10 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



and Oil the aiiteroveiitral and aiiterodorsal surfaces. 

 Samples for study were taken from a mid-point on 

 the body, below the anterior part of the dorsal fin, 

 immediately above the lateral line, and thus did 

 not include either the smallest or the largest scales 

 of the individual fish. However, scales from 

 rather limited areas show considerable variation 

 in size and shape. Scales chosen from the sample 

 for study were tliose which seemed most repre- 

 sentative of the larger symmetrical scales. 



The scales of lake trout appear during the sum- 

 mer of the first year of development. However, 

 neither the age nor the length of the fish at the 

 time of scale formation lias been determined 

 definitely for the lake trout in I>ake Michigan. 

 Both salt-water and fresh-water fishes that have 

 been studied develop platelets, the beginnings of 

 scales, when the yoinig fish are from about 18 to 

 50 millimeters in total length (Fish 1932; Hilde- 

 brand and Cable, 1930, 1934, and 1938; Cooper 

 1951 ; Brown and Bailey, 1952; and others). Fisli 

 (1932) described a lake trout larva 21.5 millimeters 

 long from Cape Vincent Hatchery, but did not 

 mention the development of scales. A yolk sac 

 was still present at this size and the appearance 

 of scales would scarcely be expected before ab- 

 sorption of the yolk. 



In 1953, young lake trout 26 to 56 millimeters 

 long, were taken in Lake Superior in the middle 

 of June and the middle of August by the Fish and 

 Wildlife Service research vessel ( 'isco. The largest 

 of those caught in August was 56 millimeters or 

 2?i6 inches long. It had a band of scale pockets 

 containing platelets along the entire length of the 

 lateral line. This band consisted of several rows 

 of platelets on either side of the lateral line. The 

 sizes of the platelets were graduated; the larger 

 ones were adjacent to the lateral line; the others 

 became smaller and farther apart with each suc- 

 cessive row. Only in tiie lateral line did the scale 

 structures take alizarin stain readily. These 

 structures were concave ovoids, two in each 

 pocket, one dorsal to and the other ventral to the 

 lateral-line organ, forming partial sidewalls to it. 

 The platelets, situated in dermal pockets, were 

 protected from immediate <'ontact witli tiie 

 alizarin. Consequently, the scale pockets stood 

 out as clear areas after staining. The largest 

 scale platelets, when teased out of the pockets, 

 measured about 0.2 millimeter long. Some were 

 clear and smooth; the first circulus was formed on 



others. Although some fish such as biook trout 

 form scales first along the posterior ])art of the 

 lateral line (Cooper 1951), a .young lake trout 53 

 millimeters long had platelets scattered in one or 

 two interrupted rows and in small groups here 

 and tiiere along the anterior end only of the 

 lateral line. The lateral line itself was not in 

 evidence posteriorly. The largest i)latelets on this 

 lake trout were about 0.1 millimeter long and 

 lacked circuli. Probably scales begin to form on 

 lake trout in Lake Superior when the fish are 

 about 50 millimeters long but no histological 

 sections were made to determine this jxiint. 



It is not known whether young lake trout 

 growing in Lake Michigan develop scales at the 

 same size as those in Lake Superior. One hundred 

 fingerlings, all of the same age but ranging in 

 length from 35 to 85 millimeters, which were 

 reared in the fish hatchery at Charlevoix, Mich., 

 in 1948 and preserved on Septenil)er 17, were 

 examined. The smallest of these lake trout having 

 scales was 47.5 mm. long. This fish liad scales 

 with as many as 4 circuli the full length of tiu' 

 lateral fine. Other specimens 35 to 43 mm. long 

 were without scales and no evidence of a lateral 

 line was seen. Although these young lake trout 

 grew under artificial conditions, development of 

 the scales began at about the same l)0<ly length as 

 on young fish that had grown under natural 

 conditions in Lake Superior. Tlie lake trout from 

 tlie iiatchcry were caught about a month later 

 tiian tliose from Lake Superior, which may account 

 for the presence of scales on somewhat smaller fish. 

 Season of the year and age as well as i)ody size 

 may be factors in determining the time for the 

 formation of scales. 



The average total length of the lake trout 

 marked by removal of fins and i)lanted in Lake 

 Michigan in early September 1944, 1945, and 1946 

 was 81 mm. or 3.2 inches (range 2.1-4.3 inches"). 

 It probably is safe to assume, therefore, that lU'arly 

 all were fully scaled when planted and would not 

 pass through the first year without the formation 

 of an annulus. As soon as the scales appear on 

 the fish, squamation proceeds rapidly to comple- 



I' Measurements were of random samples of the general stock of lake trout 

 reared in the lisli hatchery at Charlevoix, Mich., for the 1(14.1 experiments. 

 The range for I.IXK) unmarked lake trout used as contnds was .W-lllS mm.; 

 that for 1,0110 marked lake trout also used as controls was, ')4-in9 mm.; and the 

 range for 499 lake trout lield for studies on regeneration of fins was .13-1(1.') mm. 

 I thank David S. .Shelter, Michigan Institute for Fisheries Research, for 

 permission to publish this infoimation. 



