140 STUDIES IN INSECT LIFE, ETC. 



The chief possible causes of an impoverishment 

 of the sea are three, (i) As the great crops of 

 the central United States exhausted the accu- 

 mulated richness of a virgin soil, so when fishing 

 began on the North Sea an accumulated wealth 

 was drawn upon. When the Dogger and Iceland 

 grounds were first opened up the plaice taken 

 were bigger than has since been the case. The 

 accumulated stock has been fished out. (2) Like 

 a given area of land, so a given area of sea can 

 support but a limited quantity of produce. There 

 is a definite amount of food for fishes in a definite 

 volume of sea, and so a limit is set to the number 

 of fish in that volume of water. By use of the 

 quantitative analysis introduced by Professor 

 Hensen, he and Professor Brandt, of Kiel, have 

 shown that a square metre of the Baltic pro- 

 duces an average of 150 grammes of dry organic 

 material in the shape of diatoms, copepods, and 

 other floating organisms. A similar area of land 

 produces 180 grammes of ultimate food-substance. 

 The productivity of the sea is then judged on 

 this basis about 20 per cent, less than the land. 

 Much caution, however, should be used in using 

 these figures. (3) The excessive destruction of 

 young fish is another, and perhaps the greatest, 



