Conservation and Sustainable Use of the Biosphere - WBGU

164 E Diversity of landscapes and ecosystems
2,500
2,000
Spawning fish biomass
Figure E 3.4-1
Development of herring
stocks in the North Sea and
how they were fished
between 1960 and 1998.
Source: Hubold, 1999a
Biomass [1,000 t]
1,500
1,000
500
Landings
0
1960 1970 1980 1990
120
1.6
100
Young
Mortality through
fisheries
1.4
Young at age 0 [billion]
80
60
40
1.2
1.0
0.8
0.6
0.4
Mortality through fisheries F
20
0.2
0
1960
1970 1980 1990
0
greater selectivity of catching methods in order to
protect fish stocks and the environment.
Effects of overfishing on the continued
viability of species
In spite of the heavy overfishing, the risk that species
of pelagic bony fish will become extinct is rather low
because of the wide geographical distribution of
these species. However, more recent studies show
that there are widespread pelagic fish species from
isolated populations that can vary greatly in genetic
terms. It can therefore not be ruled out that some of
these are already true species. Overfishing can therefore
lead to a loss of genetic diversity (Malakoff,
1997). In any case, fishing results in a shift in the
species spectrum from long-lived to short-lived
species and to a reduction in biological diversity.
The situation among fish in coastal waters is much
more critical. For example, some coral fish species
are already endangered from heavy overfishing (Section
E 2.4).The situation is similar with regard to fishing
for cartilaginous fish, which reproduce slowly and
make up for their small numbers of young with intensive
brood care (K selection). Worldwide catches of
the 400 shark species, 500 skate and 35 chimera
species rose from 272,000 tonnes in 1950 to 760,000
tonnes in 1996. Currently, 20 shark species are considered
to be overfished, including the spiny dogfish
in the North Sea and the north-west Atlantic. However,
improved statistics would be desirable for a
more detailed analysis of the situation.
As far as the marine mammals are concerned,
above all the fall in the numbers of large whales as a
result of whaling is alarming. With residual popula-