Marine Geology Phần 8 pot

the bottom of a chamber and forces it into a vertical tower, where the com￾pressed air spins a turbine that drives an electrical generator.

Tidal power is another form of energy. Gulfs and embayments along the

coast in most parts of the world have tides exceeding 12 feet, called

macrotides. Such tides depend on the shapes of bays and estuaries, which

channel the wavelike progression of the tides and increase their amplitude.The

development of exceptionally high tidal ranges in certain embayments is due

to the combination of convergence and resonance effects within the tidal

basin. As the tide flows into a narrowing channel, the water movement con￾stricts and augments the tide height.

Generating electricity using tidal power involves damming an embay￾ment, letting it fill with water at high tide, and then closing the sluice gates

at the tidal maximum when a sufficient head of water can drive the water

Figure 167 Wind

turbines at San Gorgonio,

California.

(Photo courtesy U.S.

Department of Energy)

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Marine Geology

turbines. Many locations with macrotides also experience strong tidal cur￾rents, which could be used to drive turbines that rotate with both the incom￾ing and outgoing seawater to generate electricity.

Thermonuclear fusion energy (Fig. 168) is both renewable and essen￾tially nonpolluting.The fuel for fusion is abundantly available in seawater.The

energy from the fusion of deuterium, a heavy isotope of hydrogen, in a pool

of water 100 feet on each side and 7 feet deep could provide the electrical

needs of one-quarter of a million people for an entire year. Fusion is safe. Its

by-products are energy and helium, a harmless gas that escapes into space.

Figure 168 An artist’s

rendition of the

International Fusion

Experiment (ITER) at

Princeton, New Jersey.

(Photo courtesy U.S.

Department of Energy)

225

Sea Riches

HARVESTING THE SEA

The world’s fisheries are in danger of collapsing from overfishing. The

United States created its marine sanctuaries program in 1972, when oil spills

and treasure plundering began to pose a significant threat to its offshore

resources. These sanctuaries prohibited oil drilling, salvaging, and other

activities deemed harmful to the marine ecology. Yet all sanctuaries still

allowed fishing. Most also permitted boating, mining, and other potentially

disruptive activities. However, since the program’s enactment, overfishing

has become a much greater threat than oil pollution. Dwindling fish stocks

such as cod and haddock have crashed in coastal waters, some to the brink

of extinction.

The relative abundance of various species has changed dramatically in

many parts of the world.The dangers result from a constant harvest rate of a

dwindling resource caused by fluctuating environmental conditions, resulting

in a major decline in fish catches.The composition of the catch is also chang￾ing toward smaller fish species. Even the average size of fish within the same

species is becoming smaller.

Overfishing drives populations below levels needed for competition to

regulate population densities of desired species. Therefore, under heavy

exploitation, species that produce offspring quickly and copiously have a rel￾ative advantage. The extent to which these changes are due to shifts in fish

populations, changes in patterns of commercial fishing, or environmental

effects is uncertain. What is apparent is that if present trends continue, the

world’s fisheries could become smaller and composed of increasingly less

desirable species.

The world’s annual fish catch is about 100 million tons (Table 18), with

the northwest Pacific and the northeast Atlantic yielding nearly half the

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Marine Geology

TABLE 18 Productivity of the Oceans

Primary Production Total Available

Tons per Year of Fish Tons per Year

Location Organic Carbon Percent of Fresh Fish Percent

Oceanic 16.3 billion 81.5 0.16 million 0.07

Coastal Seas 3.6 billion 18.0 120.00 million 49.97

Upwelling Areas 0.1 billion 0.5 120.00 million 49.97

Total 20.0 billion 240.16 million