subpopulations may provide beneficial new
alleles without harming local adaptation
(Allendorf 1983).
Intraspecific hybridization mixes gene
pools. The present 'Ivory Coast' orLBouake'
strain of 0. niloticus is actually a mixture
of Volta and Nile strains, which became
mixed in captivity. Its genetic identity
depends on the degree of hybridization
between strains which is not well docu-
mented (Nugent 1988).
Over one hundred combinations of
distant hybridizations have been recently
carried out in China, including crosses
between families, subfamilies, genera and
species. Five crossesbetween common carp
strains showed hybrid vigor with poten-
tial applicati.on in aquaculture (Wu 1990).
The Kurst strain of common carp is pro-
duced through hybridization to obtain
higher cold resistance, achieved through
excessive accumulation of body fat (Gall
1990).Conservation of Fish
Genetic ResourcesEx sifu conservation
Ex situ fish gene banks offer an alter-
native to in situ conservation of genetic
resources, but are difficult and costly to
establish and to maintain. They include
techniques such as rotational line crossing
and sperm cryopreservation. These are
particularly valuable where only a few
hatchery populations survive from for-
merly abundant stocks. Although requir-
ing expensive skilled labor and high tech-
nology, they may be in some cases the only
remaining means to prevent extinction of
some species and strains.
Fish gene banks should, as far as is
practical, represent the total gene pool. In
species where the spawning season is
protracted, material must be collected and
maintained to represent the entire spawn-
ing season. An adequate number of indi-
viduals must be kept to maintain high
genetic diversity. FA0 recommends effec-
tive population size, Ne, of at least 50 for
short-term conservation, and 500 for long-
term conservation (FAOLJNEP 1981).
These figures vary slightly with authors
(Tave 1986; Smitherman and Tave 1987).
Fish farms and public aquaria can play
important roles in the breeding and
management of endangered species.
In addition to stock collections, gam-
ete cryopreservation is a useful technique
in the conservation of genetic variability
and has the advantage of maintaining
high levels of genetic variability, without
maintaining large numbers of breeders
(Harvey 1987). Spermatozoa can be col-
lected from a range of known strains and
stored. Sperm banks should be managed
following the Codes of Practice already
established for livestock sperm and em-
bryo banks. Cryopreservation offish sperm
constitutes a haploid gene bank: only half
the genome is stored. Moreover, although
techniquesfor sperm cryopreservation are
establishedformany fish species, no sperm
collection has yet been established for
commercial purposes. Finfish eggs and
embryos cannot yet be cryopreserved.Examples of International, Re-
gional and National Efforts
The European Inland Fisheries Advi-
sory Commission (EIFAC), working with
FAO, has developed Codes of Practice for
fish introductions and transfers (Turner
1987). Recommendations have been for-
mulated and addressed to different audi-
ences: international organizations, gov-
ernments, aquaculturists, fisheries man-
agers, conservationists and research sci-
entists (FAOLJNEP 1981).
The Working Group on Genetics ofthe
International Council for Exploration of
the Sea (ICES) is promoting the develop-
ment of an International Register of
Available Strains of Fish and Shellfish.
ICLARM and FA0 are coordinating a
new international database (FishBase)