400 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Figure 20. — Average calculated weights 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. 



Probable Factors of the Decrease in Growth Fate 



The extensive decrease in the growth rate of 

 the Saginaw Bay yellow perch as indicated by 

 the records for the 1929-30 and 1943-55 samples 

 offers convincing evidence that the species is 

 living now in a greatly changed situation. Many 

 factors might contribute to this change: Limno- 

 logical conditions including production of food; 

 pollution; meteorological conditions; changes 

 within populations of associated species and of 

 the yellow perch itself. 



Limnological factors are omitted in this study 

 because of lack of data. The first limnological 

 observations on Saginaw Bay of consequence 

 were made in 1956. 



With pollution also we are in poor position to 

 offer quantitative estimates of differences between 

 the periods. Even if we had full knowledge of 

 sources, kinds, and quantities of pollutants, in- 

 terpretation would be difficult because of the wide 

 variety of substances and their equally varied 

 and largely unknown effects on the several spe- 



cies of fish and fish-food organisms. (For a sur- 

 vey of conditions in 1935 and 1936, see the 

 Saginaw Valley Report, Adams 1937.) Al- 

 though changes in the pollution situation in 

 Saginaw Bay cannot be described quantitatively, 

 it appears most probable that conditions in re- 

 cent years are less adverse than formerly. Con- 

 struction of sewage-disposal plants in cities and 

 great advances in the treatment of industrial 

 wastes surely have decreased the total load. Evi- 

 dence of lessening of industrial pollution comes 

 from the decrease in complaints about tainted 

 fish. 



Although certain meteorological conditions ap- 

 pear to show a degree of correlation with limited 

 annual fluctuations in growth rate (p. 101) there 

 is no evidence for a major climatic change of 

 the proportions that would have to be postulated 

 if the large drop in growth rate were to be 

 attributed to weather. 



The changes within the populations of fish in 

 Saginaw Bay are somewhat better understood, 



