368 



BULLETIN OF THE BUREAU OF FISHERIES 



Increments. — The computed annual average increments of growth of the Saginaw 

 Bay herring are shown in Table 38. The increments are derived from the lengths of 

 Table 35 and are arranged according to the year classes, each year of life being rep- 

 resented by a calendar year shown at the head of each vertical column. As in the 

 case of the total lengths of Table 35, the increments of Table 38 are rearranged in 

 Table 39 according to the age groups and in such a way that the increments of each 

 year of life are grouped together. The increments of the first year are the same as 

 the total lengths of that year and have been considered already. If we use the pro- 

 cedure employed for the study of total lengths and follow the increments of the 

 various years of life of each age group through successive calendar years, we come to 

 conclusions similar to those arrived at in the study of total lengths. (1) The herring 

 grew at approximately the same rate in corresponding years of life during the years 

 1915 to 1918, inclusive. (2) The herring grew more rapidly in their first, second, 

 and third years in 1919 and thereafter than before 1919. The scanty data indicate 

 that the increments of the fourth year of life are somewhat less in 1919 and there- 

 after than before 1919. (3) The acceleration in growth occurred suddenly in 1919 in 

 herring in their first, second, and third years, but apparently not in herring of greater 

 age. (4) The increments of the first year became suddenly larger in 1919 and with 

 one exception^ increased progressively thereafter. Those of the other years of life 

 fluctuated with the year classes during the period 1919 and thereafter, remained 

 constant, or decreased during those years. 



Table 39. — Comptited average length increments reached in different years of life by various age groups 

 of Saginaw Bay herring. The data are those of Table SS rearranged so as to group the increments 

 of the same year of life together " 



* The number of specimens employed is shown in parentheses. 

 '' Increment up to November when captured. 



< The small average first-year length (lie millimeters) of the 6-year herring of the 1919 year class appears to be consistent with the 

 corresponding lengths of the younger flsh of this year class, allowing a decrease due to Lee's "phenomenon "; but this small average 

 length (117 millimeters) of the 5- year fish of the 1920 year class does not seem to be thus consistent. It is not apparent why thei- 

 year flsh grew so much more slowly during the first year of life than the younger age groups of the same year class, while in the 

 later years of life the former grew at the same rate as the latter. This seems to be the only outstanding inconsistency in the com- 

 puted values and it may or may not be significant. It militates against the genera! conclusion that the growth rates olltheil-year 

 herring increased in 1919 and again in 1920. 



