56 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



that, in general, the lake trout gained about 30 

 percent of their growth sometime after the middle 

 of September. 



The growing season for lake trout in Canadian 

 waters is shorter. Kennedy (1954) found that the 

 lake trout in Great Slave Lake "grow only between 

 late May and the middle of September, with no 

 growth at any other time." Of the seasonal 

 growth of the lake trout in South Bay, Lake Huron, 

 Fry (1953) stated, "The lake trout . . . add about 

 1 inch to their total growth increment for the year 

 by mid-September. The total for the major year 

 class represented in 1949 (the 1944 year class) . . . 

 was estimated at 1.8 inches. This increment 

 would indicate the rapid growth observed from 

 June to September probably continued at least 

 until mid-October." 



SUMMARY 



From 1 to 1)2 million liatchery -reared lake trout 

 (average length 3.2 inches) were liberated into 

 northeastern Lake Michigan in September of 

 each of the years 1944-46. About 10 percent of 

 these fingerlings were marked by the removal of 

 fins. In the years subsequent to the plantings, 

 1947-52, fishermen captured 1,747 lake trout 

 with abnormal fins of whicli only 1,507 were 

 adequately documented. Of the latter group, 

 102 caught off South Haven, Mich., difTered so 

 much from those caught in the northern part 

 of the lake that all, or nearly all, were considered 

 to be unmarked wild lake trout with abnormal 

 fins; hence, they were excluded from the main 

 sample. The scales of the remaining 1,405 fisli 

 were studied to determine the validity of age 

 readings from scales and the rate of growth of 

 lake trout in Lake Michigan. 



Lake trout scales are small and have concentric 

 circuli. They develop first as platelets adjacent 

 to the anterior end of the lateral line when the 

 fish are about 2 inches long and rapidly cover 

 all the body except the head. Probably joung 

 lake trout in Lake Michigan are fully scaled 

 before the end of their first summer. 



Even though the scales were rather difficult 

 to interpret, simple criteria for recognition of the 

 annulus were determined. The annulus is gcn- 

 erallj' indicated by wider spacing between circuli 

 outside closely spaced circuli, but this arrange- 

 ment, usually most clearly seen in the lateral 

 fields, is seldom definite enough to be followed 



entirely around the scale. Other indications of an 

 annulus are: a V-shaped pattern in the circuli 

 of the lateral fields, a ridge across the posterior 

 field, also such irregularities as broken or crooked 

 circuli and fine accessory lines. An annidus is 

 usually located by a combination of these criteria. 



The annulus was formed on the scales of some 

 lake trout as early as the middle of March, of 

 the majority during June and July, and of a 

 few as late as the middle of August. 



In addition to the expected number of annuli 

 for the marked fisli, a central check was found 

 within the first annulus which has been designated 

 the "0-mark." The scales of the unmarked, 

 wild-stock lake trout from Lake Miciiigan exam- 

 ined during this study also carried the central 

 check (0-mark). 



Two readings were made of the markings 

 on the scales. The ages read agreed on 96.8 

 percent of the specimens. 



The number of annuli read from the scales 

 agreed with the age of the fish indicated by the 

 deformed fin for 93.9 percent of the lake trout 

 in the sample of presumably marked fish. Most 

 of the disagreements were of 1 year but some were 

 of 2 or more years. 



The principal difficulty in the way of determin- 

 ing the accuracy of age readings from the scales 

 of the lake trout from northern Lake Michigan 

 resulted from the presence in the collections of a 

 small percentage of unmarked fish. The exact 

 number of these fish could not be comited but 

 evidence from several lines of investigation led 

 to the conclusion that nearly all the 86 fish, for 

 which the age read from the scales disagreed with 

 that indicated by the deformed fin, were unmarked 

 lake trout. The average lengths of the age 

 groups indicated by the deformed fins of the 86 

 "uimiarked" fish were very different from those of 

 the age groups of the 1,319 "marked" fish (those 

 with agreement between age indicated by the 

 fin and that read from the scales); furthermore, 

 the average length of the 86 fish decreased with 

 increase of age. On the other hand, at ages read 

 from the scales, the growth curve for these 86 

 fish was similar to that of the 1,319 "l)ona fide" 

 recoveries. It was concluded, therefore, that the 

 age read from the scales rather than tlu' age 

 indicated by the deformed fin was correct for 

 most fish. 



Tlu» evidence strongly indicates a liigli depend- 



