AGE DETERMIXATIOX FROM SCALES OF LAKE TROUT 



11 



tioii. Fiirtlicr <rro\vtli of tlie scales is approxi- 

 mutclv proportional to the growth in length of 

 the fish. 



The spacing of circuli on the scales of lake trout 

 appears to indicate periods of fast and slow growth. 

 The wider spacing is found typically at the 

 heginning of each new hand of growth. The 

 closely spaced circuli are laid down on the scale at 

 the end of the growing season. Widely spaced 

 circuli liave been found in narrow annual growtii 

 zones (fig. 10, first year), and conversely, closely 

 spaced circuli sometimes occur in wide annular 

 growtii 7ones. Both types probably are true 

 records of growth. Fish may grow a small amount 

 but grow rapidly during a short period of the year 

 and not at all or very little the rest of tlie year, or 

 they may grow at a slow rather uniform rate 

 during a much longer period of the year. 



CHARACTERISTICS OF THE ANNULUS 



Because annular markings on lake trout scales 

 are rather difTicult to locate, a detailed description 

 of tlie aiinuli and a statement of criteria for their 

 recognition are apropos. The annulus is more 

 distinct on the scales of some lake trout tlum 

 others; it is also more easily seen on some parts of 

 a scale than elsewhere. Its location is revealed 

 by one or more of the following characteristic 

 arrangements of the circuli. 



The most common and most easily recognized 

 ari-angement of the circuli in tlie normal growth 

 pattern consists of a gradual narrowing of the 

 spacing outward from the focus to the annulus, 

 then an abrupt change to wider spacing. This 

 feature is well illustrated by the scales in figures 5 

 and f) and to a varying ilegree by all otlier scales 

 re])rO(luce(l liere.'^ Tlie closely spaced circuli give 

 the appearance of incomplete bands on the scale 

 which are usually, l)ut not always, most conspicu- 

 ous in the posterolateral fields. Figure 12 shows a 

 scale on wiiich all tlie circuH ai-e widely spaced 

 and the annular narrowing, tliough barely per- 

 ceptible, offers a definite and reliable criterion. 

 However, the annuli cannot l)e traced completelv 

 around the scales by this cliaiacteristic alone. 

 Other criteria must be used in combination with it. 



'JVaces of annuli also may be observed in the 

 posterior fi eld (shown at the bottom of all figures 



'>,\ll scah'.s weri' studied ut the sinii' niagnilication (.Xs:t..1). Ulustra- 

 tions (,f I hi' scales have been reduced Xfifi.S. Sec p. 59 fur siRnificanee o( th.' 

 cheek latuled ■'O." 



of scales). Here, the annulus often is seen dis- 

 tinctly as a ridge on the scale or as a groove on 

 the impression. The groove is well illustrated 

 by the second and third annuli in figure 10, and 

 the third and fourth annuli in figure 9. Another 

 characteristic pattern in the posterior field results 

 at points where circuli of the preceding growing 

 season end and the first circulus of a new season 

 crosses their paths at angles that bring the pattern 

 to a crude V in which the angle of the V points 

 toward the annulus. These V's are in evidence 

 somewhere on nearly every scale, but on the scale 

 shown in figure 7, it is doul)tful whether the 

 fourth annulus would have been located but for 

 the V on the lefthand side, as the annulus is 

 indistinct elsewhere around the scale. The V's 

 are also clearly represented in figure 5 bv tlie 

 second and third annuli, and in figure 8 bv the 

 first, second, and third annuli. 



Frequently, part of the posterior area of the 

 scale is almost devoid of .sculpturing. Only 

 ragged bits of crooked, discontinuous circuli are 

 scattered about, but even then, circuli extend 

 farther out into this part of the scale at the annulus 

 than between annuli, pointing it out like a crooked 

 finger. 



In the anterior and lateral fields, three charac- 

 teristics of the pattern of circuli, usually occurring 

 in combination, indicate the location of the 

 annulus. First is the narrowing of the spacing 

 between circuli at the end of a growing season, 

 mentioned earlier and seen in most figures. Usually, 

 in addition, there is a broken circulus here or 

 there along the annulus with another circulus 

 crossing the ends in a "cutting-over" pattern (as 

 in the V formations of the posterior field). The 

 longer circulus which does the cutting-over is the 

 first circulus of the new growth. It is often 

 continuous through the anterior field from the 

 posterior field on one side to the posterior field on 

 the other side of the scale, and may cross or 

 extend partly across the posterior field itself, as 

 shown by the first annulus in figure 12, and by 

 all annuli in figure 11. The third characteristic 

 pattei-n results from the apj)earance of one oi- two 

 very fine, broken lines '* at the annulus. This 

 feature is illustrated by the scale shown in figure 

 10. Note especially the second and third annuli. 



The scales shown in figures 5 to 12, also 15A and 

 16A, are from fish representative of lake trout 



'• Thes*' do not appear to t'e true circuli. 



