YELLOW PERCH IN SAGINAW BAY 



411 



were almost all males. Beyond age group VI, 

 the change in the sex ratio could not be con- 

 sidered as descriptive because of the small num- 

 ber of fish. In the 1943 collection, the females 

 were more plentiful in age group V-VII than 

 in age groups III-IV and in the 1945 data and 

 the June 22, 1955, sample no trend is apparent. 

 For all spawning-run collections combined (bot- 

 tom of table 55), the percentage of males in- 

 creased continuously from 17 percent in age 

 group II to 77 percent in age group VI and then 

 dropped slightly to 65 in the VII group. The 

 4 older fish were all females. 



It is difficult to judge the degree to which the 

 trends of tables 55 and 56 are actually descriptive 

 of the sex ratio of the Saginaw Bay stock of 

 yellow perch. Samples of perch from commer- 

 cial gear are biased, and this bias, in turn, leads 

 to differential destruction in the fishery. Be- 

 cause of sex differences of growth rate (more 

 rapid growth of females), the younger males are 

 much less easily captured than are females. Even 

 before fish of either sex attain legal size, this 

 differential rate of capture leads to a greater 

 mortality of females since numbers of undersized 

 fish are killed in the handling and sorting of the 

 catch. As the fish reach legal size this differen- 

 tial destruction is intensified, since females grow 

 to 8V2 inches in about 4i/ 2 years, as compared 

 with 6-years for males. 



SUMMARY 



(1) The yellow perch is an important fish for 

 both commercial fishermen and anglers because 

 of its wide distribution and its frequent great 

 abundance. 



(2) In Saginaw Bay, the average commercial 

 catch of yellow perch has decreased from 1,961,- 

 309 pounds in 1891-1916 to 499,938 pounds in 

 1917-55. Since 1938 the commercial production 

 of yellow perch has been below 500,000 pounds 

 except in 1943, 1944, and 1945. Statistics of the 

 commercial fishery prove that the recent low out- 

 put has resulted from reduced fishing intensity, 

 not from a scarcity of fish. 



(3) The trap net is the principal gear for 

 catching yellow perch in Saginaw Bay (75.9 per- 

 cent of the catch ) . Production is concentrated in 

 the fall (75 percent of the catch in September, 

 October, and November). 



(4) The present study of age and growth was 

 based on the determination of age and the calcu- 



lation of growth histories of 4,285 fish, 3,407 of 

 them collected during the spawning seasons of 

 1943-55 (no collections, 1945 and 1952). In 1955 

 additional collections were made outside the 

 spawning periods. 



(5) In the combined 1943-55 spawning-run 

 collections, age group IV contributed 48.6 per- 

 cent of the total, followed by age group V (29.9 

 percent) and age group III (15.9 percent). The 

 remaining age groups (II and VI— IX) together 

 contributed only 5.6 percent. The mean of the 

 average ages for the 1943-55 spawning-run sam- 

 ples was 4.3 years. 



(6) In 1955 the age composition changed sea- 

 sonally. Average ages on various dates were: 

 April 18, 4.8 years; May 18, 4.4 years; June 7, 

 3.8 years; June 22, 4.3 years; October 19, 3.7 

 years. 



(7) Saginaw Bay yellow perch collected in 

 1943-55 averaged older than fish from the same 

 Bay collected in 1929-30 and from other Great 

 Lakes stocks (southern Green Bay, northern 

 Lake Michigan, and Lake Erie). 



(8) The estimation of year-class strength for 

 1939-52 was based on a series of comparisons of 

 the percentage representation at various age 

 groups. The strongest year classes were those 

 of 1939 and 1952. The weakest were those of 

 1941 and 1945. The estimated year-class strength 

 was correlated significantly with production 4, 

 5, and 6 years later, but the corresponding corre- 

 lation with later commercial availability was not 

 significant. 



(9) It was not possible to establish a relation 

 between year-class strength and the abundance of 

 legal-sized fish in the year of hatching or with 

 temperature, precipitation, water level, and tur- 

 bidity. 



(10) Length frequencies of the spawning-run 

 samples were typically unimodal. The modal 

 lengths (V2-inch intervals) fluctuated from year 

 to year but mostly lay within the range 6.0-6.9 

 inches, total length. The length-frequency dis- 

 tribution also varied seasonally in 1955. From 

 April 18 to October 19 the modal intervals ranged 

 from 5.5-5.9 inches (June 7) to 7.5-7.9 inches 

 (October 19). 



(11) The length distributions of successive age 

 groups overlapped extensively. Fish of a par- 

 ticular length might belong to 2 to 5 age groups 

 (mostly 4 age groups). The overlap made the 

 length of Saginaw Bay perch a poor index of age. 



