THE AGE OF FISHES 187 



the numerous detailed routine tasks that go to make up scien- 

 tific research have shown that in spring a great plankton out- 

 growth takes place, using up the chemical food within the 

 zone of active plant life. Toward summer the surface water 

 becomes heated by the sun, but only in the upper layers — 

 since water conducts heat rather slowly. These hot upper layers 

 of water are lighter than those below, and therefore, like oil, 

 they mix with them very poorly — particularly during summer 

 when the winds are light and prevent the replenishment of 

 surface fertilizer by mixing from below. During this time the 

 rain of plankton carries chemical food down to the deeper 

 water out of the zone of plant growth, until finally the surface 

 supply of fertilizer is depleted and further growth is stopped. 



As autumn comes and as the surface again becomes cooler, 

 the different layers are able to mix again. The autumn gales 

 may trouble the fisherman, but together with the flow of water 

 they conspire to stir up the lower layers with their store of 

 chemical food and so provide for a new growth of plankton at 

 the surface. The second crop is shortlived, for light and heat 

 begin to fail. As winter approaches the upper layers cool fur- 

 ther, until they become heavier than the water beneath and 

 tend to sink. This adds further to the mixing process, and fer- 

 tilizer is restored to the surface. During the depths of winter, 

 both the poor light and a temperature too low for rapid growth 

 together with other factors play their part in holding back any 

 new outburst of plankton until the warmth and sunlight of 

 spring set the cycle of life once more on its rounds. 



The Ocean River, with its branches and its tributary streams, 

 acts like a gigantic pump, continually replenishing the de- 

 pleted fertilizer in this natural hydroponic system from the 

 deep sea reservoir. Wherever surface water is blown by winds 

 away from the land, or where it streams apart in diverging 

 currents, the underlying colder water, with its rich store of 

 chemical food, is drawn up to replace it. The west coast of 



