AG:: AND GRO'T'I OF JUVSMILE PILCHARDS AS- 

 JUDGED FROI'! SCALES AND OTOLITHS 



The Use of Scales for Determining ^ige 



The scales of pilchards are typical of fishes in the fanily Clupeidae. 

 Like herring scales, they are sciilptured on the upper surface of the 

 unexposed part with low transverse folds, (Figure 12) which are in most 

 specimens variously interrupted ^.Tith rather coarse radii; and they are, 

 with fevf exceptions, alnost entirely unsculpttired on -their exposed part. 

 Like herring scales, they differ from scales of many other species that 

 are used for age determination, such as cod, haddock, or salmon, in 

 that the surface folds — "circuli" — are tjiDically not circular or concen- 

 tric, but transect the scale from the dorsal to the ventral margins. 

 Careful study has shown no irregularity in spacing between these folds, 

 extending clear along their length, that can be related to age; there 

 is no indication of winter narrowing of spaces, or of spring vifidening; 

 nor any periodic change in texture that involves entire folds. 



Close inspection of an adult pilchard scale, however, reveals other 

 marks, concentric v^ith the margin , along the line of which the trans- 

 verse folds are more or less distorted and irregularly spaced. These 

 are similar to the year marks on the scales of European herring, and 

 are what Dr. Thompson counted as annuli in his study (1926) reviewed 

 above, and also what the authors of this paper have considered as such. 



If it could be demonstrated that they are really annual marks, 

 the problem of determining the age of pilchards might then become a 

 mere matter of counting them. However, they are not distinct in ordinary 

 preparations, and special mounting is necessary to bring them out. 



It Tfas found during the present study that unless a pilchard scale 

 be perfectly clean it is almost invariably useless fbr age determination. 

 Because of the tenacity of the dried mucous, blood, guanin, etc., to the 

 delicate surface structure, dirty pilchard scales cannot be cleaned, 

 once they have dried, by washing in water and rubbing. Consequently, 

 the scales must be cleaned at the time they are removed from the fish , 

 before they are preserved in envelopes. For" dirty, dried scales, a 

 moderately satisfactory method of cleaning is the folloviingj Soak the 

 scale until thoroughly moistened in a 10 percent solution of sodium hy- 

 droxide, then rinse in water, rub vj'ith fingers to remove offal, rinse 

 in 10 percent acetic acid to neutralize, and then rinse in clean water. 



For mounting, many liquid media were tried; glycerine, water, mix- 

 tures of the two, and various kinds of oils and glues. These substances 

 all rendered the scale too transparent for use, seriously decreasing the 

 visibility of the surface structure, an effect found to be more or less 

 proportional to the refractive index of the mounting medium (see figures 

 25 and 26). 



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