40 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



T.\BLE 20. — Tnlnl length at capture and lengths calculated by direct proportion from diameters of annuli on the scales of marked 



lake trout 



[Lengths in inches] 



I Figures in this column are computed lengths at time of formation of the 0-mark in the field of first-year growth. 



Calculated Lengths 



The growth history of each fish was taken as 

 the average of the calculated lengths computed 

 independently from the measurements of two 

 scales selected as representative of a sample. ^^ 

 Averages of these lengths for all marked lake trout 

 of each of the three year classes (1944-46) are 

 shown in table 20 where they are arranged by age 

 group. Distribution of the calculated lengths for 

 corresponding age groups of different year classes 

 was fairly random and agreement between them 

 sufficiently close to warrant combination of the 

 data from year classes. The small discrepancies 

 that did exist among the means of different 

 age groups are not believed to influence adversely 

 the general mean. 



The lengths for early years of life, calculated 

 from age-groups III to VI, exhibited Lee's phenom- 

 enon of gradually decreasing values with increasing 

 age (fig. 19), Differences in the estimates of the 

 lengths calculated from the first three of these 



M The larger scale in a pair from the same fish often gave a smaller calculated 

 length for the early years of life than did the smaller scale, .\lthough the 

 difference hetwecn this distrihution and the 60-50 relationship expected (on 

 the theory that all scales on the fish give the same calculated lengths) was 

 highly significant, the mean dilTerences in calculated lengths for the sample 

 were slight (0.05 inch in formation of the 0-mark and 0.04 inch at the first 

 annulns). The dilTerences in the sizes of the paired scales were small, how- 

 ever (ranging from 0.12 to 0.96 millimeter). Because of this bias inherent 

 to the data, and the necessity for studying the scales at high magnification 

 (so close measurements are difficult to make), it is desirable that lengths 

 calculated for the early years be made from measurements of two or more 

 scales, especially when the number of fish in the sample is small. Consistent 

 selection of either very large or very small scales could result in appreciable 

 error. 



age groups were much smaller than the differences 

 between these and the estimates calculated from 

 age-group VI. The 6-year-olds were not only 

 smaller than the 5-ycar-olds at time of capture 

 but, according to tlieir calculated lengths, they 

 started life (at planting) in the lower half of the 

 range of length. The early disadvantage in size 

 at formation of the 0-mark was not compensated 

 in later life.^^ All values for lengths at various 

 ages, except those calculated from age-group VI, 

 were rather closely grouped about a common mean 

 value, hence could be combined. As age-groups 

 II to V were represented by 1,306 lake trout and 

 age-group VI by only 13, the averages were 

 scarcely influenced by the latter group. Effects 

 of selective fishing by gill nets used in the com- 

 mercial fishery are discussed in the following sec- 

 tion of this paper. 



The growth history of the entire sample of 

 marked lake trout has been described by two 

 common methods (bottom of table 20). By tlie 

 first method, and the one believed to yield the 



" Reibisch (1899) in writing of the ditTerence in size of young plaice, 

 Pldironectes plalessa, caused by a long hatching period during which those 

 that had hatched early were aheady large at the time others were hatching, 

 toward the end of the season, stated that the time of hatching continues to 

 exert its elTect on the size of the individual throughout later years. More 

 receiilly. Hodgson (1929) concluded from his studies that so-called compen,sa- 

 tory growth is simply explained as the natural result of comparing the growth 

 of fishes which are at different ages. Ford (1933) points out that with increase 

 of age fi.sh have less ability to increase length, but the "curve of ability to 

 grow," which has the form of a geometric regression, may be subject to 

 variation from fish to fish, and from population to population with difTer- 

 ent conditions of growth. 



