YELLOW PERCH IN SAGINAW BAY 



377 



1939 1940 1941 1942 1943 1944 1946 1946 1947 1948 1949 1950 



YEAR CLASS 

 Figure 8.— Relation between year-class strength (solid line) and the abundance after 4 and 5 years (dashes) and 



production (dots and dashes) 4, 5, and 6 years later. 



reach legal size in less than 4 years and fish 

 older than the VI group are rare. 



Except for the 6-year interval all coefficients of 

 correlation between year-class strength and abun- 

 dance as estimated from records of catch per unit 

 effort were positive, but all of them were far 

 below significant values. Correlations between 

 year-class strength and later production, on the 

 other hand, were all significant, three of them at 

 the 5-percent and 2 at the 1 -percent level. 



Discussion of possible factors of fluctuation in 

 year-class strength is given in a later section 

 along with the treatment of factors of fluctua- 

 tions in growth rate. 



SIZE AT CAPTURE 



Length Distribution of Samples 



The length-frequency distribution of Saginaw 

 Bay yellow perch during the 1943-55 period 

 (table 14) was typically unimodal. The 1945 

 collection showed some tendency toward bimo- 

 dality but this might be due to the accidental 

 capture of some large fish and to the small num- 

 ber in the sample (only 97 fish). With the ex- 

 ception of 1943 and 1945, the modal lengths lay 

 within the range of 6.0-6.9 inches total length. 

 Most common was at 6.5—6.9 inches. In 2 years 



489035 O— 59 3 



only (1947 and 1955), the modal lengths were 

 at 6.0-6.4 inches. In 1943 and 1945, on the other 

 hand, the modes were 7.5-7.9 and 8.0-8.4 inches. 

 This annual variation in the length may be at- 

 tributed largely to fluctuations in the age com- 

 position of the stock and to the change of growth 

 from year to year (these fluctuations are dis- 

 cussed in earlier and later sections). Differences 

 from year to year in the percentage of males and 

 females also had a certain effect on this variation. 

 Jobes (1952) included the selectivity of the dif- 

 ferent kinds of nets among the causes of annual 

 variation of length. The effect of gear selectivity 

 was minimized in the present study because all 

 samples came from one kind of net (trap net). 

 The percentage of legal-sized yellow perch 

 (total length 8i/ 2 inches or longer) of the com- 

 bined samples was 11.0. This percentage varied 

 considerably, however, from year to year. The 

 highest value was recorded in 1945 (66 percent). 

 In 1948 the sample included no legal-sized fish. 

 Although it is not to be concluded that the legal- 

 sized perch were unavailable in Saginaw Bay 

 during that year, it is obvious that in the early 

 season the fisherman must have had to do tedious 

 sorting to find a marketable catch. 



