changing environment during their evolution (Fryer and Iles 1972). Lately,
the Ein Hamifrats fish farmers (unpublished) have observed that when a
spawning pond becomes overcrowded with fry, the spawning activities of
parent fish cease completely. In these conditions, there are no mouthbrooding
females and the few nests present seem to be unattended.Intraspecific SpawningA lot of work has been published on the reproductive behavior of tilapia
spp., but very little has been said about the individual spawning potential of
females. The only possible way to study this closely is under aquarium
conditions, where the females are kept in separate aquaria or in separate
compartments of the same aquarium, because in spawning groups a hierarchy
may become established in aquaria and cause some dominant females to
spawn more than others (Fishelson 1966a; Mires 1973; Rothbard 1979).
During the first working period in the Ein Hamifrats hatcheries, some
valuable data were gathered on the individual spawning potential of S.
niloticus B and S. vulcani (Mires 1977). A few females weighing 400 to 500 g
were introduced into separate compartments in aquaria and kept at a con-
stant temperature of 25 to 28°C year-round, fed with 25% protein pellets.
The day on which each female was ready to spawn was recognized by the
form of the genital papilla being much larger and more erect than usual.
Such a female was transferred to an aquarium where one male of the same
species was held. If the female was ready to spawn, the male would imme-
diately start courting her; but if not he would start chasing her around and
eventually bite her severely. In such a case the female was removed and
placed back in her compartment.
After spawning, the female was separated from the male by a screen,
and was left to mouthbrood her eggs for a few days. The larvae were then
removed from her mouth, and cared for in Zuger bottles for the rest of
their larval development. When a female spawned without the presence of a
male the spawn was recorded, but unfortunately the eggs were not counted.
The results for one group of S. vulcani and another group of S. niloticus B
are given in Tables 1 and 2. Although the females were kept in separate
compartments of the same aquarium, it is possible that slight environmental
differences existed (corners versus center). Their spawning frequency was
very variable and they did not spawn year-round despite the controlled
temperature. In both groups the maximum number of individual spawns was
7: represented in both cases by only one female. It is interesting that the
proportions of prolific and poor spawners were similar for both S. niloticus
and $. vulcani. The same phenomenon exists with interspecific spawns.
McBay (1961) gathered data on the spawning of what was then called
T. nilotica, later re-identified as S. aureus. The fish were paired in 40-liter
aquaria and the egg or fry production was recorded and counted. Out of 17
females, one spawned 6 times, two spawned 5 times, one spawned 4, and all
the rest spawned three times or less.