LIFE HISTORY OF LAKE HERRING OF LAKE HURON 



361 



RATE OF GROWTH 



METHOD OF DETERMINATION 



By Petersen's method of age determination the individuals of a large collection 

 are grouped according to their size and the size-frequency distribution is plotted 

 on graphs. Each prominent mode or hump in the resultant curve is then assumed 

 to represent an age group, the first mode representing the youngest age group. An 

 examination of the length-frequency curves of Figure 37 shows that they are virtually 

 unimodal, so that this method can not be employed for the herring of my samples, 

 although, if material were available, it might be found applicable to the fish of the 

 two youngest age groups (1 and 2). In the study of growth rates, therefore, I have 

 restricted myself to the methods based on scales. We may assort the individuals 

 of the collections of different years according to their year classes, as determined by 

 reckoning back from the year of capture, according to the number of annuli on the 

 scales. We may then separate the various age groups of each year class by using the 

 annuli as an indication of age. Having assorted the fish, two procedures are possible. 

 First, we may compare the actual length measurements of these various age groups. 

 Second, we may compute, from the scale diameters (or other dimension) of all the 

 specimens, the lengths at the end of each year of life and thus determine for each 

 year class its rate of growth throughout life. The second method affords a much 

 larger series of data. By combining both actual and computed lengths of fish of the 

 same ages of all the year classes a general norm of growth characteristic of the 

 species may be obtained. The actual measurements, both lengths and weights, 

 of all the age groups of all my comparable samples are summarized in Table 34. 

 The grand averages are given near the bottom of the table. Similarly, the estimated 

 lengths for each year of life of all age groups, computed from the measurements of 

 scales, are summarized in Table 26. The length values of fish of the same age groups 

 of both of these tables are combined in Table 43. 



Table 34. — Rale of growth of all year classes of Saginaw Baij herring, in terms of length and weight, as 

 determined by direct measurement and weighing ' 



' Number upon which an average is based is shown in parentheses. 

 ' Gravelly Point herring. 



3 Tobico and Nayanquing herring. 

 ' 2, 3, and Au Ores herring combined. 



