FISHERY BULLETIN: VOL. 85, NO. 4 

 Table 5. — Comparison of ageing variability of Pacific cod by ANOVA. 



significant at 1% level. 



Table 6 — Tests for differences between mean ages of Pacific cod using various 

 ageing methods. Bracket and underline: not significantly different at 5% level. 



Dorsal 



fin-ray 



age 



N Reading 



Dorsal 

 fin ray 



Otolith 



Pectoral 

 fin ray 



Scale 



SD 



df 



traced from the length-frequency distributions 

 from 1978 to 1983 (Fig. 2). Using the method of 

 Macdonald and Pitcher (1979), the mean lengths 

 for ages 1-6 were 22, 35, 43, 52, 61, and 64 cm 

 respectively (Lai 1985) and were very close to the 

 modal length of length-frequency distributions. 

 Figure 2 shows the mean lengths at age esti- 

 mated from the 1983-84 samples by the five age- 

 ing methods and the comparison to the modal 

 lengths. It is apparent that the mean lengths at 



age estimated from dorsal fin rays were closest to 

 the modal lengths. Also, the variability around 

 mean length at age estimated from dorsal fin rays 

 was generally smaller than for the other methods. 

 We also used the index of variation^ (IV, Lai 



61 V = 100% • Vscy, -X,m{n - l)(X + Y)/2], where X, is 

 the first age reading, Y, js the second age reading, n is the 

 sample size, and X and Y are mean of the X, and y, (Lai 

 1985). This indicates the degree of variation between the two 



718 



