LIFE HISTORY OF LAKE HERRING OF LAKE HURON 355 



the fall of 1913. Hjort (1914) found that in the marine herring (Olupea harengiis) 

 of the North Sea one year class predominated heavily in the commercial catches for 

 many successive years. Jarvi (1920) found a similar phenomenon among "Die 

 kleine Marane" {Coregonus albula) of the Keitelesee, an inland lake of Finland. This 

 dominance of one year class in the commercial catches for two or more consecutive 

 years was explained by both Hjort and Jarvi as due to the unusually favorable con- 

 ditions for hatching in that year in which the dominating year class was hatched. 

 No such phenomenon appears in the lake-herring samples. The 1917/1918 year 

 class formed the bulk of the 1921 sample, the 1918/1919 year class that of the 1922 

 sample, the 1919/1920 year class that of the 1923, and the 1920/1921 year class that 

 of the 1924 samples. Each year class drops off rapidly in the years following the 

 year of its dominance, which, as shown in the lower part of Table 30, was the fom'th. 



The data of Table 30 show that 87.2 to 97.4 per cent of the commercial catches 

 are composed of 3, 4, and 5 year fish, and that no one year class predominates for a 

 longer period than a year. 



The data of Tables 29 and 30 suggest that commercial fishing for herring is very 

 intense. The first sj'mptom of this heavy fishing is the paucity of old individuals. 

 Herring are known to reach an age of 11 years (p. 358, fig. 36), and if permitted to live 

 they would probably attain a greater age. Yet, in Saginaw Bay extremely few indi- 

 viduals reach their sixth year of life (Table 30), the second year after the majority of 

 them first join the commercial schools. Table 30 shows that the majority of herring 

 do not even reach their fifth year of hfe. The percentages of this table indicate that 

 on the average the 5-j"ear herring are, roughly, not quite one-third as numerous as 

 the 4-year fish, in spite of the fact, as I shall show later (p. 384), that some of the 

 former are recruited from the immature stock — that is, from fish that are still imma- 

 ture in their fourth year and that join the sexually mature commercial schools in their 

 fifth year of life. In terms of fishing intensity, the above facts suggest that relatively 

 few 4-year herring escape the nets to comprise, a year later, the 5-year group. As 

 stated on page 353, we are not absolutely certain that the high mortahty among the 

 older age groups is due to commercial fishing. It is possible, though not probable, 

 that the life span of the hening terminates in the fourth or fifth year of life. It is 

 difficult to believe, however, that in this species old age and sexual maturity are 

 reached in the same year. 



That fishing intensity is the important factor is suggested further by the shifting 

 in the age composition of the samples (p. 354). In 1921 the 5-year fish were more 

 numerous than the 3, but since this year the former became progressively less, the 

 latter progressively more abundant. The percentages of the 5-year fish of Table 30 

 seem to tell us that since 1921 fishing intensity each year grew more severe, permit- 

 ting fewer and fewer 4-year fish to complete their fifth year of life. So intense does 

 commercial fishing appear to be that a year class is practically wiped out during its 

 year of dominance — the fourth (Table 30). Briefly stated, the history of the major- 

 ity of adult individuals of a year class seems to be as follows: They are spawned m 

 the fall, hatched in the spring, grow as immature fish for two or three years, attain 

 sexual maturity in the third or fourth year, and are captured by the fishermen before 

 or during their fifth year of life. Each year class predominates for one year only; 

 it is rapidly depleted. 



