AGE DETKRMIXATIOX FROM SCALES OF L,\KE TROUT 



35 



Table 15. — Comparison of the average lengths of lake Iroiil 

 whose scale readings disagreed with their supposed age 

 uith the average lengths of lake trout for which the reading 

 agreed with the supposed age ' 



(In all readings, It was assumed that the central check was a O-mark] 



Departure of age read from 

 expected age 



5 yean; more: 



Age from scales.. 



.\verage length.. 



Number of fish_. 

 4 years more: 



.\gi' from scales.. 



.\vorape length _. 



Number of fish.. 

 3 years more: 



.\ge from scales. 



.\verage length.. 



Number of fish., 

 2 years more; 



.\ge from scales,, 



.\verage length.. 



Number offish.. 

 1 year more: 



.\ge from scales. 



.\verage length. 



Number of fish. 

 .\s expected: 



.\ge from scales. 



.\verage length. 



Number of fish. 



1 year less: 



Age from scales. 

 .\verage length. 

 Number of fish. 



2 years less: 



.\ge from scales. 

 .\verage length. 

 Number of fish. 



3 years less: 



.^ge from scales. 

 Average length. 

 Number of fish. 



Age indicated by fln mark reported 



III 



12.3 



1 



I 



'5.6 

 1,319 



VII 



19.0 



1 



VI 



18.2 



2 



V 



16.4 

 2 



IV 



21.2 



1 



III 



13.4 



3 



II 



10.0 



39 



HI IV 



VII 



20.0 



1 



VI 



20.0 



1 



V 



16.1 

 6 



IV 



13.4 



15 



III 

 12.8 

 255 



II 



10.5 



3 



VII 



19.2 



2 



VI 



18.8 



2 



V 



15.4 

 13 



IV 

 14.3 

 732 



III 



13.5 



4 



II 

 9.3 



I 



VII 



22.0 



1 



VI 



17.8 

 11 



V 

 15.8 

 280 



IV 



13.0 



10 



III 



12.5 



3 



II 



12.4 



1 



VI 



VI 



15.6 

 13 



IV 



13,0 

 1 



' In addition to the fish listed in the table, the collection contained 1 lake 

 trout. 31,8 inches long, which, according to the fin. should have belonged to 

 age-group I but the scales indicated it to be a member of age-group IX. 



• A mean calculated length bii.'^ed on all lake trout for which ages from scales 

 and fin marks agreed. The samples contained no fisli whose scales indicated 

 that they belonged to age-group I. 



scale readings give a reasonable estimate on growtli 

 of tlie lake trout for which ages from scales and 

 fins disagreed. In tliis table age groups, as 

 established from the scales, have been combined 

 regardless of discrepancies between supposed age 

 and age as read. For age groups II to V, the 

 differences in average lengths between the lake 

 trout with and without agreement on age fell 

 within the range of 0.1 inch (age-group V) to 0.8 

 inch (age-group IV). The difference was fairly 

 large (2. .5 inches) for age-group VI, but here the 

 average length for trout with agreement (15.6 

 inches) must be viewed with skepticism as it was 

 0.2 inch below the mean lengtli for age-group V 

 (15.8 inches). Despite this discrepancy, the 

 data, as a whole, show that the scale readings gave 

 reasonable estimates of the growth of tlie 86 fish 

 with disagreement on age, and hence provide still 

 further evidence of a high percentage of unmarked 

 lake trout among them. 



Table 16. — Comparison of average lengths of lake trout for 

 which the ages as indicated by scales and fins were the 

 same, with average lengths as indicated by scale readings 

 for the 86 lake trout foi which ages indicated by fins and 

 scales were different 



[Data from table 15. Number of fish in parentheses.) 



III. 



12.8 

 (255) 

 14.3 

 (732) 

 15.8 

 (280) 

 1 15.6 

 (13) 



13.1 

 (11) 

 13.5 

 (27) 

 15.7 

 (21) 

 18.1 

 (16) 



' This low figure is probably due to selective destruction of the lake trout 

 population in Lake Michigan. 



CONCLUSIONS AS TO THE DEPENDABILITY OF 

 SCALE READINGS 



The study of tlie scales of lake trout presumably 

 of known age has proved scale readings to be 

 highly dependable over the age span represented 

 in the sample. In the original collection of 1,405 

 recaptured lake trout from northern Lake Michi- 

 gan, ages as read from scales agreed with ages as 

 indicated by fin marks for l,.'n9 or 9:5.9 percent 

 of the individuals. The actual degree of de- 

 pendability is much greater, however, than this 

 percentage suggests. The evidence is strong that 

 the 86 fish, for which ages were in disagreement, 

 actually included many unmarked individuals on 

 wliich fin development had been abnormal. Nine 

 lake trout could be designated with confidence as 

 "unmarked" because of their unreasonably large 

 size in relation to their supposed age. Criteria 

 were lacking for an objective decision as to whether 

 any one individual among the remaining 77 fish 

 could have been a bona fide recovery, but a series 

 of analyses on the reflation of the disagreements to 

 appearance of the deformed fins, year and locality 

 of recapture, size and growth of fish yielded con- 

 vincing evidence of tlie presence of considerable 

 numbers of unmarked lake trout. Although an 

 exact figure can not be given, it can be stated 

 with confidence that, had the original sample 

 been composed entirely of recaptures from the 

 three plantings of marked fish, the agreement 

 between supposed ages and ages read from the 

 scales would have been well above 95 percent. 



The O-mark, a check in the field of first-year 

 growtli, was present on the scales of nearly all 



