LIFE HISTORY OF LAKE HERRING OF LAKE HURON 365 



groups III, IV, V, and VI with calculated lengths of 127, 122, 119, and 110 milli- 

 meters, respectively; that is, the calculated length at the end of year I is less the older 

 the fish from which the calculation is made. A similar relation appears in the cal- 

 culated lengths of fish of difTerent age groups in the 1918, 1920, and 1921 year classes 

 but not in the other year classes. Obviously, if the scales used for the length cal- 

 culations for the year 1919 and following years were from younger fish than those 

 used for length calculations for the year 1918 and preceding years, they would give 

 higher values. In that case, although the growth rate in 1919 might, in reality, be 

 the same as that of 1918, it would appear to be greater. In order to compare cal- 

 culated lengths attained at a given age in any two years, it is therefore necessary that 

 the scales used be taken from fisli of the same age. By such a procedure whatever 

 error is involved in Lee's "phenomenon of apparent change in growth rate" is the 

 same for all year classes in fish of the same age. 



In Table 36 the data of Table 35 are regrouped. The year classes are no longer in 

 chronological order, but the age groups of different year classes are brought together. 

 The 8 and 7 year herring of the year classes 1914 and 1917 are too few to give depend- 

 able averages. An examination of Table 36 shows that the 7-year fish of the 1915 

 j'ear class and the 6-year fish of the 1916 year class reached about the same length 

 in corresponding years throughout life. The 6-year herring of the 1917 j'ear class 

 reached the same length as those of the 1916 year class at the end of each of the first 

 two j^ears of life but exceeded them in length at the end of their third year (1919). 

 This excess in length then gradually grew less, until the actual length of the 1917 fish 

 at death was slightly less than that of the 1916 fish. The 6-year individuals of the 

 1918 year class were slightly smaller than those of the 1917 or 1916 year class at 

 the end of the first year of life but were larger at the end of the second year (1919) 

 and continued to be larger in the remaining corresponding years of life. The 6-year 

 fish of the 1919 year class attained the same average length as those of the 1916 and 



1917 year classes at the end of the first year of life but exceeded them in length at the 

 end of the second year (1920) and at the end of the later corresponding years. The 



1918 and 1919 6-year fish, however, seemed to have had similar rates of growth. The 

 5-year fish of the 1918 year class were approximately the same in length as those of 

 the 1917 year class at the end of the first year of life, then exceeded them in length 

 in the second year (1919), and continued to exceed them in corresponding years, though 

 in diminishing amounts, throughout life. The 5-year-old fish of the 1919 and 1920 year 

 classes, which had similar rates of growth, however, reached greater lengths tim'O. 

 those of the two preceding year classes in corresponding years throughouit^Iifei! ; Ifl the 

 fourth (year class 1921 excepted), third, and second age groups each younger yea« 

 class of each age group successively attained greater lengths than its predeoesscxi 

 for corresponding years throughout life. .^ICI 



Summarizing this detailed analysis, we find that the 1915 and 1916 year etasses 

 grew at corresponding rates throughout life. Although the fish used for calculating 

 the lengths of these two year classes were respectively 7 andi6 yearsiold, tiie difference 

 in age is so little that there is no evidence of apparent change ingrowth rate. We 

 find that the 6-year fish of the 1917 year class grew at tjhe same rate as the two pjeTi- 

 ousfy mentioned year classes during the firat twio years of life (1917 andi' 191-8), 'theri 

 reached a greater length in the third year in 1919 and in the corresponding later 

 years of life. The 1918 year class (age groups V and VI) reached about the same 



