198 THE OCEAN RIVER 



of migration at any moment to point the way to cooler waters. 

 These giant tunas migrate to waters rich in herring and mack- 

 erel and shad, but there is no sign on their way to tell them in 

 which direction their food will be found. During a great part of 

 their travel they are suspended in a great water stream out of 

 sight of the land or the sea floor, and there is no easy reference 

 in sight to tell them whether they are moving with the Stream 

 or not. Recently it has been discovered that there are streaks 

 or flaws along the direction of flow of the Gulf Stream; per- 

 haps the tuna align themselves with these in their northerly 

 exodus. Most likely — according to a few who have studied 

 the mystery — they may be able to use the sun to direct them- 

 selves. These are all conjectures, and the certain answer is still 

 to be found. 



Scientists who devote their attention to the great sea fish- 

 eries are concerned not only with problems of migration, of 

 food and growth, of the complicated effects of drifts and cur- 

 rents, of hot and cold, and of salt and fresh. All these are 

 necessary preliminaries to the immediately practical question 

 of how intensively we can carry our fishing without depleting 

 the natural stocks of fishes, without reducing the average 

 catch for each day away from port, and without bringing about 

 a crop of smaller or inferior fish. 



The ocean has a very much greater food potentiality than 

 the land, and so far only a small part of this is being harvested. 

 This has led in the past to false ideas about the ''inexhaustible 

 supply of food in the sea.'' It is true that each codfish will lay 

 as many as 4,000,000 eggs, but there is a huge mortality among 

 them. Eaten by the larger plankton carnivores and even by 

 their own parents, these countless billions of eggs are greatly 

 diminished in number before they have a chance to hatch. 

 The young fishes, too, are the natural food of others. Thus 

 there is a limit to the number of fish in any one part of the 

 sea. As a result of heavy fishing the British and other European 



