The Biology and Culture of Tilapias

to tilapia hybridization and then discovering methods to overcome these
barriers."
There is another area where it seems to me that fish culturists have been
slow to make use of available scientific knowledge. Genetic selection work
cannot proceed efficiently without good knowledge of the pedigree of
parental stocks, nor is it very effective if based on highly inbred stocks. In
reviewing the various experiments cited in the papers on hybridization work
in tilapias, I wondered how often contradictory reports on the effectiveness
of selection for growth and other characters have been the result of very
different levels of heterozygosity in the individuals used for the different
experiments. It is very evident among those who have been working with
experimental tilapia populations that (1) so-called pure strains have several
times been derived hom unrecognized accidental hybrids, and (2) many
research stocks have passed through one or more "genetic bottlenecks"
where the number of breeding pairs in the line has been reduced to well
below twenty-five.
It would seem to me very valuable if, in the course of these discussions,
we could identify other areas where "basic science barriers" stand in the way
of efficient development of tilapia culture. I gather that Dr. Avtalion's work
on electrophoretic markers was undertaken to determine the identity of
farmed stocks. The work is clearly encouraging and the discovery of a
male-specific protein is an exciting bonus.
I will turn now to some of the more specific aspects of fry and fingerling
production, for example, the high cost of overwintering fry in temperate
areas. Mr. Mires noted that Sarotherodon fry must be grown to at least 20 g
to be successfully overwintered. Drs. Hepher and Pruginin remark that
overwintering is economically feasible in Israel if all male fingerlings are
selected for growout in the following summer: the market prefers large fish
(400 g or more).
In Belgium, there is considerable interest in using warm water industrial
effluents for tilapia culture. Dr. Coche tells me that in Belgium the restau-
rants want tilapias weighing at least 300 g and preferably about 350 g.
There is possibly a similar market in the U.K. In many areas, however,
thermal effluents may be more profitably utilized to produce hybrid male
seed for growout elsewhere. Given that the cost of hybrid 50 g fingerlings
is about 35% of all growout costs for large tilapias, as reported by Dr.
Lovshin (quoting Tal and Ziv 1978), it would seem that the availability of
inexpensive, heated water could make specialization in seed production quite
profitable in some locations.
It was interesting to note, from Hepher and Pruginin's paper, that a late
spring warmup could prove advantageous in the combined culture of mixed
sex progeny (from natural spawnings) and hatchery-produced males, because
of the suppression of spawning below 20°C. Where cold water is readily
available to hold temperatures between 18" and 20" through the culture
season, the advantages of the resulting depression of breeding might offset
the reduced growth that would also occur.
Grading and sorting fry and fingerlings also seems to be an area where
economies could be achieved by mechanization or simplification. Dr. Lovshin
describes pen systems, as used by Pruginin in Uganda, and interconnected