AGE DETERMINATION FROM SCALES OF LAKE TROUT 



47 



" I 2 3 4 5 6 



YEAR OF LIFE 

 FiGiRE 24. — Calcuhited lengths of age groups of marked lake trout caught in -iH-inch-mesh nets (year classes combined). 

 Symbols: diamonds, age-grovip II; dumb-bells, age-grovip III; dots, age-group IV: triangles, age-group V; squares 

 age-group VI. 



inclies long, during tlieir fourth year of life. 

 Fish from earlier year classes were suhject to (lie 

 selectivity of largc-mesh nets for a longer period 

 of time than the marked fish, but to a lower level 

 of lamprey infestation because they were caught 

 before the lampreys had made apprecial)le inroads 

 into the lake-trout population in their areas of tlie 

 lake. 



The best estimates available of the growth of 

 lake trout in the lake before sea lampreys enteied 

 it in large numbers are from data provided by the 

 wild stock from the earlier year classes. In the 

 northern part of the lake, areas 4, .5, and (1, 

 sixteen individuals of year classes 1939-48 were 

 caught by large-mesh nets in 1947. These fish 

 were considerably larger at each year of life than 

 the surviving fish of year classes 1944-46 caught 

 in the same nets from 1947 to 1951. The average 



calculated length of the lake trout in the earlier 

 year classes at formation of the first annulus was 

 ().9 inches and the average annual increase in 

 lengtli to the sixth annulus was ,S.O inclies com- 

 pared witli .5.9 inches at the first annulus and an 

 annual gain of 2.8 incites for the marked fish of 

 year classes 1944-46 (tables 22 and 2.3, fig. 22). 



The early year classes of lake trout tliat lived in 

 southern I^ake Michigan were rei)resented by two 

 groups, both captured in 1947 i)y large-mesh nets. 

 The larger sample of 97 fish (82 of which were of 

 year classes 1939-43) was taken in area 7 off 

 Montague, Michigan (Van Oosten 19.")0). The 

 otiier group contained 17 lake trout of tlie same 

 year classes from the collections of fish witli 

 deformed fins caught in area 8 off South Haven, 

 Mich. The average calculated lengtlis of the 

 Iwo groups differed little from the second to the 



