exotic species into a waterbody); to
supplement natural production to increase
harvests; and to mitigate fish losses
resulting from human activities (Lannan
et al. 1989).
There may be insufficient local
broodstock for the required mass propa-
gation and other strains are often im-
ported. The growingpractice of enhancing
fisheries formerly based on wild
populations by introductions of hatchery-
raised stocks can cause severe genetic
damage. Hatchery-raised stocks are com-
monly much less variable genetically than
wild stocks. For example, hatchery-raised
salmon are used for stocking rivers flow-
inginto theBaltic. Originally, these rivers
produced about 10 million smolts annu-
ally, but the building of hydropower sta-
tions in the 1940-1960s made natural
spawning impossible. In Sweden, the
Swedish Water Law obliged the generat-
ing companies to build hatcheries to
compensate for this loss. In Norway, 10-
15 million Atlantic salmon fry are pro-
duced annually for restocking purposes
(Hansen et al. 1987a) and are released into
rivers regardless of their origin. Moreo-
ver, escapees from the sea cages of salmon
farms are 'homeless', and reproduce in a
wide range ofrivers (Hansen et al. 1987b).
It is highly probable that such escapees
from cages and land-based farms interact
with local native populations (Skaala et
al. 1990).
Introduced populations can also cause
direct genetic changes by interbreeding or
have more indirect effects through preda-
tion, competition for food, mates, spawn-
ing sites, or introduction of pathogens and
parasites. The breeding ofhatchery-raised
fish with wild populations can produce
hybrid or introgressed progeny that may
be less adapted to the local environment,
on the assumption that local populations
have evolved to be well adapted.
Over much of Europe, rivers and
streams have been and are regularly
restocked with non-native fry. Often,
hatchery programs work against the aim
of preserving natural populations, and
may cause a complete replacement of
natural populations with hatchery fish.
Natural populations ofbrown trout (Salmo
trutta), each showing specific allele fre-
quencies and at least one allele at high
frequency not previously reported, have
been identified in Spain (Garcia-Marin et
al. 1991). Sea ranching is well developed
in Japan, where seed from shore-based
hatcheries are released to the wild where
they grow and are subsequentlyharvested
by the existing fisheries (Davy 1991).Introduction of Exotic Species
Waterbodies have often been stocked
with exotic fish (Billington and Hebert
1991) without evaluating the genetic and
ecological impacts on local aquatic, ge-
netic resources and habitats. There are
many cases of successful colonizations by
exotic species [for example, common carp
(Cyprinus carpi01 and brown trout (Salno
trutta) in North America, or rainbow trout
(Oncorhynchus mykiss) in Europe and
Latin America]. However, little is known
of the genetic consequences. There are
some examples ofintroductions which have
caused severe damage to local populations
either by hybridizing with them, or by
outcompeting against them, leading even-
tually to extinction. Moreover, introduc-
tions may fail in thei~ objectives and may
lead to genetic degradation of natural
populations; for example, whitefish in
Czechoslovakia (Barus 1989). Introduc-
tions and transfers usually comprise small
numbers of fish. This can cause bottle-
necks and founder effects and reduced
genetic variation in future generations.
Introductions have increased with
demands from hatcheries and improved
communications. However, many intro-
ductions and transfers are uncontrolled
and undocumented. It is often djffkult to
determine the origin of introduced stocks.