ABSTRACT 



The average annual commercial production of Saginaw Bay yellow perch dropped from 

 1,961,309 pounds in 1891-1916 to 499,938 pounds in 1917-55. Since 1938 (1939-55), the 

 catch has exceeded 500,000 pounds in only 3 years. The small catches of 1939-55 do not 

 reflect scarcity of yellow perch. The catch of legal-sized fish per unit-of-effort has tended 

 to increase, but fishing intensity has decreased sharply. 



This paper is written around two major themes : Comparison of perch collected in 

 1929-30 with specimens collected in 1943-55, as to growth rate, age composition, size dis- 

 tribution, length-weight relation, and sex ratio ; description and inquiry into the causes of 

 fluctuations in the strength of the year classes of 1939-52, and of the growth rate during 

 1942-54. 



The growth in length and weight of Saginaw Bay yellow perch in 1943-55 was the 

 slowest yet reported from any Great Lakes waters. The decrease in growth rate in Saginaw 

 Bay was believed to have resulted from a more than sevenfold increase in the population 

 density. A "space factor" rather than competition for food may account for the decline 

 in growth rate. Fish of the 1943-55 samples gave no evidence of a scarcity of food ; on the 

 contrary, they were heavier for their length than fish caught in 1929-30. 



The weight of yellow perch in the 1943-55 samples increased as the 3.262 power of 

 the length. Seasonal changes in the length-weight relation were small. Females lost 

 12.3 percent of their weight at spawning. 



Age determination and growth calculation were based on the scales of 4,285 fish, 3,407 

 of them collected during the spawning seasons of 1943-55 and the remainder at other 

 seasons in 1955. The average age of fish in impounding-net samples collected in the spring 

 increased between 1929-30 (3.8 years) and 1943-55 (4.3 years), and growth declined 

 sharply. Saginaw Bay yellow perch of the 1929-30 samples reached legal length (8% 

 inches) in 3 years but those taken in 1943-55 required more than 5 years to attain the same 

 size. At the same time the modal length dropped from 8.5-8.9 to 6.5-6.9 inches and the 

 percentage of legal-sized fish from 74 to 11 percent. In both periods, the females averaged 

 larger than the males and grew more rapidly. 



Males were relatively more plentiful in 1943-55 (62 percent) than in 1929-30 (25 per- 

 cent). The percentage of males decreased with increase of age in 1929-30 but increased 

 in 1943-55. Both males and females attained sexual maturity at a small size (nearly all 

 males were mature at 5.0-7.5 inches; 80 percent of females at 7.0-7.4 inches). 



The strongest year classes were those of 1939 and 1952. The weakest were those of 

 1941 and 1945. Tear-class strength was correlated significantly with production 4, 5, and 

 6 years later, but it was not correlated with the abundance of legal-sized fish in the year 

 of hatching or with temperature, precipitation, water level, and turbidity. 



The annual fluctuation of growth (length) in the first and in later years of life were 

 dissimilar. First-year growth was poorest in 1942 but tended strongly to improve in 

 subsequent years. First-year growth was correlated negatively with turbidity in June 

 but was not correlated with year-class strength or other factors investigated. Fluctuations 

 of growth in later years of life were largely without trend. Growth in these years was 

 not correlated with the abundance of legal-sized fish, temperature, precipitation, or turbidity, 

 but varied inversely with the water level for May to October. 



IV 



