AGE DETERMINATION FROM SCALES OF LAKE TROUT 



33 



a miiiiinuin of 2:.i. Decreases in tlu- luimber of 

 recoveries to 2.')2 fish in 1950 and a mere 27 in 1951 

 were accompanied hy increases in percentage dis- 

 agreement to 6.7 and 33.3 percent, respectively. 

 The onU-r with respect to the size of the annual 

 total number of lake trout recaptured was practi- 

 cally tlie reverse of the order of the percentage 

 disagreements. The one exception was between 

 ranks 2 and 3 where the difTerences in percentage 

 disagreement were small but sufficient to reverse 

 the order of the ranking as shown: 



Still another significant feature of the annual 

 totals is the limited range in the number of dis- 

 agreements (from 5 in 1947 to 17 in 1950). The 

 indicated variability is much below that of total 

 recaptures for corresponding years. For example, 

 from 1947 to 1949 the catch of fish with deformed 

 fins increased 14.7 times but the number of dis- 

 agreements increased only 2.4 times. Thus it ap- 

 pears that the number of disagreements tended to 

 fluctuate about a fairly stable level and to be rela- 

 tively independent of the number of recaptures 

 of marked fish. This relation is precisely the one 

 which should obtain if a liigh percentage of the 

 disagreements were caused by the presence of un- 

 marked fisli. 



The data on tlie relation between locality of 

 capture and percentage agreement within and 

 between calendar years exhibit certain incoti- 

 sistencies most of which can be attributed to tlic 

 small numbers of fish in some entries. D(>finite 

 trends can l)e detected, nevertheless. It is seen, 

 for example, that tlie percentage disagreement 

 between supposed ages and ages read from scales 

 was invariably nil or small (0.0 to 3.4 percent) for 

 lake trout recaptured within 20 miles of tlie point 

 of release. The percentages were large, on the 

 other hand, foi' trout recaptured more than 60 

 miles from the locality of planting. Only in 1949, 

 when 13.3 percent of the fish were in disagreement 

 on age, was there evidence of consideiai)le numbers 

 of bona fide marked fish in this aiea. In the re- 

 maining 3 years in which recaptures were re[)orted 



from distances greater than 60 miles, the per- 

 centages ran from 42.8 to 100.0 (numbers of fish 

 were small but the figures probably are significant 

 because of consistently high values). 



For lake trout captured at tlie two intermediate 

 distances, the percentage disagreement was nil at 

 20 to 40 miles in 1947 and 1951, but only 5 fish 

 were reported each year. The remaining records 

 for fish captured at 20 to 40 or 40 to 60 miles indi- 

 cate a general inverse relationship between per- 

 centage disagreement and number of lake trout 

 reported. In the largest sample, 251 fish at 20 to 

 40 miles in 1949, the percentage disagreement was 

 only 0.8; the two samples in the range of 100 to 200 

 fish had percentages of 1.9 and 5.8; and the four 

 samples containing fewer than 100 fish had per- 

 centages ranging from 6.7 to 22.2. 



The data of table 12, taken as a whole, lend 

 strong additional support to tlie belief that a con- 

 siderable part of the disagreements between sup- 

 posed ages and ages read from the scales can be 

 attributed to the presence in the sample of un- 

 marked lake trout with abnormal fins. 



Relation of disagreements to size of fish 



It was stated in an earlier section that 9 of the 

 86 lake trout, for whicli the supposed ages and ages 

 read from scales did not agree, were too large for 

 their supposed age and lience almost certainly were 

 not recoveries of marked fish, but merely had ab- 

 normal fins (these fish are designated by asterisks 

 in table 9). The basis for this conclusion is to be 

 found in the length-frequency distributions of table 

 13. The 9 fish include 2 members of age-group I 

 (marked lake trout of this age seemingly were 

 still too small to be captured in commercial nets) 

 and the 7 lake trout of age-group II that lay well 

 outside the range of length for lake trout of the 

 same age for which scale reading and supposed 

 age agreed. 



For the remaining fish, length does not appear 

 to offer a safe criterion for judgment as to whether 

 any particular individual in a "no" column was or 

 was not a marked fish. The frequencies and mean 

 lengths for the paired groupings are so different, 

 however, as to leave no doubt that the lake trout, 

 for which supposed age and age as read did not 

 agree, included considerable numbers of un- 

 marked fish. Despite the wide ranges in length 

 of individual age groups and the extensive overlap 

 between successive age groups, the distribution of 



