize about reactions to environmental stimuli with subtropical
and tropical fish. In mullet it is the light stimulus that triggers oogenesis and then the
temperature takes control. And, although you cannot suppress development completely,
you can delay it by temperature control even after triggering by light. There are groups of
fish for which the opposite is the case. The temperature acts as the trigger and the light
control acts as the monitor.MORIARTY: I would like .to ask a question from the point of view of the fish culturist.
Is it going to be of any practical benefit to have a much more detailed knowledge of the
reproductive hormones of these fish, or do you think that this is so complex that we
would be better advised to study environmental and social factors?JALABERT: Although the practical applications are not evident now, we do need more
work on the reproductive hormones of fish and on their modes of action, in order to
understand how environmental and social factors interact to modulate sexual activity.
I believe that management decisions could be more efficient if based on such basic
knowledge. But, from a strategical point of view, I feel that some fundamental ques-
tions are closer to application than some others; for example, are pheromones impor-
tant in tilapia reproduction?GUERRERO: In one experiment you described the separation of the two sexes of
tilapia. You say that there was no pheromone action because the fish still spawned.JALABERT: In this experiment the isolated females spawned anyway, or, more exactly,
released their eggs, but the interval between successive spawnings was increased. When fish
were together, this interval was reduced. We cannot rule out pheromones here, among the
other factors. On the other hand, spawning in artificial environment. can be affected by
many stress factors which are not necessarily specific.