tion of all organic matter), or net production (the rate of production 

 of plant tissue alone). It is these figures that are usually presented 

 when we compare the yield of the sea with that of agricultural land. 

 In practice this is a misleading comparison, for while we can harvest 

 a field of corn directly, the plants of the sea have to pass through 

 many mouths before we can harvest them as fish, and in the food 

 chain considerable losses may take place before the product is of 

 any use to us. 



The number of fish in any area depends largely on the amount 

 of food available, and so in general we find rich fisheries where 

 there is a rich supply of plankton, and desert areas of the sea where 

 conditions do not encourage a high rate of plankton production. But 

 this is only part of the story. Many of our important food fishes 

 feed on bottom-Hving animals, so they are confined to the shallower 

 waters where these animals thrive. There are few large fisheries in 

 the Southern Hemisphere because there are few extensive areas of 

 relatively shallow water; also the great successful group of gadoid 

 fishes (cod and haddock among them), which are the basis of the 

 great northern fisheries, are very poorly represented in the Southern 

 Hemisphere. 



Fishing as a method of obtaining food is still primitive compared 

 with agriculture. In several respects, for centuries fishing has 

 remained on a level with hunting. Although we are now developing 

 electronic means of finding fish and electrical methods for catching 

 them, these are only marginal improvements; they do not go to 

 the heart of the problem. The whole principle of fishing would be 

 vastly improved if we were able to apply some of the lessons we 

 have learned from agriculture. But just how far can we go in apply- 

 ing the principles of farming to fishing? Essentially, we have 

 brought about our vastly increased agricultural yields in four ways : 

 I. by the use of fertilizers; z. by the removal of weeds; 3. by the 

 control of insect parasites ; 4. by selective breeding. 



Although experiments with sea fertilizers have been tried, they 

 are not very encouraging. If we fertiUze an enclosed arm of the sea 

 we may be successful in, say, increasing the size of fish, but at the 

 same time we may also be stimulating weed growth and feeding 

 unwanted animals, and so defeat the purpose of the fertilizer. On 

 account of this, the addition of fertilizer is hardly an economical 

 proposition. 



Like a farmer, the fisheries biologist can try to protect his "crop" 

 by removing predatory animals that take a heavy toll of the food 

 needed by fish. Starfish, for example, feed on molluscs which are 

 the food for many bottom-feeding fish, so if we could devise an 

 economical means of controlling the starfish in a given area we 

 could probably increase our yield of the fish we want to market. 



While some regions have too many fish for the amount of food 

 available, others have too few fish to make full use of the abundance 

 of food. Why not, then, transplant certain fish to such underpopu- 

 lated regions? The Dutch have tried a similar method in the culture 

 of mussels with great success. Mussels growing around the Friesian 

 Islands are transplanted to the rich waters of the western delta 

 of southwest Holland for fattening, and then after being cleaned 

 in fresh sea water are ready for marketing. 



The success of plant and animal breeding in agriculture has 

 encouraged the fisheries biologist to try to develop new hybrids 



In Southeast Asian countries farmers combine 

 rice culture witii fisti culture. The mature 

 carp these Malayan farmers have just tal<en 

 from their rice field was placed in the 

 water as a young fish at the same time that 

 the rice was planted. 



^37 



