LIFE HISTORY OF LAKE HERRING OF LAKE HURON 



333 



Table 23. — Average cumpulcd increments in length, in millimeters, of various age groups of Bay 

 City herring hatched in the years 1917 to 1921, inclusive, for each growth year 



Table 23 shows this to be true. In this table are shown the computed average 

 increments of length reached in different years of life by various year classes of lake 

 herring. It may be seen that whereas the computed increments of the first and second 

 years of life usually decrease as the age of the fish whose scales were measured increases, 

 those of the third year change little with age while those of the fourth and later years 

 increase with age. (See, for example, year class 1919). Obviously the factor or 

 factors that explain the "phenomenon" in the scale diameters and the apparent 

 contraction and expansion of the scales (Table 22) will also explain these character- 

 istics in the body lengths and increments computed from these scales. We may then 

 discuss either the measurements of the scales or those dimensions of the fish computed 

 from the scales. To avoid repetition, the former course is convenient. As the scale 

 measurements are direct and do not involve the assumption relative to the proportion- 

 ate growth of body and scale, they are to be preferred to the computed lengths even 

 though they involve fewer specimens.'* The computed values are resorted to when 

 the number of specimens involved in the averages of the scale measurements is 

 unusually small; as, for example, in the sixth and seventh age groups. 



Any factor or factors suggested to account for Lee's "phenomenon" in the 

 "annular" diameter measurements of the scales and for the apparent contraction and 

 expansion of scales with age must determine (1) why the scale increments of the 

 first and second growth years of a year class generally decrease as the age of the fish 

 whose scales are studied increases, (2) why the scale increment of the third growth 

 year increases or remains constant with age, and (3) why the scale increments of the 

 fourth and fifth years increase with age. 



With these facts in mind we may now consider Lee's first suggestion that the 

 youngest year groups may be represented in the catch only by their biggest individuals, 

 and that, as we proceed toward the older groups, more and more of those that had 

 been the smaller individuals in their earliest years appear in these older groups. If 



i< Fewer specimens were employed for scale diameter averages because non-X scales only were used. The computed length 

 values of flsh were based on non-X and X scales. 



