Atomic Radiatioti and Oceanography and Fisheries 



deep sea floor with depths of 4,000 meters or 

 more. The average width of the continental 

 shelf is about 30 miles, varying from almost 

 nothing off mountainous coasts, such as the 

 West Coast of South America, to several hun- 

 dred miles in the China Sea. The shelf is not 

 everywhere smooth, but is often intersected by 

 submarine valleys and canyons. In the deep 

 ocean basins there are high mountains and long, 

 deep trenches, features larger than any on land. 

 Some of the deeper parts are isolated by sub- 

 marine ridges which restrict the exchange of 

 water between adjacent areas. 



The waters of the oceans are stratified. Within 

 a relatively thin layer at the surface, varying in 

 thickness in different places but averaging 

 about 75 meters, vertical mixing caused by 

 winds is fairly rapid and complete. In conse- 

 quence, the temperature, salinity and density are 

 nearly uniform from top to bottom. Relatively 

 fast wind-driven currents exist in this upper 

 mixed layer; these are the "surface" currents of 

 the oceans depicted on many charts. Here also 

 the horizontal mixing is relatively rapid. The 

 mixed layer is the region of the sea in which 

 most of man's activity takes place. 



Below the mixed layer is a 2one within which 

 the temperature decreases and the density in- 

 creases rapidly with depth. This thermocline, 

 or pycnocline, separates the surface mixed layer 

 from the layers of intermediate and deep water, 

 the latter extending to the bottom, within which 

 there are gentle gradients of decreasing tem- 

 perature and increasing salinity and density with 

 depth. Vertical movement in the intermediate 

 and deep layers is much slower than in the 

 mixed layer, and horizontal currents are more 

 sluggish. The strong density gradient across the 

 pycnocline tends to inhibit physical transport 

 across it, because work is required to move wa- 

 ter vertically in either direction, and thus the 

 pycnocline acts as a partial barrier between the 

 mixed layer and the lower layers. There is, 

 however, some interchange of both living and 

 non-living elements; indeed the continued ex- 

 istence of some marine resources depends on 

 such interchange. 



MARINE RESOURCES 



Living resources 



I The most important extractive industry based 

 on the resources of the sea is the harvesting of 

 jits living resources. 



On land the cycle of life is relatively simple; 



we may describe it in four figurative stages. 

 First is the grass, which by a subtle and complex 

 chemistry captures the energy of sunlight and 

 builds organic matter. Sheep and cows live on 

 the grass; tigers and men eat them. The cycle 

 is closed by bacteria, which decompose the dead 

 bodies and the excreta of all living creatures, 

 making their constituent substances again avail- 

 able as building materials for the plants. In the 

 sea, the cycle is longer. Instead of grass there 

 are the tiny floating plants called phytoplank- 

 ton; in place of cows, the zooplankton animals 

 that eat the plants are small crustaceans, no 

 bigger than the head of a pin. Many kinds of 

 tigers eat the cows, but they are mostly also 

 zooplankton, only a fraction of an inch in 

 length. Other intermediate flesh-eaters exist 

 between them and the fishes of our ocean har- 

 vest. Because every link in this long food 

 chain is inefficient, we reap from the sea only 

 a small fraction of its organic production. 



Other characteristics of the ocean also tend 

 to limit the harvest as compared to that from 

 the land. One is its giant size; more than 70 

 per cent of all the sunlight that penetrates the 

 atmosphere falls on the sea; moreover, this 

 sunlight can act throughout the top 20 to 100 

 meters, thus the living space for plants and 

 animals is far greater than on land. This great 

 areal extent and volume, combined with the 

 fluidity of the oceans, results in a low concentra- 

 tion oif organisms per unit volume and therefore 

 inefficiency in harvesting. 



On land, the standing crop of plants and 

 animals is of the same order of magnitude as 

 the amount of organic production per year, 

 while in the ocean the crop is very small, com- 

 pared to the production, because of rapid turn- 

 over. The average rate of organic production 

 per unit area is probably about the same on land 

 and in the sea, but the efficiency of harvesting 

 depends more on the size of the crop than on 

 the total amount of organic matter produced. 



The plants of the sea, on which all other liv- 

 ing things depend, grow only in the waters 

 near the surface where bright sunlight pene- 

 trates. These waters diflfer widely in fertility. 

 Like the land, the ocean has its green pastures 

 where life flourishes in abundance, and its 

 deserts where a few poor plants and animals 

 barely survive. 



The fertility of the land depends on four 

 things: water, temperature, intensity of sun- 

 light, and available plant nutrients — substances 



