VALIDITY OF AGE DETERMINATION FROM SCALES, AND GROWTH 

 OF MARKED LAKE MICHIGAN LAKE TROUT 



BY LouELLA E. Cable, Fishery Research Biotogist 



The lake trout, Salrelinuft n. namaycush (Wal- 

 baum), was once the leading fish in the Great 

 Lakes from the standpoint of monetary returns to 

 the fishermen. The normal catch in the years 

 before the invasion of the sea lamprey was 

 15,375,000 pounds, valued at $7,688,000 by present 

 day market prices.' 



Depredations of the sea lamprey ^ had so re- 

 duced the stock of lake trout by 1953 that only 

 4,128,000 pounds were taken. Lakes Erie and 

 Ontario never supported large fisheries for lake 

 trout, and, as the 1953 catch in Lake Superior was 

 near normal, most of the 11,247,000-pound loss in 

 total production was sustained in Lakes Huron 

 and Michigan. Between 1932 and 1953 the catch 

 in Canadian waters of Lake Huron was reduced 

 gradually from an annual average of 3,596,000 

 pounds to 344,000 pounds, and in t'nited States 

 waters the catch dwindled from 1,400,000 pounds 

 in 1936 to practically none in 1953 (Hile 1949, Hile 

 and Buettner 1954). 



The collapse of the lake trout fishery in Lake 

 Michigan, though later than that in Lake Huron, 

 has been equally dramatic. Annual production 

 from 1885 to 1945 held between 5 and 9 million 

 pounds. Tlie decline was first apparent about 

 1946, but by 1953 the catch amounted to only 402 

 pounds. For a record of the annual production 

 of this fishery from 1885 to 1949, see Hile, 

 Eschmeyer, and Lunger (1951). 



It now seems probable that the sea lamprey can 

 be brought under control by the use of electrical 

 barriers (Applegate, Smith, and Nielsen 1952; 

 Applegate and Moffett 1955) placed near the 

 mouths of streams into which adidt lampreys run 

 to spawn. When this has been accomplished, 

 rehabilitation of lake trout stocks will be possible. 



I Further research and control of sea lampreys o( the Oreat Lakes area. 

 Hearings of the Subcommittee Merchant Marine and Fisheries. 82d 

 Cons., y S, IIou.se of Representatives, 1952. page 28. 



' See Applegate (1951). and Van Oosten (1949 a. b) for accounts of the inva- 

 sion and spread of the sea lamprey in the upper Great Lakes. 



Meanwhile, information about the growth and 

 habits of these fish must be gathered as a basis for 

 an intelligent program for restoration and manage- 

 ment of the fishery. 



In the study of the lake trout, it is imperative 

 first to assess the reliability of ages determined 

 from scales of the fish to validate them for the 

 many uses to which age statistics and calculated 

 lengths, based on measurements of scales, are put 

 in population studies. 



Although determination of the age of lake trout 

 from scales was considered difficult by Royce,' 

 Cooper, and Fuller (1945), and by Miller and 

 Kennedy (1948), several investigators, including 

 Greeley (1934 and 1936), Fry and Kennedy (1937), 

 Fry (1949, 1953), and Van Oosten (1950), have 

 read them with apparent assurance, but without 

 establishing the validity of their readings. 



The purpose of the present examination of the 

 scales from lake trout of known age is not to offer 

 an estimate of any person's skill in reading ages 

 of the fish, but rather to ascertain whether recog- 

 nizable markings of any kind, formed one each 

 year, may be judged to be annuli. As scales of lake 

 trout of known age have not been studied critically 

 before, criteria for distinguishing annuli as they 

 occur in this species are set forth in the paper. 

 Time of annulus formation, development of mar- 

 ginal growth, calculated lengths, and growth of the 

 marked lake trout also are discussed. 



The cooperative work of the Conservation De- 

 partments of Michigan and Wisconsin and the 

 Branch of Game-fish and Hatcheries of the United 

 States Fish and Wildlife Service provided material 

 for the present investigation. The author is in- 

 debted to Dr. Ralph HUe and Dr. James W. 

 Moffett for reading the manuscript and for valu- 

 able suggestions, to Dr. Paul Eschmeyer for per- 



' The reproduction and studies on the life history of the lake trout Cristiro- 

 mer n. namaycush (Walbaum). By William F. Royco. Doctoral thesis 

 submitted to Cornell University in 1943. .Manuscript. 



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