390 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Tahle 30. — Effect of the elimination of fish with the more 

 rapid growth on the determination of the calculated 

 lengths of female yellow perch of group IV, collected 

 on Oct. 19, 1955 



[Age croup V is included for comparison] 



i Size at capture. 



three sets of calculated lengths (all for fish col- 

 lected Oct. 19, 1955) : Age group IV, entire 

 sample; age group IV, with the more rapidly 

 growing fish excluded; age group V. It is seen 

 that exclusion of the larger IV-group fish results 

 in a growth curve closely similar to that of the 

 full sample of age group V. 



From the previous discussion it is apparent 

 that various factors bias sampling and change 

 the growth characteristics of yellow perch in 

 Saginaw Bay but that it is not possible to rank 

 these factors as to their relative importance. The 

 factors doubtless operate together to bring about 

 these consistent discrepancies among the calcu- 

 lated lengths of the different age groups. 



General growth rate 



Two estimates of general growth are given in 

 the bottom section of tables 26 and 27. One is 



based on the grand average calculated lengths 

 and the second on the summation of the grand 

 average annual increments of length. The grand 

 average calculated lengths serve best to show the 

 regression of size on age in an exploited stock, 

 and the sum of the increments is believed to indi- 

 cate approximately the average growth that yel- 

 low perch might have if the stock were not sub- 

 jected to selective destruction of individuals with 

 the more rapid growth. The present discussion 

 is based on the sums of increments since this 

 curve is held to be the more descriptive of bio- 

 logical growth potential. The selection of these 

 data was mainly due to the discrepancies in cal- 

 culated length of the age groups. This view 

 agrees with that of earlier investigators who 

 made similar use of average annual increments 

 to show the general growth of yellow perch in 

 Saginaw Bay (Hile and Jobes, 1941), Green Bay 

 (Hile and Jobes, 1942) and Lake Erie (Jobes 

 1952). Comments on general growth and a com- 

 parison of the growth of the sexes are best made 

 from table 31 which was prepared from data of 

 tables 26 and 27 (see also fig. 13). 



The lengths of the sexes were closely similar 

 in the first and second years of life (a difference 

 of 0.1 inch). Then, the curves started to diverge 

 (with the females the longer). The advantage 

 of the females increased from 0.3 inch at the end 

 of the third year to 1.8 inches at the end of the 

 seventh (no males older than age-group VII in 



4 S 



YEAR OF LIFE 



Figure 13. — General growth in length and annual increments in length of Saginaw Bay yellow perch of the 1943-55 



spawning-run samples. Males, broken line; females, solid line. 



