AGE DETKRMIXATIOX FROM SCALES OF LAKE TROUT 



31 



FACTORS OF DISAGREEMENT 



Disagroempiits hctwocn ajres as read from scales 

 and supposod ages can arise from misinterpreta- 

 tion of the scales from bona fide recovei-ies, and 

 also from the inclusion in tlie sampl(> of lake trout 

 that had not been marked. Both types of errors 

 may be represented in the disagi-eements discussed 

 in the precedino; section. Although the relative 

 importance of these factors cannot be estimated 

 closely, the data do provide some instructive in- 

 formation in tlie matter. 



Errors of Reading 



Errors of reading may originate in tlie inter- 

 pretation of scale patterns which, properlv diag- 

 nosed, could lead to a correct determination of 

 the age of the fish. Errors may arise also from 

 defective scales, that is, scales that failed to form 

 certain aimuli, developed accessory checks indis- 

 tinguishable from amudi, or had a pattern so 

 diffuse that any reading is questionable. As was 

 pointed out earlier, the present collection certaiidy 

 contained some lake trout that were not recoveries 

 from plantings of fin-clipped fish. It is impossible, 

 therefore, to attribute any individual disagreement 

 strictly to error on the part of the scah> reader. 



It is possible, however, to gain a general idea 

 of tlie clarity and dependability of scale patterns 

 from the examination of a large series of scales, a 

 high percentage of whicli must be from bona fide 

 recoveries of planted fish, even thougii the status 

 of an uidividual specimen must be recognized as 

 uncertain. Careful study of the hundreds of 

 scales from which readings agreed with supposed 

 age led to the conclusion that over the age-span 

 represented, the markings were almost always 

 clear, and that failure to form an aiumlus must 

 be rare. Some annuli were extremely faint, espe- 

 cially in the posterior field but faint year-marks 

 usually could be detected in the lateral fields. 

 The presence of an occasional indistinct annidus 

 does, nevertiieless, indicate the possibility of others 

 so weak as to be overlooked. 



Accessory checks between atniuli, other than 

 the 0-mark discussed in the preceding section, 

 were not connnon and when present caused little 

 trouble because they rarely, if ever, extended 

 completely around the scale. 



Another factor which may iiave been a source 

 of some error is the int.'rpretation of marginal 

 growth. Dining th(> period of aiuiulus formation 



it is occasioiuilly didicult to decide whetlu-r the 

 marginal band rei)resents completed growth of 

 the previous year or rapid growth of the current 

 season. 



Inclusion of Unmarked Fish With Abnormal Fins 



Overwhelming evidence was |)resented earlier 

 that the "recoveries" from southern Lake Michi- 

 gan (area 8) included few, if any, marked lake 

 trout. Since there is no reason to believe that 

 the development of a])norniaI fins among naturally 

 propagated fish is exclusively a property of tin- 

 stock of lake trout in southern Lake Michigan, it 

 was to be anticipated that the recoveries from 

 northern Lake Michigan, though principally 

 marked fish of Imtchery origin, woulil also iiu'lude 

 some naturally hatciied lake trout (and possibly 

 some unmarked hatchery-reared lake trout that 

 tleveloj)ed abnormal fins).-" 

 Relation of disagreements to appearance of the fin 



If data on the "extent of regeneration" of the 

 fins of lake trout from area 8 (tal)les 3 aiul 4) are 

 typical for abnormal fins on wild fish, then, in 

 samples from northern Lake Michigan (areas 1-6), 

 the great majority of fish with fewer than 5 rays 

 regenerated or with fins less than y, normal length 

 would be bona fide recoveries of marked speci- 

 mens, whereas most unmarked fisli with abnormal 

 fins would appear in the group sliowitig greater 

 regeneration. If these conchisions are valid and 

 if the collection of lake trout from northern Lake 

 Micliigan contains appreciable numbers of wild 

 fish, a correlation should l)e found between the 

 extent of regeneration and tlie percentage of dis- 

 agreement l)etween supjjosed ages and ages read 

 from scales. 



Tills expectation is met l)y the data of table 11, 

 for the lowest percentage disagreenu'iit (3.8 per- 

 cent) occurred among fish with fewer tiian .5 fin 

 rays regenerated less than half normal length. 

 For the other three groups in the nuiin body of 

 the table the percentages ranged from (5.3 (trout 

 with fewer than r, fin rays regenerated but iialf 

 normal length or longer) to 10.7 (fish with fins 

 less than half normal length but having 5 or more 

 fin rays re generated). The value of 6.9 percent 



■" Allhough the pvmTitaBi' of wild-stuck hiko Iiout with alinorllKil fins is 

 smiill, llir total niinihiT irport.-il hy fishmiii'ii can he con.siilcrahlc when all 

 catches arc hi'Inc .scnititlizcil for dcfornicil fins. The perccntaRC of hatchery 

 fish with ahnormal fins is also low. Dr. I'aul K.schincycr. who has hcon in 

 charge of fin-clip|iin(! o|)eraIions at the fnitcd Stales Kish and Wildlife 

 Service Fish Hatchery near Charlc'voix, .Mich,, several seiisons. states that 

 an occasional finserlinc lake trout reared in the hatchery has an accrs-sory 

 fin hut very few finperlings have deformed fins. 



