YELLOW PERCH OF LAKE ERIE 



215 



Annuhis formation appears to be completed be- 

 tween early April (1932 and 1937 collections) and 

 the middle of July (1929 collection). There is no 

 evidence from these data to show a relation be- 

 tween the time of annulus formation and sex, 

 maturity, or spa^^^ling activity. The annulus on 

 yellow-perch scales cannot be said to be a spawn- 

 ing mark despite the approximate coincidence of 

 spawning and the completion of the annulus 

 because (1) immature yellow perch form annuli 

 identical in appearance with those formed by 

 spawning fish, (2) the stage of sexual maturity- 

 appears to have no influence on the time of year 

 the annulus is completed, and (3) the annuli do 

 not show the typical spa\vning marks observed 

 in other species of fish. 



The most important characteristics of the annuli 

 on the scales of the Lake Erie yellow perch may 

 be stated briefly to be the "cutting over" in the 

 lateral fields resulting from the discontinuity 

 between scale sculpturing of the successive growth 

 areas, and the irregular or fragmented appearance 

 of the last circulus laid down each year. Usually 

 there is a narrow, clear band between the outer- 

 most circulus of one growth area and the first 

 circulus of the next. 



False (accessor}') annuli occurred not infre- 

 quently on the yellow-perch scales but are be- 

 heved not to have affected the results seriously 

 since all that were recognized were disregarded. 

 Those annuli designated as false were character- 

 ized by a decreased amoimt of "cutting over," 

 by less-well-defined discontinuity between the 

 adjacent fields of growth, and, frequently, by a 

 position that would have given inconsistent calcu- 

 lated lengths. 



BODY-SCALE RELATION 



Few calculated lengths for the American yellow 

 perch have been published. The earliest, by 

 Jobes (1933) and Schneberger (1935), were com- 

 puted by the Dalil-Lea method of direct propor- 

 tion. This method is based on the assumption 

 that the ratio of body length to scale length is 

 constant at all lengths beyond that at which the 

 first year mark or annulus is formed. The age and 

 growth of the closely related European perch, 

 Perca fiumatilis L., have been studied by this 

 method by several investigators who found that the 

 lengths calculated by direct proportion usually 



were less than the empirical lengths for the early 

 years of life. 



In spite of the wide use of the direct-proportion 

 method, numerous investigations have shown that 

 this method frequently failed to give satisfactorily 

 accurate results since the computed lengths 

 obtained often did not agree with empirical 

 lengths. Of the several methods developed to 

 obtain a closer agreement between calculated and 

 empirical lengths only that of SegerstrMe (1933) 

 for the European yellow perch will be mentioned 

 here, since the calculation of lengths in the present 

 study was by a modification of his procedure. 



Segerstrale determined the average scale lengths 

 corresponding to different body lengths through an 

 extensive series of measurements of "key" scales, 

 or "Normalschuppen," taken from a selected area 

 of the body. The body-scale relation so deter- 

 mined, expressed either in tabular form or as a 

 curve, served as the basis for calculating the 

 growth histories of individual fish. On purely 

 theoretical grounds, the method of Segerstr&le is 

 the best since it assumes no fixed mathematical 

 relation between body length and scale length, but 

 rather is based on the detailed examination of the 

 actual size of scale at different body lengths. 

 The most serious objection to the use of an 

 empirically determined relation of body length to 

 scale length in the calculation of growth histories 

 is the practical difficulty of obtaining samples with 

 adequate representation of all lengths of fish. 

 The distribution by length of a fish population 

 usually is such that individuals of certain sizes are 

 difficult or impossible to obtain. Inadequate 

 representation of these length intervals inevitably 

 leads to inaccuracies in the calcidated lengths. 



The diversity of opinions expressed and of 

 results obtained by the several investigators deal- 

 ing with presumably representative collections of 

 the same and different species leads to the con- 

 clusion that the relation of body length to scale 

 length in fishes is not a subject for generalization. 

 The proper method of calculation must be deter- 

 mined for the material at hand. Data on the 

 yellow perch from Lake Erie made possible an 

 anah-sis, for the first time,- of the relation of body 

 length to scale length in a population of American 

 yellow perch. 



2 Although circumstances prevented earlier publication of this study, Hile 

 and Jobes (1941) were able to apply the method developed here to the determi- 

 nation of the body-scale relation of the yellow perch of Saginaw Bay. 



