Conservation and Sustainable Use of the Biosphere - WBGU

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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-

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