Establishment and Conservation of Pure StrainsVery few pure strains of tilapias are available to culturists. Only where
stocks are continuously tested by electrophoretic markers and selected can
genetic purity be ensured, e.g., the S. aureus stocks at Mr. Mires' hatchery
at Kibbutz Ein Hamifrats, Israel, tested by Prof. Avtalion. Elsewhere,
culturists are relying on wild stocks, often contaminated by hybridization
with introduced species, or on introductions made many years ago from very
restricted gene pools, e,g., about 30 S. aureus individuals sent from Israel to
Auburn University, Alabama, have been the ancestors of widespread intro-
ductions for culture in Southeast Asia and Latin America, where it is not
surprising that they are showing signs of inbreeding depression.
There is a clear need for collections of known strains of tilapias for
research and culture work. The establishment of such collections was dis-
cussed and the following points were made:
- Collections would have to be very carefully maintained by painstaking
workers in high-security premises and replicated at several different
locations. - Tilapia broodstock collections are probably not a function for re-
gional aquaculture centers in tropical developing countries where such
work would be difficult to capy out and would normally take lower
priority than production-orientated research. - The location of some collections in colder countries could prove
advantageous as escapees would not survive to cause environmental
problems. - The facilities needed to accommodate large numbers of broodstock
of different species and strains would be very costly. - Tilapia researchers have much to learn from domestic animal and
plant breeders in planning such collections.
It was agreed that the methodology for maintaining pure strains needed
further development. Cryopreservation of semen could prove a useful future
technique for storing male genetic material. At present, the collections of
tilapias maintained in Israel, the United Kingdom (Stirling University), the
Philippines (Central Luzon State University) and the U.S.A. (Auburn Univer-
sity) were recognized as the most significant. In the absence of funding or
plans for larger collections, it was stressed that natural populations of tilapias
in African waters should be carefully protected. Transplantations should be
avoided wherever possible to assist in the conservation of genetic material for
the future. For example, the natural populations of S. hornorurn are probably
uncontaminated at present. There is a clear need for cataloguing critical sites,
such as the small lakes in the Cameroons and possibly Lake Bosumtwi. The
International Union for the Conservation of Nature (IUCN) is aware of the
vulnerable nature of these populations.
Three main objectives were identified concerning the genetic improve-
ment of cultured tilapias: 1. The guaranteed supply of high quality pure
strain or hybrid fry for farmers; it was recognized that these would be
expensive and that restriction of distribution to monosex progeny may be
desirable. 2. Conservation of wild tilapia stocks. 3. The absolute necessity for
all researchers to work with known material.