Environmental Engineering Reference
In-Depth Information
ponds and underwater cages, and from inland fresh-
water fishing from lakes, rivers, reservoirs, and ponds.
Figure 10-23 shows the effects of the global efforts
to boost the seafood harvest. After increasing fourfold
between 1960 and 1982, the annual commercial fish
catch (marine plus freshwater harvest, but excluding
aquaculture) has declined and leveled off (Figure 10-23,
left). After doubling between 1950 and 1956, the per
capita catch leveled off until 1983. Since then, it has
been declining (Figure 10-23, right) and may continue
to fall because of overfishing, pollution, habitat loss,
and population growth.
Kilograms of grain needed per kilogram of body weight
Beef cattle
7
Pigs
4
2.2
Chicken
Fish (catfish
or carp)
2
Figure 10-21
Efficiency of converting grain into animal protein.
Data in kilograms of grain per kilogram of body weight added.
(Data from U.S. Department of Agriculture)
Science: Effects of Overfishing and Habitat
Degradation on Fish Harvests
About three-fourths of the world's commercially
valuable marine fish species are overfished or fished
at their biological limit.
Fish are renewable resources as long as the annual har-
vest leaves enough breeding stock to renew the species
for the next year.
Overfishing
is the taking of so many
fish that too little breeding stock is left to maintain the
species' numbers.
Prolonged overfishing leads to
commercial extinc-
tion,
when the population of a species declines to the
point at which it is no longer profitable to hunt for
them. Fishing fleets then move to a new species or a
new region, hoping that the overfished species will
eventually recover.
Overfishing is not new. Historical studies indicate
that some species were overfished beginning centuries
ago. However, this trend has greatly accelerated with
the expansion of today's large and efficient global fish-
ing fleets.
According to the FAO, three-fourths of the world's
200 commercially valuable marine fish species are ei-
ther overfished or fished to their estimated maximum
sustainable yield. The Ocean Conservancy states sim-
ply, “We are spending the principal of our marine fish
resources rather than living off the interest they pro-
vide.” Some fisheries are so depleted that even if all
fishing stopped immediately, it would take as long as
20 years for stocks to recover.
Studies by the U.S. National Fish and Wildlife
Foundation show that 14 major commercial fish
species in U.S. waters have been severely depleted.
Also, degradation, destruction, and pollution of wet-
lands, estuaries, coral reefs, salt marshes, and man-
groves threaten populations of fish and shellfish.
Good news.
In 1995, fisheries biologists studied
population data for 128 depleted fish stocks and con-
cluded that 125 of them could recover with careful
management. This involves establishing fishing quo-
tas, restricting use of certain types of fishing gear and
methods, limiting the number of fishing boats, closing
fisheries during spawning periods, and setting aside
production and consumption would involve shifting
from less grain-efficient forms of animal protein, such
as beef and pork, to more grain-efficient ones, such as
poultry and herbivorous farmed fish.
Some environmentalists have called for reducing
livestock production (especially cattle) to decrease its
environmental effects and to feed more people. This
move would decrease the environmental impact of live-
stock production, but it would not free up much land or
grain to feed more of the world's hungry people.
Cattle and sheep that graze on rangeland use a re-
source (grass) that humans cannot eat, and most of this
land is not suitable for growing crops. Moreover, be-
cause of poverty, insufficient economic aid, and the na-
ture of global economic and food distribution systems,
very little (if any) additional grain grown on land once
used to raise livestock or livestock feed would reach
the world's hungry people.
Science: Harvesting Fish and Shellfish
After spectacular increases, the world's fish catch has
leveled off.
The world's third major food-producing system con-
sists of
fisheries:
concentrations of particular aquatic
species suitable for commercial harvesting in a given
ocean area or inland body of water. The world's com-
mercial marine fishing industry is dominated by in-
dustrial fishing fleets that use global satellite position-
ing equipment, sonar, huge nets and long fishing lines,
spotter planes, and large factory ships that can process
and freeze their catches.
Approximately 55% of the annual commercial
catch of fish and shellfish comes from the ocean using
harvesting methods shown in Figure 10-22, mostly
from plankton-rich coastal waters. Many commercially
valuable species are being overfished by these increas-
ingly efficient methods that “vacuum” the seas of fish
and shellfish. About one-third of the world's marine
fish harvest is used as animal feed, fishmeal, and oils.
The rest of the catch comes from using
aquaculture
to
raise fish much like livestock animals in feedlots in
