214 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



the gelatin-glycerin medium recommended by 

 Van Oosten (1929). 



The first examinations of the 1927 and of part 

 of the 1928 scale collections were made by use of 

 the projection apparatus described by Van 

 Oosten (1923). The final examinations and the 

 measurements of those scales, and all examinations 

 and the measurements of the remaining scales, 

 were made by means of the apparatus described 

 by Van Oosten, Deason, and Jobes (1934) at 

 magnification X 40.7. The measurements from 

 the focus to each successive annulus and to the 

 extreme edge of the scale were made along the 

 most anterior interradial space. Approximately 

 5 percent of the scales examined were discarded 



because the age could not be determined with 

 confidence. 



Ages are designated in roman numerals corre- 

 sponding to the number of annuli visible on the 

 scales except for those fish taken in the early 

 spring before growth began. An amiulus was 

 assiuned to be present on the edge of the early- 

 spring scales. Such an assumption is necessary 

 to avoid the assignment of one age to fish of a 

 year class whose scales were without spring 

 growth and another age to fish of the same year 

 class collected the same day whose scales had 

 begmi the current season's growth. Thus fish 

 assigned to age group I were in or just ready to 

 begin their second year of life. 



DETERMINATION OF AGE AND GROWTH OF LAKE ERIE YELLOW PERCH BY THE 



SCALE METHOD 



Since the demonstration by Hoffbauer (1898) 

 that the age of carp could be determined by 

 examination of their scales, the scales of fish 

 have been used extensively for the study of growth 

 rates and age composition of the stocks of many 

 marine and fresh-water species. Historical sum- 

 maries of the literature and discussions of the 

 validity of age determinations from scales and of 

 growth calculations from scale measurements 

 already published make it unnecessary to say 

 more here than that the major part of the evidence, 

 obtained from a wide range of species, sub- 

 stantiates the general premise that age can be 

 determined accurately from scale markings and 

 that measurements of fields of growth in scales 

 can be employed for the calculation of lengths 

 at the end of the dift'erent years of life. 



VALIDITY OF THE ANNULUS AS A YEAR MARK 



It has been assumed by the several workers 

 that ages may be determined accurately from 

 an examination of the scales of the American 

 yellow perch, since these scales showed clearly 

 the characteristics that had been used in the 

 accurate determination of the ages of certain 

 other species. The data, and observations of the 

 present study, given in the following paragraphs, 

 substantiate this assumption. 



1. The collections of 1927, 1928, and 1929 were 

 dominated by fish whose scales showed 1, 2, and 

 3 annuli, respectively. The corresponding aver- 

 age total lengths of the age groups were 7.5, 8.3, 



and 9.5 inches. (Most of the 1928 collections 

 were not made as late in the autumn as were the 

 collections of 1927 and 1929; consequently, the 

 average length of 8.3 inches does not represent 3 

 full years of growth.) That the catches of suc- 

 ceeding years were dominated by progressively 

 larger individuals which, in accordance with ex- 

 pectation, were shown by scale readings also to be 

 progressively older, is strong evidence that on(> 

 annulus is formed each year and that the scale 

 markings can be interpreted accurately for at 

 least the first three years of life. 



2. Scales collected on December 7, 1929, 

 showed no annulus on the edge. Samples obtained 

 July 1, 1929, April 11 and 13, 1932, and April 29, 

 1937, showed an aim.ulus forming on the edge of 

 the scales. On July 11, 1930, the scales showed a 

 completed annulus a short distance inside the 

 margin. The outermost annulus was farther from 

 the scale margin on September 25, 1930, than in 

 July. These observed variations, especially those 

 on the relative positions of the annulus within 

 the scale margin at different times during the 

 same year (1930), provide evidence that only one 

 annulus was formed on yellow-perch scales each 

 year. 



3. There was closer agreement between the 

 calculated and empirical lengths of fish of the 

 same age as determined from scales than between 

 those of different ages. This agreement indicates 

 a constancy in the number of annuli formed each 

 vear. 



