220 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 5. — Average weights and calculated lengths of Lake Erie yellow perch taken in impounding nets in late fall, 1927-37 



combined 



[Number of specimens in parentlieses] 



Age group 



Average 

 weight at 

 capture 

 (ounces) 



Calculated length ' (inches) at end of year— 



Uncorrected Corrected 



Male: 



Age group I... 

 Age group II-- 

 Age group Ill- 

 Age group IV.. 



3.15 

 4.25 

 5.54 

 6.15 



Average' 



Annual increment - 



Female: 



Age group I.-- 

 Age group II-- 

 Age group III.. 

 Age group IV.. 

 Age group V... 



2.96 

 4.66 

 6.57 

 7.40 

 8.50 



Average' 



Aimual increment-. 



All fish: 1 



Age group I--- 

 Age group II-. 

 Age group III. 

 Age group IV.. 

 Age group V... 



3.02 

 4.38 

 5.99 

 6.86 

 8.60 



Average' 



Annual increment. 



3. 6 (266) 



3. (532) 



3. 1 (397) 

 2. 9 (45) 



3.0(974) 

 3.0 



3. 5 (37) 

 3. 1 (490) 

 3. 2 (355) 

 2. 9 (55) 

 2. 9 (5) 



3. 1 (905) 

 3.1 



3. 6 (392) 

 3. 1 (1, 636) 



3. (895) 

 2. 8 (108) 

 2. 9 (5) 



3. 1 (2, 644) 

 3.1 



4. (266) 



3. 6 (532) 



3. 7 (397) 

 3. 5 (45) 



3. 6 (974) 

 3.6 



3. 9 (37) 



3. 7 (490) 



3. 8 (355) 

 3. 5 (65) 

 3. 6 (5) 



3. 7 (905) 

 3.7 



4. (392) 

 3. 7 (1, 636) 

 3. 6 (896) 

 3. 5 (108) 

 3. 5 (5) 



3. 7 (2, 644) 

 3.7 



2 7. 5 (266) 

 6. 7 (632) 

 6. 5 (397) 

 6. 7 (45) 



6.6(974) 

 3.0 



7. 2 (36) 

 6. 9 (490) 

 6. 7 (355) 



5. 9 (65) 



6. (6) 



6. 7 (905) 

 3.0 



7.4 (371) 

 6.8 (1.636) 

 6. 6 (895) 

 6.8 (108) 

 6.0(5) 



6.7(2,644) 

 3.0 



8. 4 (394) 

 8.4 (397) 



7. 7 (45) 



8. 4 (836) 

 1.8 



9. 3 (209) 

 8. 9 (46) 



9. 4 (254) 

 1.0 



9. 6 (23) 



10. 1 (23) 



.7 



8. 7 (310) 

 8. 7 (355) 



7, 9 (55) 

 7.9(6) 



8. 6 (725) 

 1.9 



9.9 (192) 

 9. 3 (55) 

 9. 1 (6) 



9.8 (252) 

 1.2 



10. 2 (28) 

 10. (5) 



10. 7 (33) 

 .9 



10.6 (4) 



11.3(4) 

 .6 



8. 5 (750) 



8. 5 (895) 



7. 9 (108) 



7. 9 (5) 



8. 5 (1, 768) 

 1.8 



9. 5 (409) 

 9. 1 (108) 

 9. 1 (5) 



9. 6 (622) 

 1.0 



10.0 (61) 

 10. (6) 



10. 4 (56) 

 .9 



10. 6 (4) 



11.0 (4) 

 .6 



• The calculated lengths are based on all flsh without regard for time of 

 capture and usually include more specimens than used to determine the 

 length at capture. 



> The last length shown for each age group is the length at capture late in 

 the fall. 



In fish older than age group I the discrepancies 

 just described differ from "Lee's phenomenon of 

 apparent decrease in growth rate," as most com- 

 monly encountered, in that the wider disagree- 

 ments occurred among the computed lengths for 

 the later rather than the earlier years of life. It 

 seems probable that the factors that produced the 

 discrepancies in calculated lengths of the Lake 

 Erie perch began to be effective after the first year 

 of hfe had been passed. 



Since an intensive study of the body-scale rela- 

 tion of the Lake Erie yellow perch has eliminated 

 the possibility of large errors in computed lengths 



' Age group I is not included because of selected size. Beyond the third 

 year of life the average lengths were determined by successive addition of 

 the average aimual increments of the age groups for those years. 



* Includes fish for which the sex was not determined. 



resulting from the method of calculation, the ob- 

 served discrepancies in the calculated lengths must 

 be considered real rather than apparent. In other 

 words, the older fish in the samples actually grew 

 more slowly than the younger ones. The demon- 

 stration that the discrepancies in computed growth 

 were real, however, does not justify the conclusion 

 that the data are exactly descriptive of the growth 

 in the population from which the samples were 

 taken. Consideration must be given to the possi- 

 bility that the samples were not representative of 

 the population as a whole. 



DISCREPANCIES IN THE CALCULATED GROWTH HISTORIES OF DIFFERENT 



AGE GROUPS 



Two explanations of the discrepancies in com- 

 puted growth can be offered. It may be held that 

 the samples were not representative of the popula- 

 tion in the lake, and that data based on fuUy ade- 

 quate material would not have shown a decline in 

 the growth rate with an increase in age. Or it may 

 be held that the samples were satisfactorily repre- 

 sentative but that certain factors tended to bring 

 about the gradual elimination of the more rapidly 



growing individuals from the yeUow-perch popula- 

 tion, and that the recorded data therefore represent 

 a valid description of the growth of the Lake Erie 



perch. 



SELECTION BY GEAR 



The selective action of impounding nets in tak- 

 ing samples depends on the escape of small fish 

 through the meshes. A rough approximation of the 

 maximum size of escape may be obtained by deter- 



