AGE DETERMINATION FROM SCALES OF LAKE TROUT 



41 



Year of life 2 



Length (summation 

 of calculated incre- 

 ments) 8. 7 



Age group II 



Length at capture 10. 



I 2 3 4 5 6 



YEAR OF LIFE 



Figure 10. — Calculated lengths (sums of mean increments of growth in inches) of the age groups of marked lake trout. 

 A. Mean (dot) and range (broken line) of marked lake trout at time of planting in September. 



most dependable estimate of growth rate, the 

 mean lengths at the end of successive years of 

 growth were obtained by the summation of mean 

 calculated increments of length. The second gen- 

 eral estimate of growth is composed of the weighted 

 means of the caknilated lengths. Results from the 

 two procedures agreed for the first 3 years of life 

 but in the later years the summation of the average 

 increments gave decidedly higher values. Tlie ad- 

 vantage of the summation of the increments is 

 especially apparent in the data for the sixth year of 

 life. Here tlie increment of mean length ( — 0.3 

 inch) obtained for the sixth year from the average 

 lengths falsely indicates a decrease in size of the 

 fish after tlie fifth year. This !iegative increment, 

 or decrement, is based on lake trout caught after 

 the year classes had been depleted of the larger 

 fish. The sums of the increments of growth in 

 length, on the other hand, show more reasonable 

 figures on the rate of growth of the marked lake 

 trout in Lake Michigan. Lengtiis obtained in this 

 way were, nevertheless, somewhat smaller than 

 the mean lengths of marked lake trotit at the tim(> 

 of cajjture as the following tabulation demon- 

 strates: 



The marked fish recaptured as members of 

 age-groups II, III, and IV measured 10.0, 12.8, 

 and 14.6 inches long. Calculated lengths for the 

 same years of life were 8.7, 11.2, and 13.7 inches. 

 Whereas the calculated lengths give the size of the 

 fish at the beginning of the growing season, the 

 fish were caught somewhat later in the year at 

 various times during the growing season, hence, 

 were expected to be longer. Lengths, obtained 

 by adding increments of growth, for fish in their 

 fifth and sixth years of life show that in those 

 years the lake trout actually continued to grow at 

 rates only slightly lower than those during the 

 earlier years of life (excepting the firet year). 

 The relation of the calculated lengths to tlie 

 empirical data is shown in figure 20. 



The mean annual increments of growth 

 gradually decreased as tlie lisli became older from 

 5.9 inches the first vear to 2.8 inches the second 



