feeding or addition of organic fertilizer even if a small percentage of females
are present. If larger fish (2 400 g) are needed, then additional hand-sexing
or a predatory fish may have to be stocked with the less than 100% male
hybrids to control recruitment and allow a longer growing period.
- THE EVALUATION OF TILAPIA HYBRIDS
The evaluation of hybrids for culture performance is not easy because few
comparative data on fingerling production and growout are available. Only
two crosses, S. niloticus x S. hornorum and S. mossambicus x S. hornorum
are known to consistently produce all-male offspring. The male S, hornorum
appears to be the most dependable male parent of 100% male tilapia off-
spring. Several other crosses will, however, produce more than 75% males
consistently.
The maintenance of pure genetic lines is easier for some hybrid crosses
than for others. The S. niloticus x S. hornorum hybrid, when 10 to 15 cm
long, is relatively easy to distinguish from either parent on physical appear-
ance alone by a trained biologist. Differences in physical appearance allow
hatchery workers to easily determine the presence of contaminated brood-
stocks or mature hybrids in spawning ponds. The hybrids produced by
crossing S. mossambicus x S. hornorum and S. niloticus x S. aureus are
difficult to distinguish from their parents even by the trained eye. The
chances of pure broodstock becoming contaminated with these hybrids are
therefore increased. However, it is known that female S. niloticus crossed
with male S. hornorum produces a low number of hybrid offspring. S. hor-
norum and S. mossambicus are close relatives as are S. niloticus and S.
aureus. These pairs of species have similar reproductive behavior and colora-
tion which may result in increased spawning and hybrid fingerling produc-
tion, than, for example, S. hornorum with S. niloticus.
In countries where winter temperatures are low enough to threaten
the survival of tilapias, hybrids with S. aureus as a parent should be con-
sidered because of their superior cold tolerance.
Future Research NeedsFingerling production is the area in most need of research. If all-male
hybrid tilapia culture is going to have an increased impact, then improved
methods of consistently and economically producing all-male hybrid finger-
lings on a commercial scale will have to be worked out.
A further pressing need is for information concerning the genetics, repro-
ductive behavior and physiology of tilapias in relation to hybridization. What
exactly is the genetic mechanism that produces all-male progeny from some
crosses? Why are reduced numbers of fingerlings produced in some hybrid
crosses when compared with pure species fingerling production? What are
the visual and/or chemical cues which determine spawning compatibility
between species? What other factors may be influencing the frequency with
which two tilapia species hybridize? Does close taxonomic proximity of two