186 BULLETIN OF THE BUREAU OF FISHERIES 



obviously in the O year class and not having reached one winter), 554 fish were 

 examined for age by the scale method. From 4 to 6 individual, unselected scales 

 from each of the 554 fish were mounted on a glass slide, the smooth side of the scales 

 being moistened slightly with a mixture of mucilage and glycerine to cause adhesion 

 to the glass. In all cases the scales were taken from the upper forward left side of 

 the fish. The actual counting of the annuli and measurement of the scale with its 

 annual checks was done by two distinct methods of magnification and measure- 

 ment. Thus, a rehable check was made upon each method employed, as well as 

 on the personal judgment of the writer. It should be stated, however, that knowl- 

 edge of the Ufe of the species of fish, particularly the time of spawning, assists greatly 

 in imderstanding many of the annual checks that might appear confusing to one 

 who knows and sees nothing of the fish save several scales, abstractly presented. 



The first method used consisted in examining all scales through a binocular 

 microscope at various magnifications and in selecting one typical scale for measure- 

 ment by means of an ocular micrometer at a standard magnification of about SOX . 

 Upon examination many scales were found to be useless for accurate age determina- 

 tion, some having broken edges, regenerated centers, or other abnormalities. A 

 scale was judged "typical" by the ease with which it could be read and the support 

 obtained from comparison with the other scales on the slide. 



The results of the first method, while appearing reasonable, did not seem to be 

 as accurate as would have been the case if a more refined measurement of the scale 

 and its checks had been made. Two hundred of the original 554 fish had to be 

 omitted from the calculations, either because of the imperfections of the scales or the 

 inability of the writer to distinguish the annuli. 



The second method, conducted independent of the results obtained from the 

 first method, gave more accurate readings and measurements, in the writer's opinion, 

 although a comparison of the two showed no serious differences. By means of a 

 projection apparatus the image of the scale was projected on a white wall, and each 

 scale annulus was counted and measured at a magnification of about lOOX. The 

 various scale and annuli lengths were recorded on adding-machine tape, and the 

 distances were measured later with a millimeter rule. In many cases two scales 

 from the same fish were read and measured for comparison. (See Table 22.) In 

 both methods of scale measurement the distances usually were measured from the 

 center of the focus, along the radius nearest the periphery of the scale. 



Of the 554 trout, 452 were read for age and measured for annual growth by 

 the projection method. Of the 452 individuals, 56 were discarded because of apparent 

 discrepancies in the calculated measurements; and while the inclusion of these 

 doubtful 56 fish did not affect the final results appreciably, it was believed desirable 

 to eliminate obvious errors. In the final calculations 396 trout were analyzed by 

 the second method, while 354 were analyzed by the first. The fact that about 283^ 

 per cent of the original nimiber of fish scales were omitted because the writer was 

 unable to read them can hardly be considered serious, since it was thought better 

 to omit questionable scales rather than obtain doubtful calculations. 



The spotted-trout scales were collected from April, 1926, to June, 1927. Con- 

 sequently, the various age classes of fish passed through one observed winter's growth 

 check, The fish taken from AprU, 1926, to March 1, 1927, show one less winter 



