376 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



To obtain the relative positions of all year 

 classes, the 1939 year class is given arbitrarily 

 a value of 0.0 and the positions of succeeding 

 year classes are determined by the successive ad- 

 dition of the "percentage" differences in the 

 right hand column of table 11. The series so 

 obtained is then adjusted to a mean of 0.0 to 

 give the final ranking (table 12; fig. 7). 



The year-class strength dropped from 147.4 in 

 1939 (highest recorded value) to -123.1 in 1941 

 (lowest value). The position or index value in- 

 creased until it reached 18.9 in 1943 and dropped 

 to -69.5 in 1945. From 1945 to 1952 the trend 

 #as strong toward improvement in year-class 

 strength (only exception in 1950). The indices 

 remained below average until 1947. From 1948 

 to 1952 the year-class strength was above average, 

 reaching a value of 55.9 in 1952 (second highest 

 value) . 



If the fluctuations of year-class strength are 

 substantial, it is to be expected that their effects 



will be felt in later years in the abundance of 

 fish and in the production of the commercial 

 fishery. To test this point for Saginaw Bay 

 yellow perch, coefficients of correlation r were 

 computed between year-class strength and the 

 abundance and production (see section on the 

 commercial fishery) 4 to 6 years later (table 13; 

 see also fig. 8). These intervals and combina- 

 tions of intervals were chosen because few perch 



Table 13. — Correlation between estimated strength of year 

 classes and later abundance and production of yellow perch 

 in the commercial fishery 



Table 12. — Index values of year-class strength of Saginaw Bay yellow perch, 1939-52 



(Adjusted to a 1939-52 mean of 0.0] 



150 



IOO 



x 



u 



Q 



Z 



V) 



< 



rr 



< 



UJ 



>- 



-100- 



-150 



1939 1940 1941 1942 1943 1944 I94S 1940 1947 1948 1949 I960 1991 1902 



YEAR CLASS 



Figure 7.— Fluctuation in the relative strength of year classes 1939 to 1952 of Saginaw Bay yellow perch. 



