44 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



3 4 5 6 



YEAR OF LIFE 



Figure 21. — Calculated lengths of marked lake trout (year classes combined) caught in large- and small-mesh gill nets 

 For each age group the calculated lengths of the fish caught in 4!j-inch-mesh nets are connected by a solid line and 

 those of fish caught in 2H-inch-mesh nets by a broken line. As all age groups represented in the sample are shown 

 in the same graph, the curves do not have a common base, hence none is shown. Consult table 22 for values (inches 

 of length) of points on the curves. The numbers of fish taken by each net are shown in parenthetical boxes. 



lake trout were the smaller individuals of age- 

 group II. Their mean calculated lengths were 

 all smaller than those for the same years of life 

 of the fish in age-groups III, IV, or V caught in 

 either type of net (with the exception of the 

 calculated length for the second year of age-group 

 V caught in the small-mesh nets which was just 

 0.3 inch shorter than the one for age-group II). 

 Nets of both sizes of mesh took fish of approxi- 

 mately the same size from age-group III (larger- 

 mesh nets captured only slightly larger fish). 

 The difference in the calculated lengths of fish 

 caught by large- and small-mesh nets increased 

 gradually, as the fish advanced in age, from 0.1 

 inch in the third year of life of age-group III to 

 0.9 inch in the fourth year of age-group IV, 1.3 

 inches in the fifth year of age-group V, and 5.0 

 inches in the sixth year of age-group VI. 



The large discrepancies in the older age groups 

 between the calculated growth histories of fish 

 caught in 2^^- 



/2- and 4j2-inch-mesh nets leave some 



uncertainty as to the true rate of growth. Pos- 

 sibly the samples from small-mesh nets give better 

 estimates of the growth rates for the younger age 

 groups and the fish from large-mesh nets may 

 provide better estimates for the older age groups. 

 Because of the different selectivities shown by the 

 gill nets of these two mesh sizes, the marked and 

 unmarked lake trout caught in nets of each mesh 

 size were studied separately. 



The growth rates of marked and unmarked lake 

 trout of the same year classes (1944-46) caught by 

 4}2-inch-mesh nets in northern Lake Michigan 

 were closely similar. However, with but one 

 exception, sizes ecjual at formation of the first 

 annulus, the calculated lengths of the unmarked 

 fish were somewhat lower, ranging from 0.2 inch 

 at formation of the 0-mark to 1.2 inches at the 

 sixth annulus. The average annual increment of 

 growth in length after the first year was 2.8 inches 

 for the marked and 2.5 inches for the unmarked 

 fish. The calculated lengths of the unmarked 



