AGE DETERMINATION FROM SCALES OF LAKE TROUT 



57 



ability of ago readings from lake trout scales. 

 The reader does, nevertheless, need considerable 

 experience with scales from fish of known age to 

 become proficient in recognition of the 0-mark 

 and annuli. 



The estimate obtained of the relation between 

 weight in ounces and total length of the fish is 

 expressed by the formula: 



log Tr= -2.4698+3.1 125 log L 



The range of total lengths at capture of fish 

 within an age group of marked lake trout was wide. 

 The average length for an age group of one year 

 class, however, was close to those for the same age 

 group of the other two year classes. Lake- and 

 pond-reared fish had attained about the same 

 lengths at 2, 3, and 4 years of age. 



The calculated lengths of the fish at various 

 ages prior to capture were computed by direct 

 proportion from the diameters of the annuli. The 

 calculations from 2 scales were averaged. The 

 calculated lengths (sums of the mean increments of 

 growth in length) being lengths of the fish at the 

 end of growing seasons were, as would be expected, 

 somewhat smaller than the mean lengths of the 

 fish of the same age groups at time of capture 

 which was, in most cases, after the beginning of a 

 new growing season. 



The lengths calculated from the fish in age- 

 groups III-VI exhibited Lee's phenomenon of 

 gradually decreasing values with increasing age. 

 Most of the discrepancies are explained by selec- 

 tive destruction of the most rapidly growing fish 

 by nets and sea lampreys. 



Scars and open wounds made Vjy lampreys were 

 found more often on large than on small lake trout. 

 The destruction of the large, fast-growing fish 

 could account for the small size of the fish remain- 

 ing in the older age groups which were caught after 

 the population had been materially reduced. 



Gill nets of the two sizes of mesh most com- 

 moidy used in Lake Michigan caught lake trout of 

 greatly different sizes. During tlie years marked 

 lake trout were caught, the fishermen gradually 

 shifted from use of large- to small-mesh nets. The 

 large-mesh nets caught larger fish than the small- 

 mesh nets and the difference became greater as the 

 fish grew older. It is questionable, therefore, 

 whether a general average gives a true estimate of 

 the growth of these lake trout. The fish caught 

 in the small-mesh nets may give the better esti- 



mate of the growth of the younger age groups, 

 whereas those caught in the large-mesh nets may 

 be more representative of the older age groups. 

 Lee's phenomenon, prominent iti measurements of 

 the first group, is almost lacking from the meas- 

 urements of the fish in the latter group. 



Summing the increments of growth in length 

 minimizes the efl'ects of biased sampling and selec- 

 tive destruction of the fish. 



The weights of the marked lake trout were 

 similar to the lengths in that the weights of in- 

 dividual fish at capture varied greatly within age 

 groups and the mean weights for the age groups at 

 capture were slightly larger than the calculated 

 weights. Although the most rapid gain in length 

 occurred during the first year of life, the gain in 

 weight was least in this year and much greater in 

 later years. 



Seasonal growth of the marked lake trout re- 

 flected the long period of annulus formation. The 

 growing season was extended and variable. 

 Growth for the three year classes indicated a long 

 period of slow growth in the spring, rapid growth 

 from the end of June through October, and 

 slower growth again on into December. Monthly 

 distribution of the increments of growth in length 

 of the 1945 year class suggested that lake trout 

 may occasionally have a somewhat longer season 

 of growth. The average percentage of growth 

 completed at semimonthly intervals for the sepa- 

 rate age groups showed that the growing season 

 varied consideraVjly from one year to the next. 

 Not only the time of the beginning but also of the 

 end of the growing season may vary several weeks, 

 even months. Because of this lack of uniformity 

 in the time of start and finish, growth of lake 

 trout in Lake Michigan may be expected to take 

 place in 9 or 10 months of the year. 



As large- and small-mesh nets caught fish of 

 different sizes and the destructiveness of the sea 

 lampreys increased during the years the marked 

 lake trout were in the lake, it was necessary for 

 estimation of the growth in length, to select fish 

 of the same year classes caught in the same calen- 

 dar years by nets with mesh of the same size. 

 The marked fish (year classes 1944-46) caught in 

 large-mesh nets were slightly larger than the un- 

 marked fish also caught in the northern part of the 

 lake, which suggests that the marked lake trout 

 gained some small advantage from early care in 

 the hatchery. Lake trout caught in large-mesh 



