YELLOW PERCH OF LAKE ERIE 



263 



rather closely with the empirical lengths of fish 

 shown by scale readings to have completed the 

 same number of years of life, and lengths calcu- 

 lated for the same year of life agreed more closely 

 with each other than with lengths computed for 

 any other year regardless of the age of the fish 

 employed in the calculations. 



4. The more important criteria employed to 

 determine the presence of an annulus were the 

 discontinuity between successive growth fields 

 which resulted in well-defined "cutting over" of 

 the cu-culi, particularl}' in the lateral region of the 

 scale, and the fragmented, u-regular appearance 

 of the last circulus laid down in each growing sea- 

 son. False annuli occui-rod but it is believed they 

 usually could be detected by the lack of cutting 

 over, their generally indefinite appearance, and 

 tlieir position with respect to true annuli. About 

 5 percent of the scales were discarded as unfit for 

 age determinations. 



5. Annulus formation maj^ be completed as 

 late as July 1 in some years. In spite of the ap- 

 parent coincidence of spawning and the completion 

 of the annulus in some years, the annulus cannot 

 be considered as a spawning mark since immatm-e 

 individuals form annuli identical in appearance 

 with those formed by mature fish, and the charac- 

 teristics of a typical spawning mark as found on 

 the scales of other fish are absent from yellow 

 perch scales. 



6. Detailed data are provided on the relation 

 between body length and scale length of the 

 American yellow perch. 



7. The Dahl-Lea method of calculating lengths 

 by direct proportion was applicable to the yellow 

 perch when the calculated standard lengths were 

 96 mm. (4.5 inches total length) or greater. When 

 these lengths were less than 4.5 inches they were 

 corrected by use of a table containing the corrected 

 length corresponding to each length computed 

 by direct proportion. These corrected calculated 

 lengths, derived from an empirical curve of the 

 body-scale relation, were always gi-eater than the 

 uncorrected lengths. Correction of the com- 

 puted lengths failed, however, to e'iminate the 

 discrepancies between corresponding lengths cal- 

 culated for different age groups. 



8. Discrepancies occurred between correspond- 

 ing calculated lengths in all years of life. The 

 computed lengths for any one year of life decreased 

 progressively as the fish for which the computa- 



tions were made became older. Discrepancies in 

 first-year calculated lengths were small among 

 age groups older than group I. 



9. The discrepancies in calculated lengths were 

 shown to represent real rather than "apparent" 

 differences in growth since large erroi"s could not 

 result from the method of calculation. 



10. It was concluded that the selective action of 

 gear, selection according to maturity at the time 

 of the spawning run, and selection according to 

 legal-size limit, all of which doubtless produced a 

 selective destruction of the more rapidlj' growing 

 individuals in the fishery, were the chief causes 

 of the discrepancies in the calculated growth of 

 the Lake Erie yellow perch, but that a differential 

 natural mortality, correlated with rate of growth, 

 was a possible supplementary factor. The presence 

 of discrepancies between corresponding calculated 

 lengths of different age groups of the same year 

 class proved that annual fluctuations in growth 

 rate were not an important source of the dis- 

 crepancies in calculated lengths. 



11. The females grew in length a little more 

 rapidly than the males during the first j-ear of 

 life, at the same rate in the second year, and 

 more rapidly in all later j^ears. 



12. The aimual increments of growth in length 

 decreased progressively with age in both sexes. 



13. Growth compensation occurred in the Lake 

 Erie yellow perch, but usually did not appear 

 before the third year of life. The difl'erence in 

 average length between the largest and smallest 

 yearlings was maintained or increased in the 

 second year. 



14. It was estimated that the proportions of 

 growth completed at the end of the different 

 months of the 1928 and 1929 seasons were 15 per- 

 cent for June, 50 percent for July, 80 percent for 

 August, and 100 percent for September. How- 

 ever, growth continued through October in 1927. 



15. Significant correlations could not be demon- 

 strated between armual fluctuations in growth 

 rate and precipitation, percentage of possible sun- 

 shine, and mean wind velocity. Significant posi- 

 tive correlations were determined, however, be- 

 tween growth and mean air temperatures for the 

 following combinations of mouths: May, July, 

 and September; May and September; July and 

 September. Mean air temperatures in August 

 exhibited sisrnificaiit negative correlation with 

 annual fluctuations in growtli rate. 



