270 BULLETIN OF THE BUREAU OF FISHERIES 



GENERAL CONSIDERATIONS 



DESCRIPTION OF COREGONID SCALES 



TYPICAL SCALES 



In their general features the scales of coregonid species are so much alike that 

 the detailed description of a typical whitefish scale published elsewhere (VanOosten, 

 1923) suffices for the herrmg. It may be sufficient here to reproduce the explanatory 

 photograph of this whitefish scale and to point out briefly the significant structural 

 features. Figure 2 represents a typical scale of a whitefish 197 millimeters in length 

 captured October 22, 1917, at East Tawas, Mich., on Lake Huron.. Near the center 

 of the scale is a small clear area, the focus (F), which presumably represents the 

 original scale in the young specimen. Around this focus are numerous more or less 

 relieved striations, concentric, or nearly so, with the margin. These are termed 

 circuli (C) and mark successive stages in the growth of the scale. Rvmning from the 

 focus to the periphery of the scale are four more or less conspicuous radiating ridges 

 {AR, PR), which divide the surface of the scale into four roughly triangular areas 

 or fields. When the scale is in position in the fish, the area to the right in the figure, 

 the exposed area, is directed toward the tail and therefore is designated as the caudal 

 or posterior area (caudal). The area opposite the caudal is the anterior (anterior), 

 while the two areas that separate the caudal from the anterior are the lateral, or the 

 doi-sal (dorsal), and the ventral (ventral). The borders of these four areas, which 

 form the periphery of the scale, accordingly are termed the caudal, anterior, dorsal, 

 and ventral borders. The radiating ridges are either anterolateral (AR) or postero- 

 lateral (PR). The greatest anteroposterior diameter that bisects the caudal area of 

 the scale is its length {L-L). 



By careful examination two distinct zones may be seen in this scale, an inner 

 characterized in general by more closely spaced lines and an outer in which the lines 

 are farther apart. The mner zone represents, according to current theory, the 

 entire growth of the first year, while the outer zone represents the growth 

 of the second summer. If the lines of growth in the lateral field be followed 

 from the center outward and downward along the anterolateral ridge, it may be 

 seen that the first 20 are complete and uniformly spaced. With occasional breaks 

 and irregularities, they may be traced entirely around the scale. The next sLx are 

 incomplete and the outermost of them ends (or begins) near the anterolateral ridge. 

 Following this last incomplete line to the anterior field a region is encoimtered within 

 which the mdividual circuli can no longer be traced with certainty, for they are less 

 distinct, much broken, anastomosed, and closer together. This zone of faint, approx- 

 imated, and much broken circuli, when contrasted with the preceding and succeeding 

 areas of strong, complete, and widely spaced circuli, often stands out as a rather sharply 

 defined band. This band may be traced around the whole scale and is, perhaps, 

 better defined in the posterior field, where it appears as a lighter zone vdth very 

 little detail. This band, representing retarded growth, is here called the annulus (A). 



When the scale resumes its rapid growth, a complete circulus is formed again, 

 which, in the process of uniting, as it were, the incomplete lines, bends sharply at 

 the anterolateral ridge. This circulus is considered the limit of the annulus it incloses 

 and is employed so in the measurements of scales. 



