AGE DETERMINATION FROM SCALES OF LAKE TROTIT 



9 



age that will be demonstrated later, some of the 

 ranges in table 5 cannot be considered reasonable. 

 These lines of evidence, even though they do not 

 exclude the possibility of the presence of a few 

 marked lake trout in the samples from area 8, 

 demonstrate conclusively that the great majority 

 were unmarked wild stock, and that the occurrence 

 of abnormal fins among these fish was not related 



to the age of the fish. The sample is, therefore, 

 considered unsuitable for use in the present study. 

 Samples from areas 1-6 undoubtedly also include 

 some unmarked fish with abnormal fins; and con- 

 vincing evidence of their presence will be offered. 

 There is no reason to believe they were sufficiently 

 numerous there to harm seriously the materials 

 for the purposes of this investigation. 



VALIDITY OF AGE DETERMINATIONS FROM SCALES 



The study of the scales of lake trout, presum- 

 ably of known age, ofTered the rather perple.xing 

 problem of using the same materials for two pur- 

 poses which, in a sense, are mutually exclusive. 

 It was, of course, imperative to examine carefully 

 the scale characteristics of a large series of fish of 

 known age to establish, as exactly as possible, 

 criteria for the determination of age. It was 

 equally necessary to use the same fish as the basis 

 for an objective estimate of the degree of accuracy 

 to be expected in the reading of the scales of lake 

 trout for which the ages are not known. 



With a small series of fish, accomplishment of 

 both purposes would be impossible, for the investi- 

 gator would become so well acquainted with the 

 scales of individual specimens as to remember 

 their characteristics, especially their unusual 

 features, and hence would be unable to make ob- 

 jective age determinations. In the present large 

 series of 1,405 fish from northern Lake Michigan 

 (areas 1-6), however, memory of scales of indi- 

 vidual fish probably had no biasing effect on the 

 accuracy of successive readings. Even so, pre- 

 cautions were taken to keep the tests objective. 

 A brief statement of the general procedure 

 follows. 



In a preliminary examination, designed to estab- 

 lish whether or not the scales of lake trout bear 

 markings that can be interpreted as annuli corre- 

 sponding in number to the supposed age of the 

 fish (as indicated by a deformed or missing fin), the 

 scales of several hundred lake trout were read 

 objectively. They were studied for the occurrence 

 of repetitive irregularities in the sculptured pattern 

 without reference to any information about the 

 fish except the date of its capture. When such 

 markings were found, readings and measurements 

 made from them were compared with tlie full 

 data on the individual fish. Another important 

 aspect of the first series of examinations 



was the establishment of the time of annulus form- 

 ation and the progress of the season's growth, 

 without knowedge of which it is difficult to make 

 accurate readings from scales of fish caught over 

 much of the growing season. 



After the characteristics of the annulus and the 

 time of annulus formation were well established, 

 the entire series of scales was read twice. During 

 both readings the only information available was 

 date of capture, and eacli second reading was 

 made without knowledge of the age assigned at 

 the first. After completion of the two readings, 

 a careful study was made of the scales of all lake 

 trout for which the ages assigned were not the 

 same at the first and second examination and a 

 best estimate of the correct age was made. 



EARLY GROWTH OF SCALES 



The scales of lake trout are cycloid, oval to egg- 

 shaped. Concentric ridges or circuli, arranged 

 about a focus, roughen the outer surface of the 

 scale. The focus may be central or slightly 

 anterior or posterior to the center of the scale 

 (see figs. 8 and 11). Neither radii nor transverse 

 grooves are present. The inner surface of the 

 scale lacks circuli but is not utterly smooth and 

 characterless. Annuli sometimes are clearly visi- 

 ble on this side. The scales are so small, thin, 

 and deeply embedded in the skin as to be relatively 

 inconspicuous. They are dislodged with such 

 difficulty that few are regenerated. Variation in 

 the number of scales, in series along the lateral 

 line, is large, from 180 to more than 200. Squa- 

 mation of the body is complete. Only the head, 

 which is well supplied with mucus pores, and fins, 

 are unsealed. 



The size of the scale varies greatly from one 

 location on the fish to another. In general, the 

 larger scales are on the posterolateral surfaces of 

 the body and the smaller scales about the fin bases 



