tend to trigger spawning. The food obviously has a role to play both in terms of quantity
and quality. To my knowledge there has been very little attention paid to food quality
with regard to this switching mechanism.
NOAKES: In temperate species, in highly seasonal conditions, food itself may not be SO
critical. The animal will be committed to enter a reproductive cycle. In the tropics,
however, food may be much more important.
CAULTON: I would like to make one comment. About three years ago, we tried some
high density rearing of T. rendalli. We found that they bred very easily, but we did not
find many offspring in the pond because they are very cannibalistic. We were really
recycling the gonadal products back into the food chain, and we got very good size fish
from this system. As the fish has a short reproductive cycle, not too much energy is
devoted to breeding and this is a useful way to combat it.
NOAKES: I would like to suggest that anything which alters the juvenile survivorship,
or which decreases their mortality, will induce the fish to put more effort into growth
and less into reproduction.TREWAVAS : I would like to suggest that we avoid using the terms precocial and altricial
in connection with tilapias. i do not think they help much, and they cause confusion as
we already use the word 'precocious' for tilapias which breed at a small size, whether they
are substrate-spawners or mouthbrooders--this dwarfing occurs in both groups. In Dr.
Noakes' paper these terms appear to be used in two ways: (i) equated with substrate-
spawners (altricial) and mouthbrooders (precocial), and (ii) with early and late develop-
ment, a phenomenon which occurs in both groups. Thus, for example, the dwarf popula-
tions of S. niloticus breeding precociously in lagoons have, according to Dr. Noakes'
terminology, become more altricial (hence less precocial) than the larger-growing lake
populations.
Then there is heterochrony. The contact reaction of juveniles to parents is activated at
an early stage. The reaction can then go on for some weeks. In S. melanotheron, which
has a large egg, the young are said to exhibit no contact reaction after release. But Bauer's
(1968) experiments with extra-orally reared embryos show that the reaction, though a
weak one, is initiated at the usual time, but because of the large yolk, the embryos are
normally then still in the parental mouth. This can hardly be called heterochrony.
It has been shown that certain structural and behavioral features develop at the same
time in both substrate-spawners and mouthbrooders. These are incapable of expression
in terms of function in the mouthbrooders, because the embryo is still attached to a
heavy egg and is passive in the parental mouth. Hence, the temporal factor is constant,
the key difference being the size of the egg, as Fishelson (1966b) showed.