are slow-growing. For example, they may breed for the first time at an age of
less than one year (compared to three or four years in a big lake); they
produce smaller eggs in these situations, and they may spawn repeatedly at
relatively short intervals. Inshore species in a lake tend to breed earlier (in
age) than do offshore species (e.g., in Lake Malawi), and the same species
living in smaller (and therefore more "harsh") lakes breeds at an earlier age
(and smaller size) than when living in larger lakes. She also draws attention
to an example we shall return to in some detail, that of tilapias in fish
culture ponds. Under these conditions the fish (e.g., S. mossambicus and S.
niloticus) can breed when only a few months old, at about one-half the
typical size at first reproduction in the field, and produce relatively more,
but smaller eggs than fish of comparable size would if they bred at that size
in the field.
Fryer and Iles (1972) cite a number of examples, too numerous to repeat
here in their entirety, but worth mentioning in a summary fashion. Again,
they clearly drew attention to the adaptive nature of such responses in
tilapias, for which we are proposing the mechanism. They reported the
response of fish in "confined" conditions (i.e., dams, pools, ponds) to breed
at smaller sizes (earlier ages), to breed more frequently, and to produce
smaller eggs than normal (the extremes of these sometimes being reported
from aquarium conditions). Under natural conditions, bigger species grow
faster (but these data need to be extended), mature at a later age, and
live longer (observations remarkably in accord with our description of the
precocial life style) (see also Lowe-McConnell, this volume). From the
evidence available, they concluded that these responses, especially the
"stunting" of body size, were due to a phenotypic (rather than genetic)
change in the fish (i.e.,reversible). They also concluded, quite correctly, that
the phenomenon is not one of "stunting", but of relatively earlier breeding
in the fish (they described the situation as "neoteny") (but see Balon 1981b
for a discussion of this term). They defined neoteny as the ability to breed
successfully while still in the juvenile interval of development, and elaborated
on the potential survival value of this property for tilapias. They suggested
that this property is at least in part responsible for the widespread distribution,
plasticity and success as colonizers of tilapias, particularly in shallow lakes
subject to fluctuations in water levels. Others have noted such "generalist",
"colonizing" abilities of these fishes, and have also drawn attention to the
apparent relationship between this and the relative lack of adaptive radiation
in these species (only about 100 species of tilapia sensu lato), compared to
the explosive speciation in genera adapted for stable, lake conditions (e.g.,
Haplochromis, sensu lato).
Some Comments on CultureIn a very real sense, the interests of fish culturists are working at odds
with the biology of these fishes. The culturist wishes to raise the fish in a
monoculture (or simple polyculture) system, with a high density of fish, high
growth rate, and a harvest (yield) of large individuals in as short a time as
possible.