LIFE HISTORY OF LAKE HERRING OF LAKE HURON 359 



millimeters in 1924. In each age group except the fifth and sixth the 1922 individuals 

 averaged larger than the 1921. Without exception the 1923 fish averaged larger 

 than the 1922, and with the exception of the fourth age group tlie 1924 herring 

 averaged larger than the 1923. 



The average weights of Table 31 tell a similar story. In general, the fish of 

 corresponding age groups become progressively heavier each year. In each age 

 group except the fifth ''and sixth the fish taken in 1922 averaged more in weight than 

 those taken in 1921, while in each age group without exception the 1924 fish weighed 

 more than the 1922. The herring taken in 1923 were not weighed. 



If the average length of the herring population varies as the spawning season 

 advances, as is the case in some age groups of the sockeye salmon runs (Gilbert, 

 1922, pp. 34 and 64), and if the samples of difi'erent years are taken at different 

 periods of the run, then the above averages of length and weight would not be 

 comparable for different years. However, they are believed to be comparable in 

 this case, because each sample collected in 1921-1923 was taken at about the same 

 period; that is, about two weeks after the main spawning run of herring began, and 

 because, as I shall show later (p. 385), the average length of the herring population 

 does not seem to vary much during the height of the spawning season. It will be 

 shown in another place (p. 364) that the same conclnsion (the herring are growing 

 bigger) may be drawn from other data not subject to these possible errors. 



Apparently, then, these data show that the herring of Saginaw Bay were reach- 

 ing a bigger size in 1922, 1923, and 1924 than in 1921 and in general were becoming 

 progressively larger at corresponding ages each successive year. The curves of 

 Figure 37, based on the length frequencies of Table 29, point to the same conclusion, 

 as with each year the curve moves farther toward the right. This change in the rate 

 of growth of the herring may be the third symptom of intense fishing. (For the other 

 two symptoms see p. 355.) Briefly stated, this may mean that as the number of herring 

 in the lake is being reduced competition for food among the survivors becomes less 

 severe and consequently they grow faster. The subject is discussed further in the 

 section on growth rate. 



MALES AND FEMALES 



Table 32, in which are given the actual, average, minimum, and maximum length 

 and weight for each sex of an age group, shows that males and females attain approxi- 

 mately the same length and have the same growth rate in corresponding years of 

 life; that is, no consistent differences in growth rate occur between the sexes. This 

 is brought out more clearly in Table 33 and Figure 38. Table 33 shows, for fish taken 

 in 1921 and 1922, the average total length attained at the end of each year of life by the 

 males and females of each age group. All averages but the last one of each row are com- 

 puted from the measurements of scales. In Figure 38 the average lengths of the 1921 

 fish only are plotted. The progressive divergence of the two curves of growth in the 

 fourth and later years of the 6-year fish presumably is due to the small number of 

 females in that age group. In weight the females average a little higher (see Table 32) 

 than the males, due to the greater weight of their partially developed sex organs, 

 although the difference is not as great as was expected. In a further discussion of 

 growth rates the males and females will be considered together. 

 997GO— 29 7 



