224 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 7. — Average calculated length, by age, of Lake Erie 

 yellow perch taken by impounding nets 



Year of life 



Males; 



1 year. 



2 year. 



3 year. 



4 year. 



5 year. 



6 year. 



Females: 



1 year. 



2 year. 



3 year. 



4 year. 

 6 year. 

 6 year. 



All fish: 



1 year. 



2 year. 



3 year. 



4 year. 



5 year. 



6 year. 



Standard 

 length 



Milli- 

 meters 



76 

 143 

 181 

 203 

 220 



77 

 146 

 187 

 216 

 234 

 248 



77 

 146 

 184 

 208 

 228 

 242 



The use of the average annual increments caused 

 the lengths of the fish in the later years of life to 

 be higher than the corresponding average calcu- 

 lated lengths as determined from the individual 

 age gi'oups (table 5). For example, as derived 

 from the general growth cm-ve, the length of the 

 females at the end of the fifth year was 10.7 inches 

 as compared to the values of 10.2 (length at time 

 of capture in the autumn) and 10 inches as deter- 

 mined from age-gi-oups IV and V, respectively. 

 Similarly, this length was 10.1 inches as derived 

 from the growth curve of the males but was only 

 9.6 inches at time of captm-e in the autumn. Dis- 

 crepancies occurred also in the lengths at the end 

 of the fourth and sixth years of life. Although 

 the successive additions of the average annual 

 growth increments, to determine the general 

 growth curve in the later years, introduce dis- 

 crepancies, they cannot be held with certainty to 

 represent errors in the general growth curve. On 

 the contrary, the use of the average annual 

 increments may tend to offset the distorting 

 effects of the differential destruction of the more 

 rapidly growing individuals; hence the seemingly 

 greater lengths of the general growth curve may 

 approximate the true typical growth of the Lake 

 Erie yeUow perch more closely than a cm-ve based 

 entirely on grand-average calculated lengths. 



Figure 4 is a graphic presentation of the data of 

 table 7 on length at the end of each year of life 

 and the annual growth increment. At the end of 



the first year of life the females were slightly larger 

 (0.1 inch) than the males, and they maintained this 

 advantage in length during the second year. Be- 

 gimiing in the third year the females increased in 

 length progressively faster than males of the same 

 age until at the end of the fifth year they were 0.6 

 inch longer. 



Of particular interest is the fact that the 

 minimum legal size of 8)^ inches, effective in the 

 States of Michigan and Ohio, was reached at the 

 end of the thu-d year by all fish (sexes combined). 

 It may be seen also (table 5) that the average 

 length of the males captured late in the faU at 

 the end of their third year was only slightly less 

 (8.4 inches) and that of the females only a little 

 more (8.7 inches) than the legal minimum. The 

 maxim lun length of yellow perch examined in Lake 

 Erie was 13.9 inches total length, sex not deter- 

 mined. The longest male was 11.3 inches and 

 the longest female 12.9. 



No physiological explanation can be offered for 

 the difference in the growth of the sexes. It is 

 rather certain, however, that the earlier attain- 

 ment of sexual maturity by the males was not the 

 primary cause of their poorer growth. The 

 females enjoyed the gi-eatest actual and relative ad- 

 vantage in growth in the fourth year of life (fe- 

 males 1.2 inches, males 1 inch). Yet at that size 

 86.1 percent of the females and 98.6 percent of the 

 males were matm-e (see table 36). If the poor 

 growth of the males was the result of their early 

 attainment of matiu-ity, the greatest advantage in 

 the growth of the females would be expected to 

 occur in the second year of life when 57.8 percent 

 of the males and none of the females were mature. 



In spite of the differences in the growth of the 

 sexes, the same general description of the course 

 of growth applies to the cm-ves for the females, 

 males, and the sexes combined. The most rapid 

 growth in length took place in the first year of life, 

 after which the annual increments decreased 

 continuously'. 



GROWTH IN WEIGHT 



The average weights of the age groups of yellow 

 perch taken late each fall (table 5) bring out clearly 

 that the females were heavier at each age than 

 the males with the exception of those fish assigned 

 to age group II in which the males also were the 

 longer. The best-represented age group (II), 

 which characteristically dominates the late-season 



