tend to be the lo^.^er, and that for a^e on scale length the higher value 

 of the tv/o. Obviously, judging from tables III and IV, just the oppo- 

 site of this result is deraonstrated. Thus, so far as can be deduced 

 from this evidence, there is no confirnation of a supposition that scale 

 length significantly influenced the age determinations in the present 

 study. Indeed, it appears unlikely that it vras of any influence what- 

 ever. 



The Use of Otoliths for Determining Age 



Another line of evidence by which to judge the correctness of scale 

 readings is given by a study of otolitlis. This was carried on simul- 

 taneously by Mosher, independently of the scale v/ork, and without refer- 

 ence to it. 



Otoliths were collected from 6000 fish of the same series, de- 

 scribed above, as represented by scales. These were washed and pre- 

 served in envelopes in the same manner as the scales. They were sub- 

 sequently mounted as follovra ; A piece of thin cardboard was cut in 

 the shape of a standard microscope slide, but about l/8 of an inch 

 shorter and narrovfer. This was punched with eight small round holes, 

 and laid over a drop of Canada Balsaia on a microscope slide. Into 

 each hole v;as placed an otolith, then a drop of Canada balsam; and 

 any air bubbles were burned out with a hot needle. Another microscope 

 slide was laid on the mount as a cover glass, and the preparation put 

 in a warm place to harden. 



The mounted otoliths were examined in a micro-projection apparatus 

 and the dimensions recorded on punch cards, as described on page 8, 

 v/ith the base of the ruler placed at the center, as indicated in Figure 

 28. The inner edge of each translucent zone was taken as representing 

 the year mark. These readings were later checked by examination with 

 a binocular microscope and reflected light, vrtiich brought out the year 

 marks sharply as blue-black against the white opaque intervening zones. 



Preliminary examination of otoliths from fish known to belong to 

 year class and collected over the course of 12 months established 

 the following criteria of a year mark: 



An annulus is a more or less translucent band concentric 

 with the margin of the otolith, the intervening spaces 

 being opaque. It can usually be traced entirely around 

 the otolith, although it is more easily observed at the 

 blunter anterior end than at the sides or posterior end. 

 Annul! tend to be zones rather than lines. 



Marks having the following characteristics were considered to be 

 adventitious, or otherwise unperiodic; 



Though translucent, they tend to be lines rather than zones j 

 they frequently meet an annulus at some point. 



46 



