YELLOW PERCH IN SAGINAW BAY 



399 



I4i- 



12 



10 



UJ 



I 



o 

 ? 8 



13 



bJ 6 



2 



o 



/ 



^ 



// 



3 4 



YEAR OF LIFE 



Figure 19. — Average calculated lengths at the end of each year of life for yellow perch from different Great Lakes 

 waters (sexes combined). Lake Erie, short dashes; Green Bay, dots and dashes; northern Lake Michigan, two 

 dots and dashes ; Saginaw Bay, 1929-30, long dashes ; Saginaw Bay, 1943-55, solid line. 



forcefully by the comparison of calculated 

 weights for the various samples (table 44, fig. 

 20). The growth of the earlier collection of 

 Saginaw Bay perch, for weight as well as length, 



Table 44. — Growth in weight of yellow perch from different 

 localities of the Great Lakes 



[Sources of data: Lake Erie. Jobes (1952); southern Green Bay and northern 

 Lake Michigan, Hile and Jobes (1942); Saginaw Bay 1929-30 samples, Hile 

 and Jobes (1941); Saginaw Bay 1943-55 samples, present study] 



ranked second or first among the Great Lakes 

 stocks. It was surpassed by the Lake Erie perch 

 during the first 4 years of life, but in the fifth 

 and later years (no data for Lake Erie beyond 

 the fifth year), Saginaw Bay perch were the 

 heaviest for their age of all perch populations. 

 In the 1943-55 period, the situation was com- 

 pletely reversed; the growth of Saginaw Bay 

 perch was inferior to that of all other Great 

 Lakes perch populations. This sharp drop in the 

 growth in weight affected all years of life. The 

 ratios of calculated weights of 1929-30 fish to 

 those of perch collected in 1943-55 follow: 



> Unweighted means. 



2 No data for sexes separately. 



From these ratios, it is apparent that in most 

 years Saginaw Bay yellow perch caught in 

 1929-30 were more than twice as heavy as those 

 in the 1943-55 samples. 



