UNBALANCE IN NATURE 97 



fish may produce from one to five million eggs 

 in a single year. When there is a favorable op- 

 portunity for these to develop, such fishes are 

 abundant. With codfish, the numbers of young 

 of the year found off the coast of Sweden is cor- 

 related with a high average temperature in the 

 spring months and this correlation is probably 

 due to the fact that the higher temperatures in 

 spring allow a rich development of the minute 

 animals and plants upon which the cod-fry feed. 



An almost random sample of the way a fish pop- 

 ulation depends on successful breeding years is 

 given in the case of lake-herring in Finnish lakes. 

 These fishes congregate to spawn in October and 

 November. The fertilized eggs sink to the bot- 

 tom and remain there over the winter. The 

 stock of herring in 1930 in these lakes consisted 

 almost entirely of fishes hatched within the last 

 four years, that is of the last four year classes. 

 Since 1905 there have been good year classes in 

 nine different years with a mean interval of two and 

 five-tenths years and with three years as the most 

 common interval between good year classes. In 

 bad years the newly hatched fishes are driven 

 ashore by windy weather. With favorable con- 

 ditions, an excellent year class may result from 

 a decimated stock. 



There is considerable support for the view that 



