102 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 14. — Amount of season's growth in length completed 

 by age groups, individually and combined, on various dates 

 of capture 



[Expressed as calculated increments (inches) and as percentages of the full- 

 season growth determined from samples of the same year class taken in 

 January and February of the following calendar year] 



The full-season increments given in the table were 

 determined from samples of the same year class 

 taken in January and February of the next calen- 

 dar year. A better estimate of the full-season 

 growth could not be derived from the materials 

 at hand. It will be brought out in a later section 

 on seasonal changes in style of growth that the 

 fishery removes lake herring selectively according 

 to individual growth rate. By reason of this 

 selective destruction, the early-winter samples 

 may not be truly descriptive of the course of 

 growth that would have taken place in an unex- 



ploited population. There is no reason to believe, 

 however, that this source of bias has seriously 

 affected the estimates of progress of the season's 

 growth. 



Despite the difficulties and possible sources of 

 bias just discussed, the data of table 14 can be 

 used to form a general idea of the normal course 

 of the season's growth of lake herring. That this 

 growth does follow a distinct and reasonably defi- 

 nite course is indicated by the unweighted means 

 of the percentages of growth completed for the 

 different dates of collection. These percentages 

 are given graphically in figure 4 ; in the same figure 

 a curve has been fitted by inspection to the 

 empirical data. The progress of growth probably 

 can be described best from the following percent- 

 ages obtained from the curve. 



From the preceding table it is seen that the 

 greatest amount of growth in any single month 

 took place in July (25 percent) and that the sum- 

 mer months, June to August, accounted for 64 



100 



MAY JUNE JULY 



AUC. JEPT. 

 M O NTH 



Figure 4. — Percentage of season's growth completed at 

 time of capture. The dots show empirical data; the 

 curve was drawn by inspection. 



