YELLOW PERCH IN SAGINAW BAY 



387 



discrepancies ranged from 1 to 6 mm. and aver- 

 aged 3.6 mm. There is no information now from 

 which to determine the cause of this bias. For- 

 tunately, the discrepancies affect principally a 

 single year of life and are not excessive. 



Implications as to procedure in the calculation of 

 growth 



The differences in body-scale relation between 

 stocks and between two scale positions on the 

 body of the same fish emphasize the importance 

 of an exact knowledge of the body-scale relation 



Table 25. — Calculated standard length (millimeters) of the 

 same yellow perch from key scales above and below the 

 lateral line 



1 Size of fish at capture. 



of the population under study and a high degree 

 of consistency in the field as to the point from 

 which scale samples are removed. Not only must 

 the body-scale relation be known for a particular 

 key area; routine samples of scales must be taken 

 from that area. 



Even with these precautions, precision may not 

 be so great as would be desired. The differences 

 between the body-scale curves derived for Sagi- 

 naw Bay perch by Hile and Jobes (1941) and 

 in the present study, and discrepancies between 

 lengths calculated for the same fish from meas- 

 urement of scales above and below the lateral 

 line indicate a certain amount of variability that 

 cannot yet be explained. 



Growth in Length 

 Growth in length of the age groups 



In the presentation of average calculated 

 lengths for yellow perch collected from Saginaw 

 Bay during the spawning season of 1943—55 

 (tables 26 and 27) sexes are kept separate because 

 of the more rapid growth of the females. Data 

 for the calculated growth histories of age groups 

 from different years are combined to give a best 

 estimate of average conditions. 



The calculated lengths of the males and females 

 through age group VI for the different years 

 of life show a definite tendency to decrease as the 

 fish grow older. This discrepancy in calculated 

 length is more pronounced in the later years of 

 life, particularly after the second year. Among 

 the first-year calculated lengths the values for 

 different age groups beyond the II group are 

 nearly the same. 



Table 26. — Calculated total length at the end of the different years of life for male yellow perch collected during the spawning 



seasons of 1943-55 



[The figures in this table were rounded from original records carried to the nearest 0.01 inch, hence the discrepancy between the 5th-year grand average increment 

 and the 4th- and 5th-year lengths derived from summation of the increments. Increments in parentheses] 



