LAI ET AL.: AGE DETERMINATION OF PACIFIC COD 

 12 ^ 



10 - 

 8 - 

 6 - 



4 

 2 







J 



1978 



10 n 



6 



4 

 2 



1981 



y ,- -^i--> 



T 1 1 1 



T 1 1 



1982 



Figure 2. — Length-frequency distributions collected from trawl surveys in 1978-83 (after Bakkala and Wespestad 1984). The mode 

 in 1978 represents age-1 Pacific cod of the 1977 year class, and progresses from age 2 to age 6 in the subsequent years. The estimated 

 mean lengths at ages 1-6 estimated from the five ageing methods (1, coracoids; 2, otoliths; 3, dorsal fin rays; 4, pectoral fin rays; and 

 5, scales) correspond to the age of 1977 year class. Horizontal lines indicate 95% confidence interval around means. Vertical lines 

 indicates the mean length from length-frequency analysis. 



1985) to examine the degree of precision. Among 

 the five ageing methods, the IV for dorsal fin rays 

 was the lowest ( ISVc ) when compared with other 

 methods (149c, 14%, 15%, and 16% respectively 

 for otoliths, coracoids, scales, and pectoral fin 

 rays). 



The accuracy of converting scale ages to dorsal 

 fin-ray ages was also evaluated. Mean length at 

 age and age composition (obtained by using con- 

 verted dorsal fin-ray ages) were compared with 

 corresponding results from length-frequency 

 analysis (Macdonald and Pitcher 1979). The 95% 



ages being compared taking the age distribution of the sample 

 into account. 



confidence interval (Fig. 3) for each converted 

 mean length at age in 1979 and 1980 included the 

 corresponding value estimated from the length- 

 frequency analysis. However, the mean lengths 

 at age derived from scales were significantly dif- 

 ferent from the other two. 



The age composition estimated by the scale 

 method showed a monotonic decrease with age in 

 1979, and the strong 1977 year class (age 2) was 

 not evident (Fig. 4). Nevertheless, length- 

 frequency data from surveys indicated that age-2 

 fish were dominant in 1979 (Bakkala and We- 

 spestad 1984). The age composition based on con- 

 verted dorsal fin-ray ages was dominated by age- 

 2 fish and was similar to that estimated from the 



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