98 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



modification of the population by the fishery (that 

 is, a selective destruction of the larger fish) or to a 

 differential distribution of the fish according to 

 size so that only a certain segment of the popula- 

 tion is represented in the fishery. Since the second 

 condition obviously would contribute to the first, 

 it may be assumed that the commercial fishery 

 exerts a strong modifying effect on the population. 

 Natural mortality, of course, may also play an 

 important but unmeasurable role in this process. 



A progressive increase in length of lake herring 

 of each age group in successive years from 1949 to 

 1952 (table 9) indicates that more rapid growth 

 took place in the later years. This trend is also 

 brought out in a later section on annual fluctua- 

 tions in growth rate. 



Small as variations were in the average lengths 

 of lake herring collected at different times of the 

 year (tables 7 and 8), November-May collections 

 taken at about the same time but often at con- 

 siderable distances apart, showed still smaller 

 differences of no more than 0.2 inch. This 

 similarity was not always present, however, for 

 in collections of other months (June-October) 

 large differences sometimes occurred. Examples 

 of these small differences in average lengths of 

 herring taken in different areas are given in the 

 following table: 



The weight.of Green Bay lake herring at capture 

 presents much the same picture as does length. 

 Weights of fish of a given age are distributed over 

 a wide range and each weight group is frequently 

 represented by fish of three ages (table 10). 

 Differences between age groups III and V varied 

 only 0.6 ounce to 1.2 ounces in different years as 

 would be expected when differences in length were 

 small. 



GROWTH 



BODY-SCALE RELATION AND CALCULATION 

 OF GROWTH 



Van Oosten (1929) established the validity of 

 computations of the growth of lake herring from 

 the diameters of the entire scale and of growth 

 fields within the several annuli. Since the publi- 

 cation of his work, most investigators reporting on 

 growth of this species have accepted Van Oosten's 

 conclusions. 



The relation between body length and the 

 anterior scale radius of lake herring was determined 

 for the tullibee of Lake of the Woods by Carlander 

 (1945). Carlander used the anterior radius be- 

 cause he found annuli difficult to locate in the 

 posterior field. He demonstrated that the rela- 

 tion between scale radius and standard length was 

 described satisfactorily by a third-degree equation. 

 From a comparison of results of calculations from 

 diameters and anterior radii Van Oosten (1929, p. 

 327) found that "* * * the diameter of a scale 

 grows in length more nearly proportional with the 

 body than does the anterior radius [and] * * * 

 that the diameter dimension is less variable than 

 the anterior radius * * *." Since no difficulty 

 was experienced in locating annuli in the posterior 

 field of scales of Green Bay lake herring, diameter 

 measurements were used in this study to take 

 advantage of the simple, direct-proportional rela- 

 tionship determined by Van Oosten. It was held 

 desirable, nevertheless, to study the body-scale 

 relation of the Green Bay lake herring to make 

 certain that the procedure was valid in this stock. 



If direct-proportion computations are to be 

 valid, the body-scale ratio must be the same for all 

 lengths of fish from the time of completion of the 

 first annulus. Van Oosten (1929) found that after 

 formation of this annulus the ratio of total scale 

 diameter to body length was so nearly constant in 

 the herring of Saginaw Bay that an assumption of 

 constancy could be made. In the Green Bay lake 

 herring the body-scale ratio exhibited no trend 

 with increase in fish length (table 11). A f-test to 

 determine whether such variations as did occur 

 represented a significant trend confirmed the 

 validity of the assumption that the ratio does not 

 change with length (0.8<p<0.9). Since the fish 

 from which these data were taken covered the full 

 range from those that had just completed the first 

 year of life to the largest and oldest fish collected, 



