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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



observed that small specimens of yellowfin, weigh- 

 ing about 5 pounds, had no scales except on the 

 corselet region. He therefore concluded that 

 scales were formed first on the anterior and later 

 on the posterior part of the body. He stated that 

 "if this is the case, any marks laid down in the 

 scales would be of little value until the nature of 

 scale formation and its relation with size and age 

 of fish was known." The nature of scale formation 

 on the albacore has not as yet been determined. 

 Since it was our experience that only about half 

 of the scales examined were considered readable, 

 and these were read with considerable subjectivity, 

 further work with scales was discontinued. 



VERTEBRAE 



Description 



The albacore vertebra is amphicoelous, or 

 concave at both ends of the centrum. A sectioned 

 vertebra shows two cones, the vertices of which 

 meet at or near the middle of the centrum, with 

 their bases forming the margins of the vertebra. On 

 the inner surface of these cones are concentric 

 rings running parallel to the outer margin of the 

 centrum. These rings are described by Partlo 

 (1955) as "narrow translucent zones separated by 

 broad, opaque zones similar to those described by 

 Freidenfelt (1922) for Luciopcrca. In the albacore 

 the narrow zones were observed not only as 

 translucent bands but also as eruptions or ridges 

 on the centrum surface. The innermost ring 

 differed slightly from the others but was never- 

 theless clearly marked." We also observed certain 

 rings that appeared as translucent ridges on the 

 surface of the centrum; there were other rings, 

 however, that resembled narrow, translucent 

 bands without any surface eruptions, and still 

 others that were mere suggestions of lines or bands. 



Change in the Shape of Vertebrae With Growth of Fish 



In the course of examining a wide series of 

 vertebrae, it was noted that while the two cones 

 of an amphicoelous caudal vertebra were nearly 

 the same size in small fish, the posterior cone was 

 larger than the anterior in large fish, indicating 

 that the vertebra changed its shape with growth 

 of the fish. The lengths of the two cones (vertebral 

 radii) were measured and the ratio of the two 

 measurements was plotted against the size of the 

 fish (fig. 4) . While the ratio was nearly 1 : 1 in the 

 smallest vertebrae examined (fish around 50 to 60 



70 80 90 100 



FORK LENGTH (CM) 



Figure 4. — Ratio of the average length of the posterior 

 and anterior cones plotted against fish size to demon- 

 strate the disproportionate growth of the two cones of 

 the amphicoelous vertebrae of albacore. 



cm. long), the posterior cone was definitely larger 

 than the anterior in the larger vertebrae. This 

 changing proportion is described by the rectilinear 

 regression : 



F=0.9229 + 0.0019 L 



where Y is the ratio of the two cones and L is the 

 fork length in centimeters. Although the two 

 cones of a caudal vertebra grow disproportionately, 

 each cone in itself exhibits a linear growth with 

 growth of the fish. Thus, in working with caudal 

 vertebrae from a wide size range of fish, it is 

 important that any measurements of ring or verte- 

 bral radii, for the purpose of estimating the growth 

 increments of the fish, should be confined to either 

 the anterior or the posterior cones and not to 

 averages of the two. By working with one cone, 

 growth calculations can be based on the simple 

 linear relation between size of the vertebrae and 

 size of fish. 



Criterion for an annulus 



Because of the great variability in the appear- 

 ance of the rings, it was extremely difficult to 

 formulate a satisfactory criterion for an annulus. 

 In the beginning, we followed the definition of 

 Partlo (1955), who considered as annuli those 

 rings which appeared as ridges on the centrum 

 surface. However, as will be discussed, our 

 original criterion was later expanded to include 

 rings which appeared as translucent bands with 

 no surface eruptions. 



