24 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



ducing scale impressions that was fast and at the same time gave accurate results. 

 This method has been applied with complete success to striped bass scales. Trans- 

 parent celluloid, acetate base, was obtained in sheets 20 by 50 inches and 0.050 inch 

 thick. It was cut into pieces 1 by 2)i inches so that over 100 fitted in an ordinary 

 wooden slide-box of 25-slide capacity. The scale-sample numbers were written on 

 each slide with Volger's Opaque Quick-Drying Ink. The surface of a slide was then 

 softened slightly by spreading a thin film of acetone over it with a glass slide, and 

 the scales making up that particular sample were placed outer surface downward 

 on the area that had been moistened with acetone. The slide and scales were next 

 subjected to pressure under a reinforced seal press having a die approximately 

 1M inches in diameter. The scales were then removed and the impressions of their 

 outer surfaces were left clearly imprinted on the slide. Measurements on 50 scales 

 from striped bass of all sizes were made before they had been subjected to pressure, 

 and then the impressions of these same scales on transparent celluloid were measured; 

 there was no significant difference in the two measurements. Thus it is clear that 

 no stretching takes place in the scale impression method described above. The ad- 

 vantages of this method are threefold: (1) The cast of the outer surface is easier to 



ctnllME TEftS 



Fioitre 16. — Diagram for the conversion of different types of length measurements. 



study than the scale itself because the light does not have to penetrate the fibrillar 

 layers of the scale to show the desired marking; it is also better for photographic 

 purposes. (2) The method is much faster. (3) The cost is far less. 



All scales, or scale impressions that were studied for age determinations, or on 

 which measurements were made, were first examined under a dissecting microscope, 

 a magnification of about 20 times being satisfactory for most purposes. Those that 

 were measured were then placed in a micro-projection apparatus and the necessary 

 measurements were made on the image, which was magnified 13.75 times. 



The problem of interpreting annuli correctly at all times in scales from striped 

 bass is somewhat complicated by the occasional presence of accessory, or false annuli. 

 Usually, however, these false annuli are different in structure, so that they are quite 

 often easily recognizable. The false annuli are mainly of two types. The first is a 

 broad accessory annulus that is scarlike in its appearance and is frequently seen on 

 scales from larger fish, extremely rarely on those from smaller individuals 2 or 3 years 

 old. This type of mark invariably appears just outside a true annulus or in close con- 

 junction with it. It seems likely that these are spawning marks, since striped bass 

 are anadromous and spawning occurs in the spring near the time of the formation 

 of a true annulus (pp. 20 and 22). The second type of false annulus has much the 

 same appearance as a true annulus, but is distinguishable on close examination by the 



