AGE DETERMIXATION FROM SCALES OF LAKE TROUT 



27 



the fairly close agreement between the average 

 calculated length of 3.7 inches (range 1.5-5.9) at 

 time of formation of the 0-mark (computed from 

 scale measurements of recovered marked fish) and 

 the average measured length (3.2 inches; range 

 2.1-4.3) of samples of fingcrlings at time of release 

 into the lake. 



On the other hand, the examination of scales of 

 lake trout that almost surely were not marked 

 fish (lake trout from northern Lake Michigan that 

 were unreasonably large for the ages indicated by 

 their deformed fins and fish from area 8 that 

 included few, if any, marked fish) suggested 

 strongly that naturally hatched lake trout in Lake 

 Michigan also form a 0-mark. Such a mark could 

 arise, for example, from a change in environmental 

 conditions, a change of diet, or a shift by the fish 

 to different grounds upon attainment of a particu- 

 lar length (about 3.7 inches in the northern part 

 of the lake). 



The scales of lake trout for which there was 

 disagreement between the age, as indicated by 

 scales and fin, consistently exhibited a first check 

 that resembled in every way the 0-mark on the 

 scales of marked specimens. The scales in figures 

 13 and 14 were from fish turned in as recoveries 

 of marked lake trout but they were unquestionably 

 from fishes of natural origin. Fish marked in 

 1946, averaging 3.2 inches long, could not 

 have attained lengths of 26.2 and 31.8 inches 

 before they were caught in 1947. Actually, the 

 scales showed 8 and 10 checks, respectively. The 

 central checks resemble closely the 0-marks of the 

 scales from bona fide recoveries. This is brought 

 out forcefully by figures 15 and 16 in which the 

 lefthand scales are presumably from bona fide 

 recoveries (age read from the scales and age 

 indicated by the deformed fin in agreement); 

 and the righthand scales are probably from natur- 



ally propagated fish (ages from scales in disagree- 

 ment with age indicated by fin). It is readily 

 apparent that the structure and size of the central 

 areas of these scales are similar. 



That the central check on the scales of wild- 

 stock lake trout was in fact a 0-mark and not the 

 first annulus was strongly supported by the good 

 agreement between the average calculated lengths 

 of the naturally propagated fish and the marked, 

 liatchery-reared fish at each of the first three 

 checks on the scales. A few lake trout captured 

 by large-mesh nets in northern Ijake Michigan 

 during 1947 could be identified, without question, 

 as wild stock because they were too large to have 

 belonged to any group of marked fish. The 

 calculated lengths of these fish at all three first 

 checks were greater than for the marked lake trout 

 caught in all nets over a period of years, 1947-51 

 (columns 2 and 4, table 7). The differences were 

 no larger, however, than would be expected from 

 the small number of fish in the sample and from 

 the powerful selective influences that bore on the 

 older age groups of the more recent year classes 

 in the collections. The calculated lengths of the 

 wild stock caught in nets of all mesh sizes '* 

 differed little from tlie marked fish caught in 

 similar nets (columns 2 and 6, table 7). Calcu- 

 lated lengths of wild-stock lake trout caught by 

 all nets in the southern part of the lake were 

 0.8-1.0 inch shorter at each of the first 3 checks 

 tlian those of wild stock caught in more northern 

 waters (columns 6, 8, table 7). This large dif- 

 ference between calculated lengths of lake trout 

 from the 2 sections of the lake is indicative of 

 the racial separation of the 2 pojjulations. 



'* This group of lake trout incluiles. in addition to those positively Identified 

 as wild stock, other lake trout for wtiich the age read from the scales differed 

 from that indicated by the deformeil fin. Evidence is i)resented later to show 

 tliat most, if not all, of these fisli were also wild stock. 



Table 7. — Calmlaled total lengths (inches) and increments of growth in length of marked lake trout recaptured in northern Lake 

 Michigan and of naturally propagated fish from northern and southern Lake Michigan year classes combined 



' The marked lake trout averaged 3.2 inches long at time of planting. 



' This total includes 9 fish obviously too large for their supposed age (see p. 30) and also 7 that could not be assigned to a particular planting (more thaii one 

 fin deformed or the deformed fin not one used as a mark), but wtiicti were too large to have been from any of the three plantings. .\11 fish weri' caught In 194i . 



