6 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



(fig. 4). Aiiiiuli seen first on the inner surface could 

 then be located more readily in tlic sculptured 

 pattern of the outer surface. Labels bearing 

 specimen numbers were typed on rag paper with 

 hectograph ribbon, laid face down on the gummed 

 paper, and secured at the ends by bits of Scotch 

 tape; then 3- by 5-inch sheets of cellulose acetate 

 0.020-incli thick were inserted between the label 

 and the gummed Kraft paper caid on which the 

 scales had been affixed. 



The labeled, mounted scales were impressed by 

 the exertion of about 12 tons of pressure on 8- by 8- 

 inch platens, prelieated to 230° F.^ 



The impressions wei'e studied at a magnifica- 

 tion of 83. 5X on the microprojection machine 

 described and illustrated by MoflTett (1952). The 

 annuli found on each scale were traced on the 

 viewing screen with a glass-marking pencil. The 

 diameters of the entire scale and of fields witiiin 

 the several annuli were measured along the antero- 

 posterior axis through the center of the focus. 

 Measurements of the diameters of the annuli 

 were more suitable than measurements of either 

 anterior or posterior radii for the estimation of 

 past growth. 



In order to judge the reliability of measurements 

 made of impressions of lake-trout scales, measure- 

 ments of scales mounted in gelatin were compared 

 with those of impressions of the same scales. 

 Gelatin mounts are wet scales whereas plastic 

 impressions are made from dry scales, yet the 

 difference in scale size was not significant and 

 was no greater than occurs regularly between 

 independent measurement of the same scale. It 

 appears from this comparison that dehydration 

 causes no appreciable decrease in size of lake 

 trout scales. Butler and Smith (1953), who com- 

 pared dry mounts, gelatin mou!its, and Plastacele 

 impressions of the thicker scales of the walleye, 

 Sfizostedion vitrevm, found significant difi'erences 

 among them but the differences were "reflected 

 proportionally at each annulus." 



In this paper, age groups are tlesigiuited by 

 Roman numerals corresponding to the number of 

 annuli (fish in tiieir first year are members of age- 

 group 0). A "virtual" annulus is credited at the 

 edge of the scale from January 1 to the time of 

 amiulus formation. Year classes are identified by 

 the calendar year of hatching (wliicii takes place 

 in the spring; spawning occurs the ])receding fall). 



BASIS FOR REJECTION OF SAMPLES FROM SOUTHERN LAKE MICHIGAN 



It was realized early that l)y no means all lake 

 trout represented in the 1,507 scale samples were 

 authentic recoveries of fin-clipped fish. That 

 occasional naturally pi-opagated lake trout may 

 lack fins or liave abnormally formed fins has been 

 established." Hatchery-reared lake trout rarely 

 develop deformed fins of the tv])es that would be 

 mistaken for clipped fins (see footnote 20). 



Even though the percentage of ?uiturally occur- 

 ring malformations may be small, it was antici- 

 pated that most of the fish bearing them would be 

 reported by the fishermen. The maiking experi- 

 ment was widely publicized and tiie oiJerators were 

 urged strongly both by the officers of their own 

 trade association and conservation officials to 

 cooperate by reporting all recoveiies. 



Various aspects of the data were studied in detail 



8 Details of this procedure, basic features of wliicli were developed by 

 R. A. Nesbit, unpublished. 



' John Van Oostcn reported in 1949. at the spring meeting of the Great 

 Lakes Lake Trout Committee, that Frank W. Jobes and Howard .T. Buettner, 

 examined 1.8.111 lake trout from Lake ^ficlliKan and found 4 (n.22 percent^ 

 with deformed fins. Three fish (n.2n percent) also with deformed fins were 

 found among 1,4(12 lake trout from Lake Superior. It was believed only one 

 of these fins could have been mistaken for a regenerated fin which had pre- 

 viously been removefi by ciippins. 



to obtain reasonably objective standards for dis- 

 tinguishing between marked and unmarked lake 

 trout. Among the points considered were: geo- 

 graphical distiibution of recoveries from the 3 

 years' plantings; condition of abnormal fins in 

 terms of numbers of regenerated rays and length 

 of fin; growth shown by the fish at ages indicated 

 by the deformed fins; discrepancies between ages 

 iiulicated by abnormal fins and those shown by 

 tiie scales. Data on the last of the aforementioned 

 points may be used, of course, only as indicative 

 of general relationships and trends, since a mere 

 disagreement between these ages does not in itself 

 constitute acceptable evidence that individual fish 

 had not been marked by fin clipping. 



The analyses led to the rejection of the entire 

 sample from stnitliern Lake Miciiigaii (area 8) as 

 containing few or no marked lake trout. For the 

 samples fi'om the northern part of the lake (areas 

 1-6), ol)jective standards were not furnished for 

 the separation of (lie marked from tiie unmarked 

 or wild fish. By other methods, it was |)ossible 

 to jioint out most of the unmarked fish there with 



