A Portuguese boat at Aveiro, south of 

 Oporto, gathers seaweed. Seaweeds are 

 rich in iodine and are used for 

 medicinal purposes and as fertilizer. 





A'\. 



'il 



Guano deposits, rich in nitrogen and 

 phosphates, have been valuable fertilizers 

 since prehistoric times. Islands off the 

 coast of Peru, such as the Chincha Islands 

 shown here, are the most important source, 

 yielding guano up to 150 feet thiols, laid 

 down by cormorants, boobies, and pelicans. 

 The dark areas in the photograph are 

 massed guano birds. 



of fish, and to experiment in the intensive culture of fish in fish 

 ponds, a centuries-old practice in Asian countries. 



Fish ponds range from what one might call kitchen gardening 

 to full-scale farming covering many square miles. In principle the 

 ponds are fertili2ed and stocked with fast-growing species which 

 do not compete for food but make use of all the available food. 

 There might be one species feeding on surface weeds, one feeding 

 in mid-water on plankton, and another on bottom weeds. One of 

 the most interesting examples of fish ponds is found in southeast 

 Asian countries where farmers combine rice culture with fish culture. 

 After flooding their paddy fields and planting the young rice plants, 

 the farmers place young carp in the water, and while the rice is 

 growing the carp feed on the plankton. By the time the fields are 

 drained to harvest the rice, the carp are also marketable. 



Because the quantity of plankton produced in the sea is greater 

 than the quantity of fish, experts have proposed using plankton 

 themselves as a protein-rich food source. But the difficulty is 

 catching them. On the whole, planktonic plants and animals are 

 very small. Even with fine-mesh filters we could expect an average 

 catch of not much more than one gram of plankton animals for 

 each ton of water filtered, even in the richest areas. This does not 

 mean that plankton as a food source is out of the question. We can 

 grow large-scale cultures of planktonic plants. One gigantic culture 

 covering an area the size of Kent and Sussex combined (two million 

 acres, an area smaller than the island of Hawaii) would provide nearly 

 enough protein to satisfy the requirements of the present world 

 population ! 



Century after century water washing off^ the land has been flow- 

 ing into the oceans and depositing every mineral known to man. 

 Many of them, including certain salts, are accumulating as the 

 water that originally brought them to the sea evaporates, condenses 

 over the land, and brings a fresh supply down to the sea again and 

 again. Yet other minerals seem not to be accumulating. Although 

 the rivers wash down great quantities of calcium salts, we find 

 proportionally fewer of them in the seas than in the rivers. We know 

 that a variety of marine creatures — snails, oysters, and shelled ani- 

 mals of microscopic size — extract calcium from the sea to make 

 their shells, and the tiny coral polyp uses calcium to build the vast 

 reefs in tropical seas. Some species of ascidians even manage to 

 extract and concentrate a rare element, vanadium, in their blood. 



Man has been less successful in tapping the mineral wealth of 

 the sea than the creatures who make the sea their home, yet he has 

 tried in two ways: by taking from the plants and animals the 

 chemicals they have extracted from the sea, and by "mining" the 

 ocean waters directly. 



Seaweeds have long been an important source of iodine, yet 

 iodine is one of the scarcest nonmetallic elements in the sea. Al- 

 though there is only about one gram in every twenty tons of sea 

 water, some seaweeds contain one gram of iodine in only 200 grams 

 of the dry seaweed. Today we obtain some iodine from the ashes 

 of burned seaweed, but about two-thirds of the world's production 

 comes from "fossilized" seaweed deposits, chiefly in the desert 

 regions of Chile. Apart from their use as a source of iodine and 

 fire-resistant textiles, seaweeds are a traditional source of fertilizer 

 for the land, but by far the greatest supplies of fertilizers which 



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