26 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



June 18, the date on which 50 percent of the 

 marked fish had started new growth on the scales 

 was June 26. Within the 5-percent confidence 

 limits for samples of the same size, new growth on 

 the scales of lake trout in other years would be 

 expected to reach tlie 50-percent level during the 

 last 3 weeks of June. New growth may be 

 identified, then, on the scales of individual lake 

 trout in northern Lake Michigan any time be- 

 tween the middle of March and the middle of 

 August, and about 50-percent of the lake trout will 

 show new growth on their scales by the latter 

 part of June. 



Because of the long time interval in which new 

 growth may begin, the numbers of lake trout with 

 narrow spacing between the circuli at the margin 

 of their scales diminish gradually from January 

 through August and the numbers with wide spac- 

 ing between these circuli increase correspondingly. 

 In July and August some scales, that began growth 

 early in the season, already had a wide band of 

 new growth with narrowing spacing between the 

 circuli near the edge of the scales. The age of 

 unmarked fisli would be difficult to interpret from 

 such scales. Whether the band of growth liad 

 been formed during the current or the previous 

 season would be a matter of the reader's judgment. 

 On most scales from fisli caught at this season, 

 the growth of the current season was narrower 

 than the growth of tlie previous year, but tiiere 

 were exceptions which gave difficulty. 



The end of the growing season for the scales of 

 lake trout could not be determined definitely from 

 the scales themselves. As new growth on the 

 scales of individual fish in the sample began at 

 different times during the spring and summer, 

 they may also have completed growth at different 

 times. In summer and early fall, scales having 

 wide bands of marginal growth with narrowing 

 spacing between the outer circuli had the appear- 

 ance of completed growtli, but it is not known that 

 additional circuli do not form later in the season. 

 It remains uncertain, therefore, whether the scales 

 of lake trout attain the full growth of a season 

 shortly after the begiiming of growtli or continue 

 to increase in size, however slowly, until time for 

 the next anindus to form. 



SUPERNUMERARY OR 0-MARK 



During the first examination of the scales, it was 

 a surprise to discover tliat tlie number of annulus- 



•like markings observed was almost invariably 

 greater, by one, than the number of years of age 

 indicated by the clipped fin. Upon further inves- 

 tigation, the reason for the discrepancy was found 

 in the interpretation of the mark nearest the focus. 

 Comparisons of lengths at capture of lake trout 

 of a known age group (age-groups II to V) with 

 calculated lengths for the same year of life showed 

 the outermost markings to be annuli. Although 

 no lake trout of age-group I were captured, it is 

 logically to be expected that on their scales, also, 

 the outermost mark would be an annulus, hence 

 that the central check is supernumerary. This 

 check or mark appears to have been formed dur- 

 ing the fall of tlie fish's first year when they were 

 only slightly larger than at the time of planting. 

 The innermost marking on the scales, referred to 

 hereafter as the 0-mark, is interpreted to be a line 

 of demarcation between an initial slow rate of 

 growtli and a later sudden increase in the rate as 

 indicated by a change in spacing of the circuli at 

 this point. The circuli within the central mark 

 are more broken and more closely spaced tlian cir- 

 culi laid down later (figs. 5 and 12). The mark 

 is usually fainter tiian the annular rings on the 

 scales and is not present on the scales of all 

 specimens.'^ Rarely, scales show the central 

 marking so closely approximated to the first an- 

 nulus (figs. 7 and 11) as to suggest that on other 

 scales it might coincide with the annulus and thus 

 be lacking altogether as on the scale in figure 6; 

 a few have it very close to the focus, but for most 

 specimens the inner mark is a little over halfway 

 from the focus to the first annulus. Although this 

 mark is typically indistinct (figs. 5 and 9), it some- 

 times is the most conspicuous mark on the scales 

 (figs. 10 and 12). Such outstanding marks might 

 easily be taken to be first annuli on fish of un- 

 known age unless the reader were expecting to 

 find, and looking for, a mark within the true first 

 aiuiulus. 



The 0-mark can only be surmised, at this time, 

 to record some drastic change in the young fish's 

 enivronment or habits of life. A possible explana- 

 tion is that the check results from handling (an- 

 aesthetization, removal of fin, transportation) at 

 the time of planting and the change from hatchery 

 to lake environment. In support of this view is 



" A separatp inner marking was not found on the .scales o( 4 (0.3 iwrcent) 

 of the marked specimens and it is believed the inner mark on these scales 

 coincided with llle first annulus. 



