AGE DETERMIXATIOX FROM SCALES OF LAKE TROUT 



39 



The (liffiTcnces hotwec^ii m(>asure(l and calcu- 

 lated weights for each length given did not exceed 

 0.4 ounce. The average weight of the pond- 

 reared group was only slightly lieavier, 0.08 ounce, 

 than the average calculated weight for all length- 

 intervals represented by the group. 



GROWTH IN LENGTH 



The data presented on growth in length of 

 marked lake trout include both lengths at capture 

 and calculated lengths (based on scale measure- 

 ments) at the end of the several years of life and 

 at time of foi-mation of the 0-mark in the first 

 field of growth. All calculations of length were 

 made by direct proportion, that is, on the assump- 

 tion that the ratio of length of fish to diameter of 

 scale is constant at all lengths attained by the fisli 

 after completion of the 0-mark. Although the 

 materials at hand are not suitable for a discrimi- 

 nating test of this assumption (range in lengths is 

 too short and lengths at the ends of the range are 

 represented by inadequate numbers of individ- 

 uals), such data as are available indicate that any 

 systematic errors, from the use of direct propor- 

 tion, must be extremely small. 

 Lengths at Capture 



The measured lengths of the marked lake trout 

 of each age group, at the time of capture, extended 

 over a wide range which was somewhat greater for 

 the older than for the younger fish (see figure 20). 

 The range within a single age group (year classes 

 combined) varied from 5.4 inches for age-group II 

 to 12.6 inches for age-group III with intermediate 

 ranges for the remaining age groups (table 18). 



Despite wich- ranges in lengths, the mean lengths 

 for each year of age reached by the three year 



T.^BLE IS.— Mean length (inches) and ranges of length 

 at time of capture, of the year classes of marked lake trout' 

 by age group ' 



classes of marked lake tiout were remarkably 

 close together. \o represtnitatives of age-group II 

 of the 1944 year class were taken by the fishermen, 

 but the mean lengths of the 2-year-olds of the 

 1945 and 1946 year classes differed by only 0.1 

 inch. Tile mean lengtlis for age-groups III, IV, 

 and V in all three year classes had maximum 

 differences of 1.2, 1.0, and 1.5 inches, respectively. 

 Tlie year classes of marked lake trout planted in 

 Lake Michigan not only grew at similar rates but, 

 regardless of environmental difi"ercnces, they also 

 grew at about the same rate as control groups 

 reared in ponds at tiie State Fish Hatchery, 

 Marquette, Mich. The pond-reared lake trout 

 of the 1944 year class had grown 16.6 inches in 

 lengtli by October 1948 (age-group IV). None of 

 the 1944 year class of marked, lake-reared lake 

 trout were captured in October 1948, but the 

 average length of trout in age-group IV caught 

 from April through September was 15.2 inches 

 which, as would be expected, was somewhat below 

 tlie average for the fish taken only in October. 

 The pond-reared lake trout of the 1946 year class 

 were 10.1 inches long when they were measured in 

 October 1948 (age-group II). Although no re- 

 coveries from the lake-reared fish of the 1946 year 

 class were made in October 1948, the average 

 length of 9.9 inches for fish in age-group II caught 

 from May tlirough September is not far below 

 that for the pond-reared lake trout of the same 

 year class. The best comparison of lake- and 

 pond-reared lake trout comes from the more 

 plentiful samples of the 1945 year class which 

 were measured in May 1948 when they were 

 members of age-group III. At this time the pond- 

 reared fish were 11.7 inches long and the marked, 

 lake-reared fish averaged 11.9 inclies long (table 

 19). 



Table 19. — Comparison of total lengths {inches) of take- 

 reared, marked lake trout with those of the pond-reared 

 control groups 



(.Number of fish in parentheses] 



' Young of the .vear were planted in September of each vear. 

  .All fish recovered from 1!M1 and 1SM6 plantings were included because few 

 were available. None were captured as late as October. 



