General Considerations 



that usually occur in very small amounts but are 

 essential for plant growth. In the sea, water is, 

 of course, always abundant; the plants are well 

 adapted to the narrow range of temperature; 

 the intensity of sunlight determines the length 

 of the growing season and the depth of growth, 

 but usually not the differences in fertility. These 

 depend only on the plant nutrients in the wa- 

 ters near the surface. As in any well-worked 

 soil on land, the nutrients in the waters must be 

 replenished each year. They are continually de- 

 pleted by the slow sinking of plant and animal 

 remains from the brightly lighted near-surface 

 layers into the dark waters of the depths. 



Men plow the soil to restore its fertility; the 

 fertility of the sea is restored when nutrient-rich 

 deeper waters are brought up near the surface. 

 The "plowing" of the sea is accomplished in 

 three ways. In some regions winds drive the 

 surface waters away from the coast or away from 

 an internal boundary, and nutrient-rich waters 

 well up from mid-depths. In other areas, the 

 surface waters are cooled near to freezing in the 

 winter, become heavy and sink, and mix with 

 the deep waters. Elsewhere, violent mixing 

 occurs along the boundaries between ocean cur- 

 rents, and deeper waters are thereby brought 

 into the brightly lighted zone. 



The influx of nutrients to the upper layer, 

 and the corresponding loss from this layer by 

 sinking of plant and animal remains, do not 

 directly involve the deep waters. Upwelling and 

 vertical mixing take place only in the upper 

 few hundred meters. The exchange between 

 these mid-depths and the abyssal deep is a very 

 much slower process, of the scale of hundreds 

 of years. 



Most of the commercially important marine 

 organisms are harvested in coastal waters or in 

 offshore waters not very far from land. Several 

 factors are involved: (1) Profitable fisheries 

 can be conducted more easily near ports and 

 harbors; (2) the coastal waters are of high fer- 

 tility, because of greater upwelling and turbu- 

 lent mixing and the ease of replenishment of 

 plant nutrients from the shallow sea floor, and 

 perhaps also because of the supply of nutrients 

 and organic detritus from land; (3) the stand- 

 ing crop of plants and animals attached to or 

 living on the bottom in coastal areas is large, 

 relative to the total organic production. 



None of the animals of the great depths are 

 the objects of a commercial fishery. Even the 

 truly pelagic, high seas fisheries, such as the 



great offshore fisheries for tuna, herring, red- 

 fish and whales, harvest animals that live pri- 

 marily in the surface layer. Some of these ani- 

 mals, however, do much of their feeding in 

 the deeper layers. The sperm whales, for ex- 

 ample, feed on deep-sea cephalopods at great 

 depths. Moreover, much of the food for com- 

 mercially harvested organisms consists of small 

 animals, including crustaceans, squids, and 

 fishes, that perform vertical diurnal migrations 

 from several hundred meters depth to the sur- 

 face. 



The sea fisheries produce about 25 million 

 metric tons per year of fishes and marine in- 

 vertebrates, in addition to about 4 million tons 

 of whales. The great bulk of the harvest is 

 taken, at present, ifrom the waters of the north- 

 ern hemisphere, despite the fact that the south- 

 ern oceans constitute 57 per cent of the world's 

 sea area. The following table indicates the pro- 

 duction in 1954 by latitude zones: 



TABLE 1 Harvest of Fishes and Marine 



Invertebrates in 1954, by Latitude Zones 



(From FAO, 1957) 



Millions of 



Zone metric tons % 



Arctic region 1.2 5 



Northern hemisphere-temperate 



zone 17.5 72 



Tropical zone 4.1 17 



Southern hemisphere-temperate 



zone 1.4 6 



Antarctic regions 0* 0* 



* About 4 million tons of whales were taken in the 

 Antarctic, but few fish or marine invertebrates. 



The disproportionately large yield in the 

 northern hemisphere is related to three factors: 

 (1) Human populations are heavily concen- 

 trated there; (2) the major fishing nations are 

 the industrialized maritime nations, which are 

 mostly located in the north; (3) except for 

 some of the fisheries for tuna, salmon, her- 

 ring, and whales, the important fisheries are 

 located in the relatively shallow areas along the 

 continents, and the extent of these areas is much 

 greater in the northern than in the southern 

 hemisphere. 



The sessile algae of shallow coasts are also 

 the object of important industries in Japan, the 

 United States, the United Kingdom, Norway, 

 and some other countries. Some of these plants 

 are used directly for human consumption, while 



