AGE DETERMINATION OF ALBACORE 



355 



Measurements were made from the vertex of 

 the cone to each ring (ring radius) and also to 

 the outer margin of the cone (vertebral radius) 

 along the four exposed edges. Ring radii were 

 measured with a pair of dividers under a dis- 

 secting microscope and read off to the nearest 

 0.1 mm. on a ruler graduated in 0.5-mm. intervals. 

 The anterior and posterior cone measurements 

 were treated independently. The two corre- 

 sponding ring and vertebral radii measurements 

 of each cone were averaged. 



RESULTS 



Various Skeletal Structures 



Cursory examination of the operculum, dorsal 

 and pectoral fin spines, and the hypural plate pro- 

 duced no evidence that these structures possessed 

 any markings which could be interpreted as "age 

 marks." The operculum showed distinct marks 

 only on the outer free edge; the central area was 

 invariably thickened and devoid of any markings. 



Scales 



The albacore scale is cycloid and either oval or 

 roughly rectangular in general outline. The circuli 

 are concentric with the margin of the scale. 

 Examination revealed that the smaller scales 

 generally exhibited a rather distinct focus and 

 clearly defined circuli, but that the larger scales 

 (from fish larger than 90 cm.) were mostly charac- 

 terized by obscure sculpturing and a thickened 

 central area. Furthermore, it was noted in several 

 instances that scales from the same specimen 

 (usually in large fish) were of variable size and 

 shape. A count was made of the rings on different 

 sized scales from the same fish following the defini- 

 tion of an annulus as given by Nose and others 

 (1955), who state: "The ring appears as a trans- 

 parent line, near by which the circuli become fine 

 and discontinuous . . ." Though the counts on 

 the larger scales were necessarily somewhat sub- 

 jective, because of the general opaqueness of the 

 scales, there appeared to be some trend in the 

 number of rings, the smaller scales having a lesser 

 count than the larger. This would seem to indi- 

 cate that the smaller scales had originated more 

 recently than the larger ones. 



Results of ring counts 



Scales from 100 fish were examined. Of this 

 total, the scales from 43 fish were considered 



unreadable, in large part because of opaqueness 

 of the central area. The samples from the re- 

 maining 57 fish varied widely in the degree of 

 readability. Despite the obvious difficulties, an 

 attempt was made to count the rings on scales 

 which had the greatest number of annuli for each 

 sample. The ring counts are plotted against fish 

 size in figure 3. There is a tendency for ring 



60 70 80 90 IOO 110 



FORK LENGTH (CM.) 



120 



Figure 3. — Age of 57 albacore, determined by scale read- 

 ing, plotted against fish size. (The Roman numerals 

 represent the number of rings, or "age".) 



counts to increase with increasing fish size, up to 

 8 rings. The size ranges of fish with 5 or more rings 

 were broad, however, with a great amount of over- 

 lapping between successive groups. 



Moore (1951) in working with Hawaiian yellow- 

 fin tuna encountered similar difficulties. He also 



