SIX and HORTON: ANALYSIS OF AGE DETERMINATION METHODS 



than ±1 zone. Kelly and Wolf (1959) reported 

 59.7% agreement ±1 yr for otoliths of 7-20+ yr- 

 old redfish. 



Several explanations exist for the observed 

 deviations between readings. Due to the presence 

 of split zones and the irregularity of the marginal 

 areas on older rockfish otoliths, different readings 

 may be obtained from different areas of the same 

 otolith. There are eight major marginal areas on 

 otoliths that can be used in age determination 

 (Figure 4); two or three generally give superior 

 results depending on the species in question. 

 However, these favored areas are not consistently 

 readable from one otolith to the next in any 

 sample. Therefore, there is no specific area that 

 can be used consistently on all the otoliths, 

 making it possible that two different areas could 

 be read on two independent readings of the same 

 otolith. Indeed a comparison of areas used by 

 readers A and B for yellowtail rockfish otoliths 

 showed that of the readings that disagreed, 71% 

 were made on different areas of the otolith, 

 whereas, of the readings that agreed, only 56% 

 were made on different areas. 



Discrepancies in counts of annuli also are 

 probably a function of the difficulty in defining the 

 type of outer edge on otoliths. If an otolith had two 

 opaque zones, each followed by a hyaline zone, 

 plus an additional opaque zone on the outer edge, 

 then an age of 2 was assigned. If an additional 



ANTERIOR 



ANTERODORSAL 



DORSAL 



POSTERODORSAL 



ANTEROVENTRAL 



VENTRAL 



POSTEROVENTRAL 



POSTERIOR 



FIGURE 4. — Drawing of the right otolith (sagittal from a 4-yr-old 

 black rockfish as seen under reflected light on a dark background 

 showing the marginal areas used in age determination (O- 

 opaque zone; H-hyaline zone). 



hyaline zone existed on the edge of the above 

 otolith, then an age of 3 was assigned. But since 

 the zones on the outer edge of older rockfish are 

 indistinct because of slow growth at older ages, it 

 is conceivable that discrepancies of 1 yr could exist 

 between independent readings of the same area of 

 a particular otolith. 



A third cause of discrepant counts is that entire 

 samples of otoliths were often exceptionally 

 opaque, or, conversely, transparent, possibly due 

 to the storage medium and/or length of storage. 

 Annuli on otoliths such as these are difficult to 

 distinguish. 



Because one could question the use of only two 

 readings to assess the consistency of otolith 

 readings, a sample of 198 yellowtail rockfish 

 otoliths was read independently three times with 

 a week between readings. A chi-square test for 

 independent data corrected for continuity indi- 

 cated no significant differences among the three 

 agreement statistics (P>0.75). In this case, 

 consistency of readings was not changed by the 

 addition of a third reading. 



Validity of the Otolith Method 



Until the data needed for validation can be 

 collected, it is assumed for the purposes of this 

 study that one opaque and one hyaline zone are 

 laid down each year on otoliths of rockfishes in 

 Oregon. Van Oosten (1929) and Graham (1956) 

 listed methods used to provide indirect evidence of 

 the validity of age readings of scales and other 

 structures. The commonly applied methods are 

 observation of a dominant year class over a period 

 of years, and analysis of seasonal changes of the 

 margin of some anatomical structure. Westrheim 

 (1973) was able to follow the yearly progression of 

 a dominant year class of Sebastes alutus for a 

 period of several years and also demonstrated, by 

 examination of the marginal zones on the otolith, 

 that the hyaline zone is formed annually on 

 juvenile fish. Kelly and Wolf ( 1959) found that one 

 opaque and one hyaline zone are laid down each 

 year on otoliths of young S. marinus. 



Unfortunately, similar tests could not be con- 

 ducted in this study owing to the absence of any 

 obviously dominant year classes in the fish 

 sampled and to the inadequate samples of young 

 fish from a sufficient number of months through- 

 out the year to permit demonstration of the 

 seasonal changes in the margin of the otolith. 

 Otoliths from older rockfish are not suitable for 



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