AGE DETERMINATION FROM SCALES OF LAKE TROUT 



49 



4 5 6 



YEAR OF LIFE 



Figure 25. — Calculated lengths (sums of mean increments of growth in inches) of unmarked lake trout caught in Lake 

 Michigan by 452-inch-mesh nets in 1947. Year classes 1939-43 of the fish from areas 4-6 were combined as were 

 those from areas 7 and 8. 



for determination of mean weights at capture of 

 the fisli in each age group represented. Further 

 information on growth in weight was obtained by 

 calcuhitiiig weights corresponding to calcuhited 

 lengths at the end of the several years of life and 

 and at tlie time the 0-mark was formed. These 

 calculated weights were computed by the length- 

 weight ecjuation. 

 Weights at Capture 



The range of weight in all age groups of the 

 marked lake trout was large, as would be expected 

 from fish that differed so greatly in length. Both 

 the average weights and the ranges of weight of 

 the different age groups are presented in table 24. 

 In 9 of the 12 age groups for which data are given 

 in the body of the table, the weight of the heaviest 

 fish was more than 5 times that of the lightest 

 (the advantage was more than 10-fold in age- 

 group IV of the 1945 year class). In the remain- 

 ing 3 age groups the heaviest trout weighed 2.0 

 to 4.5 times as much as the liglitest. 



Despite the great variability in weight, the mean 

 weights of certain age groups of tlH> different year 

 classes were similar. The average weight ranged 

 from 4.3 to 4.4 ounces in age-group II, from 6.6 

 to 9.7 ounces in age-group III, and from 11.2 lo 

 15.2 ounces in age-group IV. The range of the 

 mean weights was somewhat larger in age-group 

 V (15.2 to 27.4 ounces). Comparabl(> data on 



the weiglits of the fish in age-gi'oup VI were not 

 available. 



Table 24. — Mean weight (ounces), at time of capture, of the 

 year classes of marked lake trout, by age groups 



Calculated Weights 



The growth in weight of the marked lake trout 

 (as determined by the length-weight equation 

 from the calculated lengths shown in table 20) 

 was slower in the earlier than in the later years 

 of life. Wiiereas the most rapid growtli in lengtli 

 occurred during the first year, growtli in weight 

 proceeded slowly through the second year. Tiie 

 weights calculated for the first year of life were 

 typically less than 1 ounce and averaged only 3.0 

 ounces at the end of tlie second vear. The annual 



