332 THE SEAS 



that these six year old fish absolutely outnumber the fish 

 of all other ages indicates that six years before there was a 

 great survival of young herring, far and above those born 

 in the years immediately previous and succeeding. That 

 year is then known as a good " survival year," and it may 

 be found that for several years the catches will be domin- 

 ated by fish born then. 



For many consecutive years Norwegian investigators 

 studied the age composition of their shoals, and the domina- 

 tion of the catches by a single year-class derived from a 

 good "survival year" is clearly shown by the following 

 instance. In the year 1908 the 1904 year-class (i.e. fish 

 born in 1904) appeared in large quantities in the catches. 

 From that year until 1919 these 1904 fish were still pre- 

 dominant, by which time they were fifteen years of age. 

 For eight years, 1909 to 191 6, they had in fact practically 

 supported the herring fishery industry ; but in 191 9 

 there appeared in the catches a new set of fish hatched in 

 1 9 14, that is six years old, which nearly equalled the 1904 

 fish in numbers. Not until 1924 did the 1904 fish die out 

 from the catches, and about this time the 1914 class was 

 also on the wane, but in 1922 a new brood appeared which 

 was still predominating in 1926 (Fig. 63). 



Thus we are faced with the amazing conclusion that the 

 main stock of herrings on which we draw may consist 

 for several years of the production of a particular year's 

 spawning, and the outlook for the herring fisheries will be 

 serious if this stock is depleted before there has been another 

 good " survival year " to keep the supply up to the demand 



It is one of the main aims of fishery research to find 

 out what is the exact combination of conditions necessary to 

 bring about a good " survival year," and this knowledge can 

 only be gained by taking very full and detailed observations 

 over a long period of years. With such knowledge it would 



