eutrophication, other forms of pollution,
or changes in the environment caused by
humans (such as siltation of spawning
sites or introduction of exotic species) which
may render a stock more vulnerable to the
stress of overexploitation (Smith 1968;
Regier 1973). Selective elimination of
subpopulations or stocks has attracted the
most attention (STOCS 1981). Within a
stock, Nelson and Soul6 (1 987) distinguish
between undirected loss of genetic vari-
ation (inbreeding, genetic drift) and direc-
tional changes (selection). These are dis-
tinct problems. Genetic erosion is the loss
of genes within a populationlspecies, re-
sulting in a much smaller gene pool in the
individuals surviving from the original
populationlspecies, which had formerly
much higher genetic diversity.
Enuironmenfal Change
Human activities cause severe habitat
alterations and produce djffferent forms of
pollution, e.g., eutrophication, toxins from
industrial wastes and thermal pollution.
In some regions, water quality has been
severely affected by precipitation acidifi.ed
by combustion of fossil fuels. Some fish
species, especially their early life history
stages, are extremely sensitive to water
acidification (FAO/LINEP 1981). In south-
ern Norway, this has caused the total
disappearance of fish from hundreds of
lakes, and of salmon in many river sys-
tems (Gjedrem1981). Hydroelectric power
development has also affected fish
populations through the building of dams
which created barriers to spawning mi-
grations and destroyed spawningsites and
nurseries. New dams have led to the
extinction of many salmon stocks
(Saunders 1981).
Cap f ure Fisheries
Capture fisheries can cause the loss
of aquatic genetic resources as a conse-
quence of overexploitation or from selec-
tive fishingpractices. In fisheries, the firststocks to disappear are those with prop-
erties most desirable to the fisheries or to
future enhancement or aquaculture ef-
forts, e.g., rapid growth and high
catchability (Thorp and Koonce 1981).
Recent technological improvement in fish-
ing gear has resulted in increasing har-
vesting efficiency, which in some cases has
caused the depletion of some fish stocks
and species. Overexploitation may also
cause genetic drift, where only a limited
gene pool survives hamesting. Fisheries
management may also have signifxant
but involuntary effects on genetic re-
sources, by setting up restrictions for time
and location of capture and type of gear
used. In particular, mesh size can be very
selective, affecting a particular size of the
population.
In the salmon fisheries ofWest Green-
land, different genetic stocks are pooled
together in fished stocks, as they gather
in common pelagic feeding areas (Stahl
1987). These 'mixed-stock fisheries" are
therefore difficult to protect, but harvest-
ing may cause overexploitation of the
numerically smaller stocks. Overfishing
of Baltic salmon has resulted in slower
growth, smaller size at maturity andearlier
homing. In Lake George, fishing pressure
reduced the size and maturation size of
Oreochromis niloticus over an eleven-year
period (Gwahaba 1973; Lowe-McConnell
1982).
Overfishing can also affect an entire
fish community. In Lake MalaGi, inten-
sive fishing of cichlids has caused the
decrease of large species, which domi-
nated before, and now small species pre-
dominate (Turner 1976).Enhanced Pisheries
There is a growing need to enhance
fisheries, particularly inland fisheries, by
stocking of hatchery-raised juveniles
followed by harvesting through fishing.
Fisheries enhancementhas threepurposes:
to create new fisheries (often introducing