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Fishery Bulletin 103(1) 



different methods on the same otoliths, the residuals 

 were correlated and thus this result can be considered 

 only approximate. 



Predicting growth increments of fish length from 

 tagged fish 



Using the von Bertalanffy curve fitted with the data 

 from the tagged fish sample, we estimated the value of 

 t to be 0.147. 



The amount that each tagged fish grew after tagging 

 was calculated three times by using fish age at recovery 

 and the von Bertalanffy equation (Fig. 4). The calcu- 

 lated sums of squared deviations for the three sets of 

 predicted values are as follows: 433,955 when fish age 

 is scaled by 0.75, 419,477 when fish age is scaled by 1.0, 

 and 761,545 when fish age is scaled by 1.25. The lowest 

 sum of squared deviations accompanied ages that were 

 scaled by 0.86. Assuming that the residuals of the esti- 

 mated growth increments have independent chi-square 

 distributions, an F-test indicates that residu- 

 als were significantly larger (P<0.05) when 

 ages were scaled 25% older and there was 

 no significant difference (P<0.05) between 

 reader-determined ages and ages scaled 25% 

 younger. The three sets of residuals came 

 from the same otoliths and would be corre- 

 lated; therefore, this result can be considered 

 only approximate. 



Another test of our reading criteria was 

 performed through a more direct compari- 

 son: simply "aging" the tagged fish from esti- 

 mated age at tagging (based on length), plus 

 the time after tagging (Table 1). Out of 106 

 samples, 75% of these fish were within one 

 year of the age that we had determined from 

 otolith readings, and 94% were within two 

 years. The average percent error (Beamish 

 and Fournier, 1981) was 8.70, and the aver- 

 age deviation from tagged-based age was 

 -0.075. Results of a Z-test indicated that 

 the average deviation was not significantly 

 different from zero (P= 0.724) and indicated 

 no bias in the age estimates. 



Discussion 



Beamish and McFarlane (1983) noted that 

 "validating a method of age determination is 

 as important in fishery biology as standard- 

 izing solutions or calibrating instruments are 

 in other sciences." Age determination must 

 reflect the actual age of each fish in order to 

 be a useful tool for use in stock assessments. 

 Although much effort has been devoted in the 

 past to finding an appropriate aging struc- 

 ture for Pacific cod, particularly with dorsal 

 fin rays (Beamish, 1981; Lai et al., 1987; 

 Kimura and Lyons, 1990), scales and otoliths 

 (Lai et al., 1987; Kimura and Lyons, 1990), a 

 directly validated method of age determina- 

 tion has yet to be found ( Westrheim, 1996). 

 The otolith seems to be the most promising 

 structure for production (large-scale) age 

 reading of Pacific cod (Kimura and Lyons, 

 1990); however it is not without weaknesses 

 (i.e., the faint patterns of some translucent 

 zones can lead to low precision between read- 

 ers and are a constant concern in regard to 



