334 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



onterior 

 margin 



. annulus iv 



annulus in 

 annulus n 



focus 



ventral 

 margin 



posterior 

 margin 



Figure 20. — Scale of a 4-year-old hybrid trout (total length 12.7 ins.) captured July 13, 1954, showing terminology used 

 in identifying various regions. Fish did not have scales at the start of its second summer and annulus I is missing. 



k was 2.07 and in 1954, 2.40. The procedure of 

 averaging scale measurements follows that sug- 

 gested by Van Oosten (1953). 



Measurements of grayling scales (magnification 

 41.5) were made from the estimated center of the 

 focus to a ventral scale margin (expressed as if in 

 situ on the fish). Back calculations were made 

 from scale averages using the above formula and a 

 correction factor (k) of 0.86 in 1953 and 1.14 in 

 1954. 



Problems in Age Assessment of Grebe Lake Grayling 



Definition and validation of the annulus 



The initial problem in age assessment of gray- 

 ling scales is the definition and validation of the 

 annulus. The interpretation of all of the marks on 

 the scales of grayling collected during 1952, 1953, 

 and 1954 from Grebe Lake was not clear even 

 after a detailed analysis. The following information 

 was developed during the study. 



The first and second annuli on grayling scales 

 were formed when growth resumed or accelerated 

 in the spring (figs. 21, 22). A clear hyaline border 

 first formed around the scale and was followed by 

 a shadowy circulus which started near the antero- 

 lateral border and developed both in the anterior 

 field and caudally until it was continuous around 

 the margin. This year mark exhibited strong 

 "cutting-over" of the outermost incomplete circuli 

 in the posterolateral corners of the scale. 



Annuli later than the first two were preceded 

 in the anterolateral corners by a clear hyaline 

 area and usually by broken circuli along the 

 anterior border of the scale. Posterolaterally 

 such year marks might cross only one or two 

 circuli and were seldom complete in the posterior 

 field after the third annulus. 



The time of completion of annulus formation 

 for yearlings was found to be in the third week of 

 June, by the following procedure. In 1954, collec- 

 tions of grayling, which hatched the previous 



