used in fish biology and fisheries manage-
ment (Ferris and Berg1 987; Bermingham
et al. 1991; Feryson and Thorpe 1991).
In particular, it is an extremely useful tool
in the detection of intraspecific variability
(Avise et al. 1990; Seyoum and Kornfield
1992; Crosetti et al., in press).Cytogenetics and Karyotyping
Fish chromosomes are small and
homologies are difficult to detect without
refined techniques to show specific stain-
ing and banding. However, there are
groups which differ markedly in their
karyotypes and cytogenetic studies can
provide significant markers. The Atlantic
salmon, for instance, has a diploid number
varying from 55 to 60, and differences in
chromosome arm numbers have been
found among its European populations
(Davidson et al. 1989). Polymorphisms in
chromosome number have been detected
in different populations and among indi-
viduals of the same population in Seriola
dumerili CVitturi et al. 1986).Fish Gene Pools
The gene pool of a given population
or species is the set of genotypes of indi-
viduals that form that particular popula-
tion or species (Rab 1989). Being a dy-
namic open system, it varies with time and
can be easily disturbed. At a given time,
a particular gene pool has evolved to
become adapted to the local physical,
chemical and biotic conditions of its en-
vironment. The gene pools of today are the
result of evolutionary processes that have
affected those taxa over thousands or
millions of years. For domesticated organ-
isms, however, humans have bypassed
natural selection with rapid and very
strong selection for specific traits.
Many fish species have a complex
structure of subpopulations more or less
genetically differentiated. Some are com-
posed of populations which are spatially
and genetically isolated and cannot mix
their gene pools. This applies particularly
to freshwater fish species that extend across
different watersheds.
The concept of a fish stock has been
well discussed to define its different
meanings in fisheries and aquaculture
(STOCS 1981). Particular care has been
devoted to discriminating between fish
stocks for fishingpurposes (more precisely
defined as mixed fisheries stocks), and
stocks in the genetic sense, i.e.,
subpopulations more orless reproductively
isolated by time or spawning locations
(Nelson and Soul& 1987).Genetic Impoverishment
in Fish PopulationsThere are many degrees of genetic
impoverishment, as evident by the scale
of danger criteria established by IUCN for
taxa threatened with extinction. Very
often, only the threat of species extinction
is strong enough to induce some kind of
human reaction towards protection. How-
ever, in the long term, reduced genetic
variability or the extinction of a single
stock could be very serious.
Human activities are the primary
cause of genetic impoverishment in many
fish stocks and species and trends towards
their extinction. Aquaticgenetic resources
can be affected by environmental changes
(particularly pollution); fisheries; artifi-
cial selection and domestication in
aquaculture; and transfers and introduc-
tion of species (which may result in hy-
bridization, introgression and founder and
bottleneck effects) (FAOIUNEP 1981;
Wohlfarth 1986).
In natural fish stocks, loss of diversity
is most evident in changes in species
composition in intensive and selective
fisheries, although these changes may be
confounded with losses brought about by