The Biology and Culture of Tilapias

In the immediate future it seems that the mass production of hybrid
tilapia fry will still depend on the number of spawners used for any given
cross.

Acknowledgments

I would like to thank Dr. G. Hulata and the genetic team in Dor station,

for allowing me to use some of their data. I would also thank Dr. Hulata, Dr.

Ernesto and Mr. Aldridge for reading the manuscript and for their remarks.

Discussion

NOAKES: In your Tables 1 to 3 giving the spawning performance of females, are these

individual females in isolated aquaria or are they all in the same tank?

MIRES: Tables 1 and 2 refer to individual females in individual cells.

NOAKES: Then these were in separate water bodies with no contact between them?

MIRES: No, they were separated by partitions but in a common water body. There

would be the possibility of chemical contact.

NOAKES: The reason for my question is that the apparent pattern suggested to me some

kind of hierarchical arrangement, or some kind of inhibition of spawning of some females

by others. Whatever the reason, the pattern of spawning by females is quite repeatable.

MIRES: I would say that there was no hierarchical arrangement. A hierarchy would

probably show in the amount of eggs spawned, whereas here, successful spawning females

always showed full spawning aggression and spawned normally.

NOAKES: It just occurred to me that the other females may sense chemically that a

given female has spawned close by and may be inhibited by this.

MIRES: If this is so, I have no knowledge of it. There was of course a water change in

this system.

PHILIPPART: Have you found differences in the number of spawn obtained from

different-sized females?

MIRES: The data we are discussing here were from comparisons with females of equal

size. They show a big variability in spawning frequency. I can say, however, that within

a spawning family in aquaria in my hatchery it always happens that a few of the females

are doing all the work and the remainder are not doing very much.

CHERVINSKI: Do you think that by draining your spawning ponds you are getting

higher fry production than by not draining?

MIRES: I believe definitely yes, and I would like to suggest why. We know that there are

definite differences in fecundity between species. Also within a species there are differing

individuals with higher or lower fecundity. Pond drainage and water change will therefore

improve production both from inter- and intraspecific spawning.