SIX and MORTON: ANALYSIS OF ACE DETERMINATION METHODS 



E 

 I 



2 4 6 8 10 12 14 16 18 20 22 



ESTIMATED AGE (yr) 



FIGURE 8. — Age-length relationships for black rockfish derived 

 from two independent readings of their otoliths collected from 

 Oregon samples, 1973-75. 



estimates for L„, slight overlap for k, and 

 considerable overlap for t Q . As was found for 

 yellowtail and canary rockfishes, sexual differ- 

 ences in growth of black rockfish are apparent. 



Further support of the otolith method may be 

 evidenced by a comparison of mean lengths-at-age 

 obtained in this study with those of other 

 investigators. Phillips ( 1964) and Westrheim and 

 Harling (1975) reported mean lengths similar to 

 those obtained in this study for yellowtail rockfish 

 (Table 8). A similar correspondence of canary 

 rockfish lengths does not exist, where an increase 

 of values from north to south is noted. This 

 analysis is limited by small sample sizes and could 

 further be complicated by geographical differ- 

 ences in growth reported to exist for other species 

 of rockfishes in the Northeast Pacific (Westrheim 

 and Harling 1975). 



In summary, the observed deviations between 

 otolith readings produced slightly different esti- 

 mates of survival and of age-length relationships, 

 although these differences were not statistically 

 significant. The otolith method is the most 

 reliable of those analyzed and we believe, with 

 some reservations, that it can be used reliably for 

 management purposes. The reader should be 

 cautioned that contrary to the results of the 

 statistical test, some of the survival estimates 

 appear to be substantially different (Table 6). 

 Possibly a Type II error exists (Snedecor and 

 Cochran 1967), i.e., the statistical test shows no 

 significant difference when, in fact, one exists. We 

 believe that, for the most part, the observed 

 deviations between readings are minor; moreover, 

 with the collaboration of two or more trained 

 readers, consistency of age determinations can be 

 improved. 



Further studies establishing the validity of the 

 technique are warranted. This may be made 

 possible by analysis of the marginal growth of the 

 otoliths of juvenile rockfish. By providing evi- 

 dence that an opaque and an adjacent hyaline 

 zone truly constitute an annulus, accuracy of 

 otolith age determinations will be ensured. 



ACKNOWLEDGMENTS 



We thank the following individuals and organi- 

 zations for their willing and generous support: 

 personnel of the Oregon Department of Fish and 

 Wildlife provided financial support, advice, and 

 samples — especially J. M. Meehan, J. G. Robin- 

 son, and R. L. Demory. Ruth Mandapat and 

 Sandra Oxford, Washington Department of Fish- 

 eries, provided some of the age determinations of 

 yellowtail rockfish; and Alfred Soeldner, Oregon 

 State University, helped with electron micros- 

 copy. R. G. Peterson, D. G. Chapman, and S. J. 

 Westrheim provided statistical advice; N. J. 



TABLE 8.— Mean length (centimeters) at selected ages of yellowtail rockfish and canary 

 rockfish from British Columbia, Oregon, and California. Numbers of fish are shown in 

 parentheses. 



British Columbia 

 (Westrheim and Harling 1975) 



Oregon 

 (This study — reading 1 ) 



California 

 (Phillips 1964) 



413 



