AGE OF ATLANTIC SHAD FROM SCALES 



191 



Fk.i re 6. Scale from a 19-inch female shad weighing 4 pounds. The single spawning mark (S. M.) is very clear on 

 this particular scale. Note that the fourth transverse groove is branched. The Roman numerals Indicate annuli 

 and spawning mark. 



A Recordak projector, designed for microfilm 

 reading, and a binocular microscope of low power 

 were tried as sources of magnification. The Re- 

 cordak projector was found to be satisfactory for 

 projecting and reading the easily read scales, but 

 where difficulties occurred, it was found that the 

 microscope gave a better image of the scale mark- 

 ings, especially of the spawning marks. Most of 

 the impressions were read by the latter method. 

 The illustrations used in this paper are negatives 

 made from the image on the ground-glass screen 

 of the Recordak projector. 



An attempt was made to read 400 selected scales 

 by the conventional method of counting annuli 

 and spawning marks. Agreement in both re- 



reading and between 2 independent readings by ~l 

 persons was obtained on 164, or 11 percent of the 

 scales. Agreement between independent readings 

 l>\ two persons on the other 236 scale samples was 

 poor because of poorly defined annuli and numer- 

 ous false annuli. Because of this high percentage 

 of disagreement it was considered necessary to 

 develop criteria to distinguish the true from the 

 fa he annuli or otherwise indicate the true age of 

 the fish. 



CRITERIA FOR DETERMINING AGE 



Counting transverse grooves 



Only those 164 scales that were clearly marked 

 and on which consistent agreement had been 



