YELLOW PERCH IN SAGINAW BAY 



375 



class appeared in the collections at various ages 

 contributing to the samples in different calendar 

 years as follows: 1946, III group, 24.5 percent; 

 1947, IV group, 70.4 percent; 1948, V group, 51.0 

 percent; 1949, VI group, 9.9 percent; 1950, VII 

 group, 0.9 percent. The sum of the percentages 

 is 156.7. The 1944 year class appeared as fol- 

 lows: 1947, III group, 19.6 percent; 1948, IV 

 group, 38.5 percent; 1949, V group, 38.5 percent; 

 1950, VI group, 5.0 percent; 1951, VII group, 

 0.0 percent; sum of percentages, 101.5. 



The 2 sums of percentages contributed to 



annual samples by the 2 year classes under com- 

 parable sampling conditions are taken as repre- 

 sentative of their relative strengths (table 11). 

 The change of year-class strength from 1943 to 

 1944 is computed as 



2 (101.5-156.7) 

 101.5 + 156.7 



X 100 = -42.8 



(The use of the mean of the two percentage 

 totals rather than the total for the earlier year 

 as a base for the estimation of the change is 

 essential if systematic bias is to be avoided. See 

 Hile 1941, p. 253, footnote 23.) 



Table 11. — Data employed in the determination of year-class fluctuation of Saginaw Bay yellow perch 



